3: GENETICS

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 CHAPTER OVERVIEW

3: Genetics
Covers Mendelian and non-Mendelian genetics, human genetics, and biotechnology.

Topic hierarchy
3.1: Mendel's Pea Plants
3.2: Mendel's First Set of Experiments
3.3: Mendel's Second Set of Experiments
3.4: Mendel's Laws and Genetics
3.5: Probability and Inheritance
3.6: Punnett Squares
3.7: Non-Mendelian Inheritance
3.8: Human Genome
3.9: Human Chromosomes and Genes
3.10: Genetic Linkage
3.11: Mendelian Inheritance in Humans
3.12: Genetic Disorders
3.13: Biotechnology
3.14: Biotechnology Applications
3.15: Ethical, Legal, and Social Issues of Biotechnology

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3.1: Mendel's Pea Plants

What's so interesting about pea plants?

These purple-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. The plants are
common garden pea plants, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. With his careful
experiments, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring.
You may not care much about heredity in pea plants, but you probably care about your own heredity. Mendel’s discoveries apply to
you as well as to peas—and to all other living things that reproduce sexually.

Mendel and His Pea Plants

People have long known that the characteristics of living things are similar in parents and their offspring. Whether it’s the flower
color in pea plants or nose shape in people, it is obvious that offspring resemble their parents. However, it wasn’t until the
experiments of Gregor Mendel that scientists understood how characteristics are inherited. Mendel’s discoveries formed the basis
of genetics, the science of heredity. That’s why Mendel is often called the "father of genetics." It’s not common for a single
researcher to have such an important impact on science. The importance of Mendel’s work was due to three things: a curious mind,
sound scientific methods, and good luck. You’ll see why when you read about Mendel’s experiments.
An introduction to heredity can be seen at http://www.youtube.com/watch?v=eEUvRrhmcxM(17:27).

Gregor Mendel was born in 1822 and grew up on his parents’ farm in Austria. He did well in school and became a monk. He also
went to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through
experimentation and to use math to make sense of his results. Mendel is best known for his experiments with the pea plant Pisum

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sativum (see Figure below). You can watch a video about Mendel and his research at the following link:
http://www.biography.com/people/gregor-mendel-39282.

Gregor Mendel. The Austrian monk Gregor Mendel experimented with pea plants. He did all of his research in the garden of the
monastery where he lived.

Blending Theory of Inheritance

During Mendel’s time, the blending theory of inheritance was popular. This is the theory that offspring have a blend, or mix, of the
characteristics of their parents. Mendel noticed plants in his own garden that weren’t a blend of the parents. For example, a tall
plant and a short plant had offspring that were either tall or short but not medium in height. Observations such as these led Mendel
to question the blending theory. He wondered if there was a different underlying principle that could explain how characteristics are
inherited. He decided to experiment with pea plants to find out. In fact, Mendel experimented with almost 30,000 pea plants over
the next several years! At the following link, you can watch an animation in which Mendel explains how he arrived at his decision
to study inheritance in pea plants:http://www.dnalc.org/view/16170-Animation-3-Gene-s-don-t-blend-.html.

Why Study Pea Plants?

Why did Mendel choose common, garden-variety pea plants for his experiments? Pea plants are a good choice because they are fast
growing and easy to raise. They also have several visible characteristics that may vary. These characteristics, which are shown in
Figure below, include seed form and color, flower color, pod form and color, placement of pods and flowers on stems, and stem
length. Each characteristic has two common values. For example, seed form may be round or wrinkled, and flower color may be
white or purple (violet).

Mendel investigated seven different characteristics in pea plants. In this chart, cotyledons refer to the tiny leaves inside seeds. Axial
pods are located along the stems. Terminal pods are located at the ends of the stems.

Controlling Pollination
To research how characteristics are passed from parents to offspring, Mendel needed to control pollination. Pollination is the
fertilization step in the sexual reproduction of plants.Pollen consists of tiny grains that are the male gametes of plants. They are
produced by a male flower part called the anther (see Figure below). Pollination occurs when pollen is transferred from the anther
to the stigma of the same or another flower. The stigma is a female part of a flower. It passes the pollen grains to female gametes in
the ovary.

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Flowers are the reproductive organs of plants. Each pea plant flower has both male and female parts. The anther is part of the
stamen, the male structure that produces male gametes (pollen). The stigma is part of the pistil, the female structure that produces
female gametes and guides the pollen grains to them. The stigma receives the pollen grains and passes them to the ovary, which
contains female gametes.
Pea plants are naturally self-pollinating. In self-pollination, pollen grains from anthers on one plant are transferred to stigmas of
flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-
pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand
with pollen from other parent plants of his choice. When pollen from one plant fertilizes another plant of the same species, it is
called cross-pollination. The offspring that result from such a cross are called hybrids.

Summary
Gregor Mendel experimented with pea plants to learn how characteristics are passed from parents to offspring.
Mendel’s discoveries formed the basis of genetics, the science of heredity.
Cross-pollination produces hybrids.

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Use this resource to answer the questions that follow.
Children resemble their parents at http://www.dnaftb.org/1/bio.html.
1. What did Gregor Mendel discover about "factors", which are genes?
2. Briefly state Mendel's three laws.

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Explore More II
Gregor Mendel and pea plants at http://www.dnalc.org/view/16002-Gregor-Mendel-and-pea-plants.html.

Review
1. What is the blending theory of inheritance? Why did Mendel question this theory?
2. List the seven characteristics that Mendel investigated in pea plants.
3. How did Mendel control pollination in pea plants?
4. What are hybrids?

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3.2: Mendel's First Set of Experiments

Peas. Some round and some wrinkled. Why?

That's what Mendel asked. He noticed peas were always round or wrinkled, but never anything else. Seed shape was one of the
traits Mendel studied in his first set of experiments.

Mendel’s First Set of Experiments

Mendel first experimented with just one characteristic of a pea plant at a time. He began with flower color. As shown in Figure
below, Mendel cross-pollinated purple- and white-flowered parent plants. The parent plants in the experiments are referred to as the
P (for parent)generation. You can explore an interactive animation of Mendel’s first set of experiments at this link:
http://www2.edc.org/weblabs/Mendel/mendel.html.

This diagram shows Mendel's first experiment with pea plants. The F1 generation results from cross-pollination of two parent (P)
plants, and contained all purple flowers. The F2 generation results from self-pollination of F1 plants, and contained 75% purple
flowers and 25% white flowers. This type of experiment is known as a monohybrid cross.

F1 and F2 Generations
The offspring of the P generation are called the F1 (for filial, or “offspring”) generation. As you can see from Figure above, all of
the plants in the F1 generation had purple flowers. None of them had white flowers. Mendel wondered what had happened to the
white-flower characteristic. He assumed some type of inherited factor produces white flowers and some other inherited factor
produces purple flowers. Did the white-flower factor just disappear in the F1 generation? If so, then the offspring of the F1
generation—called the F2 generation—should all have purple flowers like their parents.

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To test this prediction, Mendel allowed the F1 generation plants to self-pollinate. He was surprised by the results. Some of the F2
generation plants had white flowers. He studied hundreds of F2 generation plants, and for every three purple-flowered plants, there
was an average of one white-flowered plant.

Law of Segregation
Mendel did the same experiment for all seven characteristics. In each case, one value of the characteristic disappeared in the F1
plants and then showed up again in the F2 plants. And in each case, 75 percent of F2 plants had one value of the characteristic and
25 percent had the other value. Based on these observations, Mendel formulated his first law of inheritance. This law is called the
law of segregation. It states that there are two factors controlling a given characteristic, one of which dominates the other, and
these factors separate and go to different gametes when a parent reproduces.

Summary
Mendel first researched one characteristic at a time. This led to his law of segregation. This law states that each characteristic is
controlled by two factors, which separate and go to different gametes when an organism reproduces.

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Use this resource to answer the questions that follow.
Mendel's First Law of Genetics at http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel1.htm.
1. What is a pure line?
2. What did Mendel always see in the F1 generation?
3. What did Mendel always see in the F2 generation?
4. Summarize Mendel's conclusions from these experiments.

Explore More II
Pea Experiment at http://sonic.net/~nbs/projects/anthro201/exper/.

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Review
1. Describe in general terms Mendel’s first set of experiments.
2. State Mendel's first law.
3. Assume you are investigating the inheritance of stem length in pea plants. You cross-pollinate a short-stemmed plant with a
long-stemmed plant. All of the offspring have long stems. Then, you let the offspring self-pollinate. Describe the stem lengths
you would expect to find in the second generation of offspring.

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3.3: Mendel's Second Set of Experiments

Round and green, round and yellow, wrinkled and green, or wrinkled and yellow?
Can two traits be inherited together? Or are all traits inherited separately? Mendel asked these questions after his first round of
experiments.

Mendel’s Second Set of Experiments

After observing the results of his first set of experiments, Mendel wondered whether different characteristics are inherited together.
For example, are purple flowers and tall stems always inherited together? Or do these two characteristics show up in different
combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. For example, he
crossed plants with yellow round seeds and plants with green wrinkled seeds. The results of this cross, which is a dihybrid cross,
are shown in Figure below.

This chart represents Mendel's second set of experiments. It shows the outcome of a cross between plants that differ in seed color
(yellow or green) and seed form (shown here with a smooth round appearance or wrinkled appearance). The letters R, r, Y, and y
represent genes for the characteristics Mendel was studying. Mendel didn’t know about genes, however. Genes would not be
discovered until several decades later. This experiment demonstrates that in the F2 generation, 9/16 were round yellow seeds, 3/16
were wrinkled yellow seeds, 3/16 were round green seeds, and 1/16 were wrinkled green seeds.

F1 and F2 Generations
In this set of experiments, Mendel observed that plants in the F1 generation were all alike. All of them had yellow and round seeds
like one of the two parents. When the F1 generation plants self-pollinated, however, their offspring—the F2 generation—showed

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all possible combinations of the two characteristics. Some had green round seeds, for example, and some had yellow wrinkled
seeds. These combinations of characteristics were not present in the F1 or P generations.

Law of Independent Assortment

Mendel repeated this experiment with other combinations of characteristics, such as flower color and stem length. Each time, the
results were the same as those in Figure above. The results of Mendel’s second set of experiments led to his second law. This is the
law of independent assortment. It states that factors controlling different characteristics are inherited independently of each other.

Summary
After his first set of experiments, Mendel researched two characteristics at a time. This led to his law of independent
assortment. This law states that the factors controlling different characteristics are inherited independently of each other.

Making Connections

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Use this resource to answer the questions that follow.
Mendel's Law of Independent Assortment at http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel3.htm.
1. What is a dihybrid?
2. What is a dihybrid cross?
3. What were the parental phenotypes for the seeds Mendel used in his dihybrid cross?
4. What were Mendel's results in the F2 generation of his dihybrid cross?
5. What is Mendel's second law? State this law.

Explore More II
The Geniverse Lab at www.concord.org/activities/geniverse-lab.

Review
1. What was Mendel investigating with his second set of experiments? What was the outcome?
2. State Mendel’s second law.
3. If a purple-flowered, short-stemmed plant is crossed with a white-flowered, long-stemmed plant, would all of the purple-
flowered offspring also have short stems? Why or why not?

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3.4: Mendel's Laws and Genetics

Do you look like your parents?

You probably have some characteristics or traits in common with each of your parents. Mendel's work provided the basis to
understand the passing of traits from one generation to the next.

Mendel’s Laws and Genetics

You might think that Mendel's discoveries would have made a big impact on science as soon as he made them. But you would be
wrong. Why? Because Mendel's work was largely ignored. Mendel was far ahead of his time and working from a remote
monastery. He had no reputation among the scientific community and no previously published work.
Mendel’s work, titled Experiments in Plant Hybridization, was published in 1866, and sent to prominent libraries in several
countries, as well as 133 natural science associations. Mendel himself even sent carefully marked experiment kits to Karl von
Nageli, the leading botanist of the day. The result - it was almost completely ignored. Von Nageli instead sent hawkweed seeds to
Mendel, which he thought was a better plant for studying heredity. Unfortunately hawkweed reproduces asexually, resulting in
genetically identical clones of the parent.
Charles Darwin published his landmark book on evolution in 1869, not long after Mendel had discovered his laws. Unfortunately,
Darwin knew nothing of Mendel's discoveries and didn’t understand heredity. This made his arguments about evolution less
convincing to many people. This example demonstrates the importance for scientists to communicate the results of their
investigations.

Rediscovering Mendel’s Work

Mendel’s work was virtually unknown until 1900. In that year, three different European scientists — named Hugo De Vries, Carl
Correns, and Erich Von Tschermak-Seysenegg — independently arrived at Mendel’s laws. All three had done experiments similar
to Mendel’s. They came to the same conclusions that he had drawn almost half a century earlier. Only then was Mendel’s actual
work rediscovered.
As scientists learned more about heredity - the passing of traits from parents to offspring - over the next few decades, they were
able to describe Mendel’s ideas about inheritance in terms of genes. In this way, the field of genetics was born. At the link that
follows, you can watch an animation of Mendel explaining his laws of inheritance in genetic
terms.http://www.dnalc.org/view/16182-Animation-4-Some-genes-are-dominant-.html

Genetics of Inheritance
Today, we known that characteristics of organisms are controlled by genes on chromosomes(see Figure below). The position of a
gene on a chromosome is called its locus. In sexually reproducing organisms, each individual has two copies of the same gene, as
there are two versions of the same chromosome (homologous chromosomes). One copy comes from each parent. The gene for a
characteristic may have different versions, but the different versions are always at the same locus. The different versions are called
alleles. For example, in pea plants, there is a purple-flower allele (B) and a white-flower allele (b). Different alleles account for
much of the variation in the characteristics of organisms.

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Chromosome, Gene, Locus, and Allele. This diagram shows how the concepts of chromosome, gene, locus, and allele are related.
What is the different between a gene and a locus? Between a gene and an allele?
During meiosis, homologous chromosomes separate and go to different gametes. Thus, the two alleles for each gene also go to
different gametes. At the same time, different chromosomes assort independently. As a result, alleles for different genes assort
independently as well. In these ways, alleles are shuffled and recombined in each parent’s gametes.

Genotype and Phenotype

When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent.
The alleles an individual inherits make up the individual’s genotype. The two alleles may be the same or different. As shown in
Tablebelow, an organism with two alleles of the same type (BB or bb) is called a homozygote. An organism with two different
alleles (Bb) is called a heterozygote. This results in three possible genotypes.

Alleles Genotypes Phenotypes

BB (homozygote) purple flowers

B (purple) Bb (heterozygote) purple flowers

b (white) bb (homozygote) white flowers

The expression of an organism’s genotype produces its phenotype. The phenotype refers to the organism’s characteristics, such as
purple or white flowers. As you can see from Tableabove, different genotypes may produce the same phenotype. For example, BB
and Bbgenotypes both produce plants with purple flowers. Why does this happen? In a Bbheterozygote, only the B allele is
expressed, so the b allele doesn’t influence the phenotype. In general, when only one of two alleles is expressed in the phenotype,
the expressed allele is called the dominant allele. The allele that isn’t expressed is called the recessive allele.

How Mendel Worked Backward to Get Ahead

Mendel used hundreds or even thousands of pea plants in each experiment he did. Therefore, his results were very close to those
you would expect based on the rules of probability (see "Probability and Inheritance" concept). For example, in one of his first
experiments with flower color, there were 929 plants in the F2 generation. Of these, 705 (76 percent) had purple flowers and 224
(24 percent) had white flowers. Thus, Mendel’s results were very close to the 75 percent purple and 25 percent white you would
expect by the laws of probability for this type of cross.
Of course, Mendel had only phenotypes to work with. He knew nothing about genes and genotypes. Instead, he had to work
backward from phenotypes and their percents in offspring to understand inheritance. From the results of his first set of experiments,
Mendel realized that there must be two factors controlling each of the characteristics he studied, with one of the factors being
dominant to the other. He also realized that the two factors separate and go to different gametes and later recombine in the
offspring. This is an example of Mendel’s good luck. All of the characteristics he studied happened to be inherited in this way.
Mendel also was lucky when he did his second set of experiments. He happened to pick characteristics that are inherited
independently of one another. We now know that these characteristics are controlled by genes on nonhomologous chromosomes.
What if Mendel had studied characteristics controlled by genes on homologous chromosomes? Would they be inherited together? If

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so, how do you think this would have affected Mendel’s conclusions? Would he have been able to develop his second law of
inheritance?
To better understand how Mendel interpreted his findings and developed his laws of inheritance, you can visit the following link. It
provides an animation in which Mendel explains how he came to understand heredity from his experimental
results.http://www.dnalc.org/view/16154-Animation-2-Genes-Come-in-Pairs.html

Summary
Mendel’s work was rediscovered in 1900. Soon after that, genes and alleles were discovered. This allowed Mendel’s laws to be
stated in terms of the inheritance of alleles.
The gene for a characteristic may have different versions. These different versions of a gene are known as alleles.
Alleles for different genes assort independently during meiosis.
The alleles an individual inherits make up the individual’s genotype. The individual may be homozygous (two of the same
alleles) or heterozygous (two different alleles).
The expression of an organism’s genotype produces its phenotype.
When only one of two alleles is expressed, the expressed allele is the dominant allele, and the allele that isn’t expressed is the
recessive allele.
Mendel used the percentage of phenotypes in offspring to understand how characteristics are inherited.

Making Connections

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Use these resources to answer the questions that follow.
Mendelian laws of inheritance at http://www.scienceclarified.com/Ma-Mu/Mendelian-Laws-of-Inheritance.html#b.
1. What is a gene?
2. The gene for flower color in pea plants can occur in the white or red form. What are these two different forms of the same gene?
3. How many copies of a gene are in a gamete?
4. How many copies of a gene are in a zygote?
5. State Mendel's law of segregation.

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Explore More II
Modern Genetics at http://www.concord.org/activities/modern-genetics.

Review
1. If Darwin knew of Mendel’s work, how might it have influenced his theory of evolution? Do you think this would have affected
how well Darwin’s work was accepted?
2. Explain Mendel’s laws in genetic terms, that is, in terms of chromosomes, genes, and alleles.
3. Explain the relationship between genotype and phenotype. How can one phenotype result from more than one genotype?

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3.5: Probability and Inheritance

What are the odds of landing on 25 again?

Not as high as inheriting an allele from a parent. Probability plays a big role in determining the chance of inheriting an allele from a
parent. It is similar to tossing a coin. What's the chance of the coin landing on heads?

Probability
Assume you are a plant breeder trying to develop a new variety of plant that is more useful to humans. You plan to cross-pollinate
an insect-resistant plant with a plant that grows rapidly. Your goal is to produce a variety of plant that is both insect resistant and
fast growing. What percentage of the offspring would you expect to have both characteristics? Mendel’s laws can be used to find
out. However, to understand how Mendel’s laws can be used in this way, you first need to know about probability.
Probability is the likelihood, or chance, that a certain event will occur. The easiest way to understand probability is with coin
tosses (see Figure below). When you toss a coin, the chance of a head turning up is 50 percent. This is because a coin has only two
sides, so there is an equal chance of a head or tail turning up on any given toss.

Tossing a Coin. Competitions often begin with the toss of a coin. Why is this a fair way to decide who goes first? If you choose
heads, what is the chance that the toss will go your way?
If you toss a coin twice, you might expect to get one head and one tail. But each time you toss the coin, the chance of a head is still
50 percent. Therefore, it’s quite likely that you will get two or even several heads (or tails) in a row. What if you tossed a coin ten
times? You would probably get more or less than the expected five heads. For example, you might get seven heads (70 percent) and
three tails (30 percent). The more times you toss the coin, however, the closer you will get to 50 percent heads. For example, if you
tossed a coin 1000 times, you might get 510 heads and 490 tails.

Probability and Inheritance

The same rules of probability in coin tossing apply to the main events that determine thegenotypes of offspring. These events are
the formation of gametes during meiosis and the union of gametes during fertilization.

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Probability and Gamete Formation
How is gamete formation like tossing a coin? Consider Mendel’s purple-flowered pea plants again. Assume that a plant is
heterozygous for the flower-color allele, so it has the genotypeBb (see Figure below). During meiosis, homologous chromosomes,
and the alleles they carry, segregate and go to different gametes. Therefore, when the Bb pea plant forms gametes, theB and b
alleles segregate and go to different gametes. As a result, half the gametes produced by the Bb parent will have the B allele and half
will have the b allele. Based on the rules of probability, any given gamete of this parent has a 50 percent chance of having the B
allele and a 50 percent chance of having the b allele.

Formation of gametes by meiosis. Paired alleles always separate and go to different gametes during meiosis.

Probability and Fertilization

Which of these gametes joins in fertilization with the gamete of another parent plant? This is a matter of chance, like tossing a coin.
Thus, we can assume that either type of gamete—one with the B allele or one with the b allele—has an equal chance of uniting
with any of the gametes produced by the other parent. Now assume that the other parent is also Bb. If gametes of two Bb parents
unite, what is the chance of the offspring having one of each allele like the parents (Bb)? What is the chance of them having a
different combination of alleles than the parents (either BB or bb)? To answer these questions, geneticists use a simple tool called a
Punnett square, which is the focus of the next concept.

Summary
Probability is the chance that a certain event will occur. For example, the probability of a head turning up on any given coin toss
is 50 percent.
Probability can be used to predict the chance of gametes and offspring having certain alleles.

Making Connections

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Use this resource to answer the questions that follow.
Rules of Probability for Mendelian Inheritanceat www.boundless.com/biology/textbooks/boundless-biology-
textbook/mendel-s-experiments-and-heredity-12/mendel-s-experiments-and-the-laws-of-probability-94/rules-of-probability-for-
mendelian-inheritance-413-11640/.
1. Distinguish between the product rule and the sum rule.
2. Define probability.
3. How can you determine the probability of two independent events that occur together?

Review
1. Define probability. Apply the term to a coin toss.
2. How is gamete formation like tossing a coin?
3. With a BB homozygote, what is the chance of a gamete having the B allele? The b allele?

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3.6: Punnett Squares

What do you get when you cross an apple and an orange?

Though the above fruit may not result, it would be nice to scientifically predict what would result. Predicting the possible
genotypes and phenotypes from a genetic cross is often aided by a Punnett square.

Punnett Squares
A Punnett square is a chart that allows you to easily determine the expected percentage of different genotypes in the offspring of
two parents. An example of a Punnett square for pea plants is shown in Figure below. In this example, both parents are
heterozygous for flowercolor (Bb). The gametes produced by the male parent are at the top of the chart, and the gametes produced
by the female parent are along the side. The different possible combinations of alleles in their offspring are determined by filling in
the cells of the Punnett square with the correct letters (alleles). At the link below, you can watch an animation in which Reginald
Punnett, inventor of the Punnett square, explains the purpose of his invention and how to use it. http://www.dnalc.org/view/16192-
Animation-5-Genetic-inheritance-follows-rules-.html
An explanation of Punnett squares can be viewed at http://www.youtube.com/watch?v=D5ymMYcLtv0 (25:16). Another example
of the use of a Punnett square can be viewed athttp://www.youtube.com/watch?v=nsHZbgOmVwg (5:40).

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This Punnett square shows a cross between two heterozygotes, Bb. Do you know where each letter (allele) in all four cells comes
from? Two pea plants, both heterozygous for flower color, are crossed. The offspring will show the dominant purple coloration in a
3:1 ratio. Or, about 75% of the offspring will be purple.

Predicting Offspring Genotypes

In the cross shown in Figure above, you can see that one out of four offspring (25 percent) has the genotype BB, one out of four
(25 percent) has the genotype bb, and two out of four (50 percent) have the genotype Bb. These percentages of genotypes are what
you would expect in any cross between two heterozygous parents. Of course, when just four offspring are produced, the actual
percentages of genotypes may vary by chance from the expected percentages. However, if you considered hundreds of such crosses
and thousands of offspring, you would get very close to the expected results, just like tossing a coin.

Predicting Offspring Phenotypes

You can predict the percentages of phenotypes in the offspring of this cross from their genotypes. B is dominant to b, so offspring
with either the BB or Bb genotype will have the purple-flower phenotype. Only offspring with the bb genotype will have the white-
flower phenotype. Therefore, in this cross, you would expect three out of four (75 percent) of the offspring to have purple flowers
and one out of four (25 percent) to have white flowers. These are the same percentages that Mendel got in his first experiment.

Determining Missing Genotypes

A Punnett square can also be used to determine a missing genotype based on the other genotypes involved in a cross. Suppose you
have a parent plant with purple flowers and a parent plant with white flowers. Because the b allele is recessive, you know that the
white-flowered parent must have the genotype bb. The purple-flowered parent, on the other hand, could have either the BB or the
Bb genotype. The Punnett square in Figure below shows this cross. The question marks (?) in the chart could be either B or b
alleles.

Punnett Square: Cross Between White-Flowered and Purple-Flowered Pea Plants. This Punnett square shows a cross between a
white-flowered pea plant and a purple-flowered pea plant. Can you fill in the missing alleles? What do you need to know about the
offspring to complete their genotypes?
Can you tell what the genotype of the purple-flowered parent is from the information in the Punnett square? No; you also need to
know the genotypes of the offspring in row 2. What if you found out that two of the four offspring have white flowers? Now you

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know that the offspring in the second row must have the bb genotype. One of their b alleles obviously comes from the white-
flowered (bb) parent, because that’s the only allele this parent has. The other b allele must come from the purple-flowered parent.
Therefore, the parent with purple flowers must have the genotype Bb.

Punnett Square for Two Characteristics

When you consider more than one characteristic at a time, using a Punnett square is more complicated. This is because many more
combinations of alleles are possible. For example, with two genes each having two alleles, an individual has four alleles, and these
four alleles can occur in 16 different combinations. This is illustrated for pea plants in Figure below. In this cross, known as a
dihybrid cross, both parents are heterozygous for pod color (Gg) and pod form (Ff).

Punnett Square for Two Characteristics. This Punnett square represents a cross between two pea plants that are heterozygous for
two characteristics. G represents the dominant allele for green pod color, and g represents the recessive allele for yellow pod color.
F represents the dominant allele for full pod form, and f represents the recessive allele for constricted pod form.

Summary
A Punnett square is a chart that allows you to determine the expected percentages of different genotypes in the offspring of two
parents.
A Punnett square allows the prediction of the percentages of phenotypes in the offspring of a cross from known genotypes.
A Punnett square can be used to determine a missing genotype based on the other genotypes involved in a cross.

Making Connections

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Explore More I
Use this resource to answer the questions that follow.
http://www.hippocampus.org/Biology → Non-Majors Biology → Search: The Punnett Square
1. What is a Punnett square?
2. What do the boxes in a Punnett square represent?
3. What is the size of a Punnett square used in a dihybrid cross?
4. Define the following terms: alleles, genotype, phenotype, genome.

Review
1. What is a Punnett square? How is it used?
2. Draw a Punnett square of an Ss x ss cross. The S allele codes for long stems in pea plants and the s allele codes for short
stems. If S is dominant to s, what percentage of the offspring would you expect to have each phenotype?
3. What letter should replace the question marks (?) in this Punnett square? Explain how you know.

4. How do the Punnett squares for a monohybrid cross and a dihybrid cross differ?
5. What are the genotypes of gametes of a AaBb self-pollination?
6. Mendel carried out a dihybrid cross to examine the inheritance of the characteristics for seed color and seed shape. The
dominant allele for yellow seed color is Y, and the recessive allele for green color is y. The dominant allele for round seeds is R,
and the recessive allele for a wrinkled shape is r. The two plants that were crossed were F1 dihybrids RrYy. Identify the ratios of
traits that Mendel observed in the F2 generation. Create a Punnett square to help you answer the question.

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3.7: Non-Mendelian Inheritance

Green, blue, brown, black, hazel, violet, or grey. What color are your eyes?
Of course human eyes do not come in multi-color, but they do come in many colors. How do eyes come in so many colors? That
brings us to complex inheritance patterns, known as non-Mendelian inheritance. Many times inheritance is more complicated than
the simple patterns observed by Mendel.

Non-Mendelian Inheritance
The inheritance of characteristics is not always as simple as it is for the characteristics that Mendel studied in pea plants. Each
characteristic Mendel investigated was controlled by one gene that had two possible alleles, one of which was completely dominant
to the other. This resulted in just two possible phenotypes for each characteristic. Each characteristic Mendel studied was also
controlled by a gene on a different (nonhomologous) chromosome. As a result, each characteristic was inherited independently of
the other characteristics. Geneticists now know that inheritance is often more complex than this.
A characteristic may be controlled by one gene with two alleles, but the two alleles may have a different relationship than the
simple dominant-recessive relationship that you have read about so far. For example, the two alleles may have a codominant or
incompletely dominant relationship. The former is illustrated by the flower in Figure below, and the latter in Figure below.

Codominance
Codominance occurs when both alleles are expressed equally in the phenotype of the heterozygote. The red and white flower in
the figure has codominant alleles for red petals and white petals.

Codominance. The flower has red and white petals because of codominance of red-petal and white-petal alleles.

Incomplete Dominance
Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a
completely dominant allele does not occur. For example, when red snapdragons (CRCR) are crossed with white snapdragons

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(CWCW), the F1hybrids are all pink heterozygotes for flower color (CRCW). The pink color is an intermediate between the two
parent colors. When two F1 (CRCW) hybrids are crossed they will produce red, pink, and white flowers. The genotype of an
organism with incomplete dominance can be determined from its phenotype (Figure below).

Incomplete Dominance. The flower has pink petals because of incomplete dominance of a red-petal allele and a recessive white-
petal allele.

Multiple Alleles
Many genes have multiple (more than two) alleles. An example is ABO blood type in humans. There are three common alleles for
the gene that controls this characteristic. The alleles IAand IB are dominant over i. A person who is homozygous recessive ii has
type O blood. Homozygous dominant IAIA or heterozygous dominant IAi have type A blood, and homozygous dominant IBIB or
heterozygous dominant IBi have type B blood. IAIB people have type AB blood, because the A and B alleles are codominant. Type
A and type B parents can have a type AB child. Type A and type B parents can also have a child with Type O blood, if they are
both heterozygous (IBi, IAi).
Type A blood: IAIA, IAi
Type B blood: IB IB, IB i
Type AB blood: IAIB
Type O blood: ii

Polygenic Characteristics
Polygenic characteristics are controlled by more than one gene, and each gene may have two or more alleles. The genes may be
on the same chromosome or on nonhomologous chromosomes.
If the genes are located close together on the same chromosome, they are likely to be inherited together. However, it is possible
that they will be separated by crossing-over during meiosis, in which case they may be inherited independently of one another.
If the genes are on non homologous chromosomes, they may be recombined in various ways because of independent
assortment.
For these reasons, the inheritance of polygenic characteristics is very complicated. Such characteristics may have many possible
phenotypes. Skin color and adult height are examples of polygenic characteristics in humans. Do you have any idea how many
phenotypes each characteristic has?

Human Adult Height. Like many other polygenic traits, adult height has a bell-shaped distribution.

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Effects of Environment on Phenotype
Genes play an important role in determining an organism’s characteristics. However, for many characteristics, the individual’s
phenotype is influenced by other factors as well. Environmental factors, such as sunlight and food availability, can affect how
genes are expressed in the phenotype of individuals. Here are just two examples:
Genes play an important part in determining our adult height. However, factors such as poor nutrition can prevent us from
achieving our full genetic potential.
Genes are a major determinant of human skin color. However, exposure to ultraviolet radiation can increase the amount of
pigment in the skin and make it appear darker.

Summary
Many characteristics have more complex inheritance patterns than those studied by Mendel. They are complicated by factors
such as codominance, incomplete dominance, multiple alleles, and environmental influences.

Explore More
Use this resource to answer the questions that follow.
http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Exceptions to the Rules
1. Flower color in carnations demonstrates what type of inheritance?
2. What is the genotype of a pink carnation?
3. What are the alleles for blood type in humans?
4. How is skin color in humans determined?
5. Define pleiotropy.

Review
1. A classmate tells you that a person can have type AO blood. Do you agree? Explain.
2. Mendelian inheritance does not apply to the inheritance of alleles that result in incomplete dominance and codominance.
Explain why this is so.
3. Describe the relationship between environment and phenotype.
4. Mendel investigated stem length, or height, in pea plants. What if he had investigated human height instead? Why would his
results have been harder to interpret?

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via source content that was edited to the style and standards of the LibreTexts platform.

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3.8: Human Genome

All these ACGTs. What are they?

Over three billion of them from a human form the human genome - the human genetic material - all the information needed to
encode a human being. It would take about 9.5 years to read out loud - without stopping - the more than three billion pairs of bases
in one person's genome.

The Human Genome

What makes each one of us unique? You could argue that the environment plays a role, and it does to some extent. But most would
agree that your parents have something to do with your uniqueness. In fact, it is our genes that make each one of us unique – or at
least genetically unique. We all have the genes that make us human: the genes for skin and bones, eyes and ears, fingers and toes,
and so on. However, we all have different skin colors, different bone sizes, different eye colors and different ear shapes. In fact,
even though we have the same genes, the products of these genes work a little differently in most of us. And that is what makes us
unique.
The human genome is the genome - all the DNA - of Homo sapiens. Humans have about 3 billion bases of information, divided
into roughly 20,000 to 22,000 genes, which are spread among non-coding sequences and distributed among 24 distinct
chromosomes (22autosomes plus the X and Y sex chromosomes) (below). The genome is all of the hereditary information
encoded in the DNA, including the genes and non-coding sequences.

Human Genome, Chromosomes, and Genes. Each chromosome of the human genome contains many genes as well as noncoding
intergenic (between genes) regions. Each pair of chromosomes is shown here in a different color.
Thanks to the Human Genome Project, scientists now know the DNA sequence of the entire human genome. The Human
Genome Project is an international project that includes scientists from around the world. It began in 1990, and by 2003, scientists
had sequenced all 3 billion base pairs of human DNA. Now they are trying to identify all the genes in the sequence. The Human
Genome Project has produced a reference sequence of the human genome. The human genome consists of protein-coding exons,
associated introns and regulatory sequences, genes that encode other RNA molecules, and other DNA sequences (sometimes
referred to as "junk" DNA), which are regions in which no function as yet been identified.

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You can watch a video about the Human Genome Project and how it cracked the "code of life" at this link:
http://www.pbs.org/wgbh/nova/genome/program.html.
Our Molecular Selves video discusses the human genome, and is available athttp://www.genome.gov/25520211 or
http://www.youtube.com/watch?v=_EK3g6px7Ik.Genome, Unlocking Life's Code is the Smithsonian's National Museum of
Natural History exhibit of the human genome. See http://unlockinglifescode.org to visit the exhibit.

ENCODE: The Encyclopedia of DNA Elements

In September 2012, ENCODE, The Encyclopedia of DNA Elements, was announced. ENCODE was a colossal project, involving
over 440 scientists in 32 labs the world-over, whose goal was to understand the human genome. It had been thought that about 80%
of the human genome was "junk" DNA. ENCODE has established that this is not true. Now it is thought that about 80% of the
genome is active. In fact, much of the human genome is regulatory sequences, on/off switches that tell our genes what to do and
when to do it. Dr. Eric Green, director of the National Human Genome Research Institute of the National Institutes of Health which
organized this project, states, "It's this incredible choreography going on, of a modest number of genes and an immense number of
... switches that are choreographing how those genes are used."
It is now thought that at least three-quarters of the genome is involved in making RNA, and most of this RNA appears to help
regulate gene activity. Scientists have also identified about 4 million sites where proteins bind to DNA and act in a regulatory
capacity. These new findings demonstrate that the human genome has remarkable and precise, and complex, controls over the
expression of genetic information within a cell.
See ENCODE data describes function of human genome athttp://www.genome.gov/27549810 for additional information.

Summary
The human genome consists of about 3 billion base pairs of DNA.
In 2003, the Human Genome Project finished sequencing all 3 billion base pairs.

Explore More
Use this resource to answer the questions that follow.
http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Human Genome Project
1. What were 3 goals of the Human Genome Project?
2. How many genes are on chromosome #1?
3. How big is the human genome?
4. How much genetic variation is there among people?
5. How much of the genome is not part of any gene?

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Review
1. Describe the human genome.
2. What has the Human Genome Project achieved?
3. Describe the makeup of the human genome.

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content that was edited to the style and standards of the LibreTexts platform.

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3.9: Human Chromosomes and Genes

Coiled bundles of DNA and proteins, containing hundreds or thousands of genes. What are these things?
Chromosomes. These ensure that each cell receives the proper amount of DNA during cell division. And usually people have 46 of
them, 23 from each parent.

Chromosomes and Genes

Each species has a characteristic number of chromosomes. Chromosomes are coiled structures made of DNA and proteins called
histones (Figure below). Chromosomes are the form of the genetic material of a cell during cell division. See the "Chromosomes"
section for additional information.

The human genome has 23 pairs of chromosomes located in the nucleus of somatic cells. Each chromosome is composed of genes
and other DNA wound around histones (proteins) into a tightly coiled molecule.
The human species is characterized by 23 pairs of chromosomes, as shown in Figure below. You can watch a short animation
about human chromosomes at this link:http://www.dnalc.org/view/15520-DNA-is-organized-into-46-chromosomes-including-sex-
chromosomes-3D-animation.html.

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Human Chromosomes. Humans have 23 pairs of chromosomes. Pairs 1-22 are autosomes. Females have two X chromosomes, and
males have an X and a Y chromosome.

Autosomes
Of the 23 pairs of human chromosomes, 22 pairs are autosomes (numbers 1–22 in Figureabove). Autosomes are chromosomes that
contain genes for characteristics that are unrelated to sex. These chromosomes are the same in males and females. The great
majority of human genes are located on autosomes. At the link below, you can click on any human chromosome to see which traits
its genes control.http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/chooser.shtml

Sex Chromosomes
The remaining pair of human chromosomes consists of the sex chromosomes, X and Y. Females have two X chromosomes, and
males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr
body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.
As you can see from Figure above and Figure above, the X chromosome is much larger than the Y chromosome. The X
chromosome has about 2,000 genes, whereas the Y chromosome has fewer than 100, none of which are essential to survival. (For
comparison, the smallest autosome, chromosome 22, has over 500 genes.) Virtually all of the X chromosome genes are unrelated to
sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-
determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a
female, so you can think of female as the default sex of the human species. Can you think of a reason why the Y chromosome is so
much smaller than the X chromosome? At the link that follows, you can watch an animation that explains
why:www.hhmi.org/biointeractive/g...evolution.html.

Human Genes
Humans have an estimated 20,000 to 22,000 genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as
many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions
encoded in a single gene. Of the 3 billion base pairs in the human genome, only about 25 percent make up genes and their
regulatory elements. The functions of many of the other base pairs are still unclear. To learn more about the coding and noncoding
sequences of human DNA, watch the animation at this link: www.hhmi.org/biointeractive/d...sequences.html.
The majority of human genes have two or more possible alleles, which are alternative forms of a gene. Differences in alleles
account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in
individual DNA bases within alleles.

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Summary
Humans have 23 pairs of chromosomes. Of these, 22 pairs are autosomes.
The X and Y chromosomes are the sex chromosomes. Females have two X chromosomes, and males have one X and one Y.
Human chromosomes contain a total of 20,000 to 22,000 genes, the majority of which have two or more alleles.

Explore More
Use this resource to answer the questions that follow.
Chromosomes at http://www.genome.gov/26524120.
1. What is a chromosome?
2. What is the role of chromosomes during cell division?
3. Do all living things have the same types of chromosomes?
4. What are centromeres? What is their role?
5. What are telomeres? What is their role?

Review
1. Describe human chromosomes.
2. Compare and contrast human autosomes and sex chromosomes.
3. What is SRY?
4. Why are females the "default sex" of the human species?

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Foundation via source content that was edited to the style and standards of the LibreTexts platform.

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3.10: Genetic Linkage

What does it mean to be linked?

For a pair of hands, the above image may suggest a certain type of linkage. For genes, it might suggest that they are very hard to
separate.

Linkage
Genes that are located on the same chromosome are called linked genes. Alleles for these genes tend to segregate together during
meiosis, unless they are separated by crossing-over.Crossing-over occurs when two homologous chromosomes exchange genetic
material during meiosis I. The closer together two genes are on a chromosome, the less likely their alleles will be separated by
crossing-over. At the following link, you can watch an animation showing how genes on the same chromosome may be separated
by crossing-over:www.biostudio.com/d_%20Meioti...ed%20Genes.htm.
Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair color and eye color are
linked, so certain hair and eye colors tend to be inherited together, such as blonde hair with blue eyes and brown hair with brown
eyes. What other human traits seem to occur together? Do you think they might be controlled by linked genes?

Sex-Linked Genes
Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome, because the
Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes, but the term sex-
linked is often used to refer to them.
Sex-linked traits are discussed at http://www.youtube.com/watch?v=-ROhfKyxgCo (14:19).

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Mapping Linkage
Linkage can be assessed by determining how often crossing-over occurs between two genes on the same chromosome. Genes on
different (nonhomologous) chromosomes are not linked. They assort independently during meiosis, so they have a 50 percent
chance of ending up in different gametes. If genes show up in different gametes less than 50 percent of the time (that is, they tend
to be inherited together), they are assumed to be on the same (homologous) chromosome. They may be separated by crossing-over,
but this is likely to occur less than 50 percent of the time. The lower the frequency of crossing-over, the closer together on the same
chromosome the genes are presumed to be. Frequencies of crossing-over can be used to construct a linkage map like the one in
Figure below. A linkage map shows the locations of genes on a chromosome.

Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some
of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders. Which pair of genes would
you expect to have a lower frequency of crossing-over: the genes that code for hemophilia A and G6PD deficiency, or the genes
that code for protan and Xm?

Summary
Linked genes are located on the same chromosome.
Sex-linked genes are located on a sex chromosome, and X-linked genes are located on the X chromosome.
The frequency of crossing-over between genes is used to construct linkage maps that show the locations of genes on
chromosomes.

Explore More
Explore More I
Use these resources to answer the questions that follow.
Recombination and Estimating the Distance Between Genes at
http://www.ndsu.edu/pubweb/~mcclean/plsc431/linkage/linkage2.htm.
1. What is recombination?
2. What determines the amount of recombination between two genes?
3. What are recombinant gametes?
4. What is a centimorgan?

Explore More II
T. H. Morgan at www.dnalc.org/resources/nobel/morgan.html.

Review
1. What are linked genes?
2. Explain how you would construct a linkage map for a human chromosome. What data would you need?
3. People with red hair usually have very light skin. What might be a genetic explanation for this observation?
4. How often does crossing-over occur between non-linked genes? Explain your answer.

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3.11: Mendelian Inheritance in Humans

What number can you see?

Red-green colorblindness is a common inherited trait in humans. About 1 in 10 men have some form of color blindness, however,
very few women are color blind. Why?

Mendelian Inheritance in Humans

Characteristics that are encoded in DNA are called genetic traits. Different types of human traits are inherited in different ways.
Some human traits have simple inheritance patterns like the traits that Gregor Mendel studied in pea plants. Other human traits
have more complex inheritance patterns.
Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be dominant
to the other. Not many human traits are controlled by a single gene with two alleles, but they are a good starting point for
understanding human heredity. How Mendelian traits are inherited depends on whether the traits are controlled by genes on
autosomes or the X chromosome.

Autosomal Traits
Autosomal traits are controlled by genes on one of the 22 human autosomes. Consider earlobe attachment. A single autosomal gene
with two alleles determines whether you have attached earlobes or free-hanging earlobes. The allele for free-hanging earlobes (F)
is dominant to the allele for attached earlobes (f). Other single-gene autosomal traits include widow’s peak and hitchhiker’s thumb.
The dominant and recessive forms of these traits are shown in Figure below. Which form of these traits do you have? What are
your possible genotypes for the traits?
The chart in Figure below is called a pedigree. It shows how the earlobe trait was passed from generation to generation within a
family. Pedigrees are useful tools for studying inheritance patterns.
You can watch a video explaining how pedigrees are used and what they reveal at this link:http://www.youtube.com/watch?
v=HbIHjsn5cHo.

Having free-hanging earlobes is an autosomal dominant trait. This figure shows the trait and how it was inherited in a family over
three generations. Shading indicates people who have the recessive form of the trait. Look at (or feel) your own earlobes. Which

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form of the trait do you have? Can you tell which genotype you have?
Other single-gene autosomal traits include widow's peak and hitchhiker's thumb. The dominant and recessive forms of these traits
are shown in Figure below. Which form of these traits do you have? What are your possible genotypes for the traits?

Widow's peak and hitchhiker's thumb are dominant traits controlled by a single autosomal gene.

Sex-Linked Traits
Traits controlled by genes on the sex chromosomes are called sex-linked traits, or X-linked traits in the case of the X
chromosome. Single-gene X-linked traits have a different pattern of inheritance than single-gene autosomal traits. Do you know
why? It’s because males have just one X chromosome. In addition, they always inherit their X chromosome from their mother, and
they pass it on to all their daughters but none of their sons. This is illustrated in Figurebelow.

Inheritance of Sex Chromosomes. Mothers pass only X chromosomes to their children. Fathers always pass their X chromosome to
their daughters and their Y chromosome to their sons. Can you explain why fathers always determine the sex of the offspring?
Because males have just one X chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele
is always expressed in males. Because females have two X chromosomes, they have two alleles for any X-linked trait. Therefore,
they must inherit two copies of the recessive allele to express the recessive trait. This explains why X-linked recessive traits are less
common in females than males. An example of a recessive X-linked trait is red-green color blindness. People with this trait
cannot distinguish between the colors red and green. More than one recessive gene on the X chromosome codes for this trait, which
is fairly common in males but relatively rare in females (Figure below). At the following link, you can watch an animation about
another X-linked recessive trait called hemophilia A:http://www.dnalc.org/view/16315-Animation-13-Mendelian-laws-apply-to-
human-beings-.html.

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Pedigree for Color Blindness. Color blindness is an X-linked recessive trait. Mothers pass the recessive allele for the trait to their
sons, who pass it to their daughters.

Summary
A minority of human traits are controlled by single genes with two alleles.
They have different inheritance patterns depending on whether they are controlled by autosomal or X-linked genes.

Explore More
Explore More I
Use these resources to answer the questions that follow.
http://www.hippocampus.org/Biology → Non-Majors Biology → Search: A Case Study
1. A homozygous freckled man marries a non-freckled woman. If freckles are dominant, will their children have freckles? Explain
your answer.
2. Using F and f, what are the genotypes of the parents? What are the genotypes of their gametes?

Explore More II
Pedigree Analysis

Review
Describe the inheritance pattern for a single-gene autosomal dominant trait, such as free-hanging earlobes.
Draw a pedigree for hitchhiker’s thumb. Your pedigree should cover at least two generations and include both dominant and
recessive forms of the trait. Label the pedigree with genotypes, using the letter H to represent the dominant allele for the trait
and the letter h to represent the recessive allele.
Why is a recessive X-linked allele always expressed in males?
What is necessary for a recessive X-linked allele to be expressed in females?
What is an example of a recessive X-linked trait?

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3.12: Genetic Disorders

Is being short-statured inherited?

It can be. Achondroplasia is the most common form of dwarfism in humans, and it is caused by a dominant mutation. The mutation
can be passed from one generation to the next.

Genetic Disorders
Many genetic disorders are caused by mutations in one or a few genes. Other genetic disorders are caused by abnormal numbers
of chromosomes.

Genetic Disorders Caused by Mutations

The Table below lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by
mutations in autosomal genes, others by mutations in X-linked genes. Which disorder would you expect to be more common in
males than females? You can watch a video about the human genome, genetic disorders, and mutations at this
link:http://www.pbs.org/wgbh/nova/programs/ht/rv/2809_03.html.
You can click on any human chromosome at this link to see the genetic disorders associated with
it:http://www.ornl.gov/sci/techresource.../chooser.shtml.

Signs and Symptoms of the

Genetic Disorder Direct Effect of Mutation Mode of Inheritance
Disorder

heart and bone defects and

defective protein in connective
Marfan syndrome unusually long, slender limbs autosomal dominant
tissue
and fingers

sickle-shaped red blood cells

abnormal hemoglobin protein that clog tinyblood vessels,
Sickle cell anemia autosomal recessive
in red blood cells causing pain and damaging
organs and joints

soft bones that easily become

lack of a substance needed for deformed, leading to bowed
Vitamin D-resistant rickets X-linked dominant
bones to absorb minerals legs and other skeletal
deformities

internal and external bleeding

reduced activity of a protein
Hemophilia A that occurs easily and is X-linked recessive
needed for blood clotting
difficult to control

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Few genetic disorders are controlled by dominant alleles. A mutant dominant allele is expressed in every individual who inherits
even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant
dominant allele is likely to die out of the population.
A mutant recessive allele, such as the allele that causes sickle cell anemia (see Figure belowand the link that follows), is not
expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but
they carry the mutant allele and can pass it to their offspring. Thus, the allele is likely to pass on to the next generation rather than
die out. http://www.dnalc.org/resources/3d/17-sickle-cell.html

Sickle-Shaped and Normal Red Blood Cells. Sickle cell anemia is an autosomal recessive disorder. The mutation that causes the
disorder affects just one amino acid in a single protein, but it has serious consequences for the affected person. This photo shows
the sickle shape of red blood cells in people with sickle cell anemia.
Cystic Fibrosis and Tay-Sachs disease are two additional severe genetic disorders. They are discussed in the following video:
http://www.youtube.com/watch?v=8s4he3wLgkM (9:31). Tay-Sachs is further discussed at http://www.youtube.com/watch?
v=1RO0LOgHbIo (3:13) andhttp://www.youtube.com/watch?v=6zNj5LdDuTA (2:01).

Chromosomal Disorders
Mistakes may occur during meiosis that result in nondisjunction. This is the failure of replicated chromosomes to separate during
meiosis (the animation at the link below shows how this happens). Some of the resulting gametes will be missing a chromosome,
while others will have an extra copy of the chromosome. If such gametes are fertilized and form zygotes, they usually do not
survive. If they do survive, the individuals are likely to have serious genetic disorders. Table below lists several genetic disorders
that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome. Look back at the
X and Y chromosomes and you will see why. The X and Y chromosomes are very different in size, so nondisjunction of the sex
chromosomes occurs relatively often. learn.genetics.utah.edu/conte...der/index.html

Genetic Disorder Genotype Phenotypic Effects

developmental delays, distinctive facial

extra copy (complete or partial) of
Down syndrome appearance, and other abnormalities (see
chromosome 21 (see Figure below)
Figurebelow)
one X chromosome but no other sex female with short height and infertility
Turner’s syndrome
chromosome (XO) (inability to reproduce)
female with mild developmental delays and
Triple X syndrome three X chromosomes (XXX)
menstrual irregularities
male with problems in sexual development
one Y chromosome and two or more X
Klinefelter’s syndrome and reduced levels of the male hormone
chromosomes (XXY, XXXY)
testosterone

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(left) Trisomy 21 (Down Syndrome) Karyotype. A karyotype is a picture of a cell's chromosomes. Note the extra chromosome 21.
(right) Child with Down syndrome, exhibiting characteristic facial appearance.

Diagnosing Genetic Disorders

A genetic disorder that is caused by a mutation can be inherited. Therefore, people with a genetic disorder in their family may be
concerned about having children with the disorder. Professionals known as genetic counselors can help them understand the risks
of their children being affected. If they decide to have children, they may be advised to have prenatal(“before birth”) testing to see
if the fetus has any genetic abnormalities. One method of prenatal testing is amniocentesis. In this procedure, a few fetal cells are
extracted from the fluid surrounding the fetus, and the fetal chromosomes are examined.

Treating Genetic Disorders

The symptoms of genetic disorders can sometimes be treated, but cures for genetic disorders are still in the early stages of
development. One potential cure that has already been used with some success is gene therapy. This involves inserting normal
genes into cells with mutant genes. At the following link, you can watch the video ‘‘Sickle Cell Anemia: Hope fromGene
Therapy’’, to learn how scientists are trying to cure sickle-cell anemia with gene therapy.
www.pubinfo.vcu.edu/secretsof...list_frame.asp
If you could learn your risk of getting cancer or another genetic disease, would you? Though this is a personal decision, it is a
possibility. A number of companies now makes it easy to order medical genetic tests through the Web. See Genetic Testing through
the Web athttp://www.kqed.org/quest/televisi...hrough-the-web.

Summary
Many genetic disorders are caused by mutations in one or a few genes.
Other genetic disorders are caused by abnormal numbers of chromosomes.

Explore More
Use this resource to answer the questions that follow.
Genetic Disorders at http://www.nlm.nih.gov/medlineplus/g...disorders.html.
1. How do mutations affect proteins?
2. What is a single-gene disorder?
3. What is a chromosomal disorder?
4. What is a complex disorder?
5. Give an example of a chromosomal disorder.

Review
1. Describe a genetic disorder caused by a mutation in a single gene.
2. What causes Down syndrome?
3. What is nondisjunction?
4. What is gene therapy?
5. Explain why genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome.

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3.13: Biotechnology

So how does a scientist work with DNA?

It always starts with the sequence. Once the sequence is known, so much more can be done. Specific regions can be isolated,
cloned, amplified, and then used to help us.

Biotechnology Methods
Biotechnology is the use of technology to change the genetic makeup of living things for human purposes. Generally, the purpose
of biotechnology is to create organisms that are useful to humans or to cure genetic disorders. For example, biotechnology may be
used to create crops that resist insect pests or yield more food, or to create new treatments for human diseases.
Biotechnology: The Invisible Revolution can be seen at http://www.youtube.com/watch?v=OcG9q9cPqm4.
What does biotechnology have to do with me? Is discussed in the following video:http://www.youtube.com/watch?v=rrT5BT_7HdI
(10:01).
Biotechnology uses a variety of techniques to achieve its aims. Two commonly used techniques are gene cloning and the
polymerase chain reaction.

Gene Cloning
Gene cloning is the process of isolating and making copies of a gene. This is useful for many purposes. For example, gene cloning
might be used to isolate and make copies of a normal gene for gene therapy. Gene cloning involves four steps: isolation, ligation,
transformation, and selection. You can watch an interactive animation about gene cloning at this
link:http://www.teachersdomain.org/asset/...int_geneclone/.
1. In isolation, an enzyme (called a restriction enzyme) is used to break DNA at a specific base sequence. This is done to isolate a
gene.
2. During ligation, the enzyme DNA ligase combines the isolated gene with plasmid DNAfrom bacteria. (A plasmid is circular
DNA that is not part of a chromosome and can replicate independently.) Ligation is illustrated in Figure below. The DNA that
results is called recombinant DNA.
3. In transformation, the recombinant DNA is inserted into a living cell, usually a bacterial cell. Changing an organism in this
way is also called genetic engineering.
4. Selection involves growing transformed bacteria to make sure they have the recombinant DNA. This is a necessary step because
transformation is not always successful. Only bacteria that contain the recombinant DNA are selected for further use.

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Ligation. DNA ligase joins together an isolated gene and plasmid DNA. This produces recombinant DNA.
Recombinant DNA technology is discussed in the following videos and animations:http://www.youtube.com/watch?v=x2jUMG2E-
ic (4.36), http://www.youtube.com/watch?v=Jy15BWVxTC0 (0.50), http://www.youtube.com/watch?v=sjwNtQYLKeU
(7.20),http://www.youtube.com/watch?v=Fi63VjfhsfI (3:59).
The experiments of Stanley Cohen and Herbert Boyer, pioneers of genetic engineering, are explained in the video at
https://www.youtube.com/watch?v=nfC689ElUVk. More on these pioneers can be found at http://www.dnalc.org/view/16033-
Stanley-Cohen-and-Herbert-Boyer-1972.html.

Polymerase Chain Reaction

The polymerase chain reaction (PCR) makes many copies of a gene or other DNA segment. This might be done in order to make
large quantities of a gene for genetic testing. PCR involves three steps: denaturing, annealing, and extension. The three steps are
illustrated in Figure below. They are repeated many times in a cycle to make large quantities of the gene. You can watch
animations of PCR at these links:
http://www.dnalc.org/resources/3d/19-polymerase-chain-reaction.html
http://www.teachersdomain.org/asset/biot09_int_pcr/
https://www.youtube.com/watch?v=0rQFnbcEsog.

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 1. Denaturing involves heating DNA to break the bonds holding together the two DNA strands. This yields two single strands of
DNA.
2. Annealing involves cooling the single strands of DNA and mixing them with short DNA segments called primers. Primers
have base sequences that are complementary to segments of the single DNA strands. As a result, bonds form between the DNA
strands and primers.
3. Extension occurs when an enzyme (Taq polymerase or Taq DNA polymerase) adds nucleotides to the primers. This produces
new DNA molecules, each incorporating one of the original DNA strands.

The Polymerase Chain Reaction. The polymerase chain reaction involves three steps. High temperatures are needed for the process
to work. The enzyme Taq polymerase is used in step 3 because it can withstand high temperatures.

Summary
Biotechnology is the use of technology to change the genetic makeup of living things for human purposes.
Gene cloning is the process of isolating and making copies of a DNA segment such as a gene.
The polymerase chain reaction makes many copies of a gene or other DNA segment.

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Explore More
Use this resource and the videos associated with this resource to answer the questions that follow.
Polymerase Chain Reaction at www.dnalc.org/resources/spotlight/index.html.
1. Who developed PCR?
2. What does PCR allow?
3. Describe the 3 steps involved in PCR.
4. Approximately how many copies of a specific segment of DNA can be made by PCR?

Review
1. Define biotechnology.
2. What is recombinant DNA?
3. Identify the steps of gene cloning.
4. What is the purpose of the polymerase chain reaction?
5. Describe the three steps of PCR.

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3.14: Biotechnology Applications

Why would anyone grow plants like this?

Developing better crops is a significant aspect of biotechnology. Crops that are resistant to damage from insects or droughts must
have a significant role in the world's future. And it all starts in the lab.

Applications of Biotechnology
Methods of biotechnology can be used for many practical purposes. They are used widely in both medicine and agriculture. To see
how biotechnology can be used to solve crimes, watch the video "Justice DNA—Freeing the Innocent" at the following
link:www.pubinfo.vcu.edu/secretsof...list_frame.asp.

Applications in Medicine
In addition to gene therapy for genetic disorders, biotechnology can be used to transform bacteria so they are able to make human
proteins. Figure below shows how this is done to produce a cytokine, which is a small protein that helps fight infections. Proteins
made by the bacteria are injected into people who cannot produce them because of mutations.

Genetically Engineering Bacteria to Produce a Human Protein. Bacteria can be genetically engineered to produce a human protein,
such as a cytokine. A cytokine is a small protein that helps fight infections.
Insulin was the first human protein to be produced in this way. Insulin helps cells take up glucose from the blood. People with type
1 diabetes have a mutation in the gene that normally codes for insulin. Without insulin, their blood glucose rises to harmfully high
levels. At present, the only treatment for type 1 diabetes is the injection of insulin from outside sources. Until recently, there was no

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known way to make insulin outside the human body. The problem was solved by gene cloning. The human insulin gene was cloned
and used to transform bacterial cells, which could then produce large quantities of human insulin.

Pharmacogenomics
We know that, thanks to our DNA, each of us is a little bit different. Some of those differences are obvious, like eye and hair color.
Others are not so obvious, like how our bodies react to medication. Researchers are beginning to look at how to tailor medical
treatments to our genetic profiles, in a relatively new field called pharmacogenomics. Some of the biggest breakthroughs have
been in cancer treatment. For additional information on this “personalized medicine,” listen to www.kqed.org/quest/radio/pers...ed-
medicineand see www.kqed.org/quest/blog/2009/...ized-medicine/.

Synthetic Biology
Imagine living cells acting as memory devices, biofuels brewing from yeast, or a light receptor taken from algae that makes
photographs on a plate of bacteria. The new field of synthetic biology is making biology easier to engineer so that new functions
can be derived from living systems. Find out the tools that synthetic biologists are using and the exciting things they are building at
www.kqed.org/quest/television...thetic-biology.

Applications in Agriculture
Biotechnology has been used to create transgenic crops. Transgenic crops are genetically modified with new genes that code for
traits useful to humans. The diagram in Figure below shows how a transgenic crop is created. You can learn more about how
scientists create transgenic crops with the interactive animation "Engineer a Crop: Transgenic Manipulation" at this link:
http://www.pbs.org/wgbh/harvest/engineer/transgen.html.

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Creating a Transgenic Crop. A transgenic crop is genetically modified to be more useful to humans. The bacterium transfers the T-
DNA (from the Ti plasmid) fragment with the desired gene into the host plant's nuclear genome.
Transgenic crops have been created with a variety of different traits, such as yielding more food, tasting better, surviving drought,
and resisting insect pests. Scientists have even created a transgenic purple tomato that contains a cancer-fighting compound and
others that have high levels of antioxidants (see Figure below). Seehttp://extension.oregonstate.edu/...tomato-debuts-‘indigo-rose’
for more information. To learn how scientists have used biotechnology to create plants that can grow in salty soil, watch the video
"Salt of the Earth - Engineering Salt-tolerant Plants" at this link:http://www.sosq.vcu.edu/videos.aspx.

Transgenic Purple Tomato. A purple tomato is genetically modified to contain a cancer-fighting compound. A gene for the
compound was transferred into normal red tomatoes.
Biotechnology in agriculture is discussed at http://www.youtube.com/watch?v=IY3mfgbe-0c(6:40).

Applications in Forensic Science

Biotechnology has also had tremendous impacts in the forensic sciences. Can DNA Demand a Verdict
(http://learn.genetics.utah.edu/content/science/forensics/) discusses how DNA analysis is used to solve crimes. Also see Gel
Electrophoresis athttps://www.youtube.com/watch?v=Pk5hCRE_28g to see how biotechnology helps with solving crimes.

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Summary
Biotechnology can be used to transform bacteria so they are able to make human proteins, such as insulin.
It can also be used to create transgenic crops, such as crops that yield more food or resist insect pests.

Explore More
Explore More I
Use this resource to answer the questions that follow.
Modern Biotechnology at www.biotechlearn.org.nz/theme..._biotechnology.
1. Give an example of early biotechnology.
2. Give an example of modern biotechnology.
3. Describe one use of biotechnology in:
1. medicine,
2. agriculture,
3. forensics.

Explore More II
Craig Venter at http://www.youtube.com/watch?v=Ce8ZVyUqY-I

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Review
1. What are transgenic crops?
2. Make a flow chart outlining the steps involved in creating a transgenic crop.
3. Explain how bacteria can be genetically engineered to produce a human protein.

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3.15: Ethical, Legal, and Social Issues of Biotechnology

Right or wrong? Good or bad? Legal or illegal?

The completion of The Human Genome Project is one of the most important scientific events of the past 50 years. However, is
knowing all of our DNA a good thing? The advancement of biotechnology has raised many interesting ethical, legal and social
questions.

Ethical, Legal, and Social Issues

Imagine someone analyzes part of your DNA. Who controls that information? What if your health insurance company found out
you were predisposed to develop a devastating genetic disease. Might they decide to cancel your insurance? Privacy issues
concerning genetic information is an important issue in this day and age.
ELSI stands for Ethical, Legal and Social Issues. It's a term associated with the Human Genome project. This project didn't only
have the goal to identify all the genes in the human genome, but also to address the ELSI that might arise from the project. Rapid
advances in DNA-based research, human genetics, and their applications have resulted in new and complex ethical and legal issues
for society.

Concerns from Biotechnology

The use of biotechnology has raised a number of ethical, legal, and social issues. Here are just a few:
Who owns genetically modified organisms such as bacteria? Can such organisms be patented like inventions?
Are genetically modified foods safe to eat? Might they have unknown harmful effects on the people who consume them?
Are genetically engineered crops safe for the environment? Might they harm other organisms or even entire ecosystems?
Who controls a person’s genetic information? What safeguards ensure that the information is kept private?
How far should we go to ensure that children are free of mutations? Should a pregnancy be ended if the fetus has a mutation for
a serious genetic disorder?
Addressing such issues is beyond the scope of this concept. The following example shows how complex the issues may be:
A strain of corn has been created with a gene that encodes a natural pesticide. On the positive side, the transgenic corn is not eaten
by insects, so there is more corn for people to eat. The corn also doesn’t need to be sprayed with chemical pesticides, which can
harm people and other living things. On the negative side, the transgenic corn has been shown to cross-pollinate nearby milkweed
plants. Offspring of the cross-pollinated milkweed plants are now known to be toxic to monarch butterfly caterpillars that depend
on them for food. Scientists are concerned that this may threaten the monarch species as well as other species that normally eat
monarchs.
As this example shows, the pros of biotechnology may be obvious, but the cons may not be known until it is too late. Unforeseen
harm may be done to people, other species, and entire ecosystems. No doubt the ethical, legal, and social issues raised by
biotechnology will be debated for decades to come. For a recent debate about the ethics of applying biotechnology to humans,
watch the video at the link below. In the video, a Harvard University professor of government and a Princeton University professor
of bioethics debate the science of “perfecting humans.” http://www.youtube.com/watch?v=-BPna-fSNOE

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Summary
Biotechnology has raised a number of ethical, legal, and social issues. For example, are genetically modified foods safe to eat,
and who controls a person’s genetic information?

Explore More
Use this resource to answer the questions that follow.
What were some of the ethical, legal, and social implications addressed by the Human Genome Project? at
http://ghr.nlm.nih.gov/handbook/hgp/elsi.
1. What is the ELSI program focus of the Human Genome Project?

Review
1. Identify three ethical, legal, or social issues raised by biotechnology.
2. State your view on an ELSI issue, and develop a logical argument to support your view.

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