Genetics

Definitions
• Heredity: This is about how traits or characteristics are passed down from parents
to offspring. It's like inheriting features from your mom and dad.
• Variation: This is the differences we see in traits among individuals of the same
species. For example, some people have brown eyes, while others have blue.
• Genetics: This is the study of how traits are passed down from one generation to
the next. It's like understanding the instructions or codes that determine how living things
develop and function.
• Character: This refers to a speci c feature or trait that an organism has. For
example, hair color or ower shape are characters.
• Trait: This is a speci c version of a character. For instance, if we're talking about the
character "hair color," the trait could be "brown hair."
• Gene: Think of genes as the instructions or blueprints for a particular trait. They are
made up of DNA and tell your body how to develop and function.
• Allele: These are different forms of a gene. For example, for the gene that
determines eye color, one allele might code for blue eyes, while another allele codes for
brown eyes.
• Variation: This refers to the small differences we see between individuals of the
same species due to inheritance. It's why siblings might look similar but not exactly the
same.
• Mutation: This is a sudden change in one or more genes or in the structure of
chromosomes in offspring. It's like a genetic surprise that might not have been in the
parents' or ancestors' genes. For example, albinism, where an organism lacks skin pigment,
is caused by a mutation.
• Dominant Allele: This is an allele that masks or hides the effect of its counterpart
when both are present. It's like the stronger version that shows up in the trait.
• Recessive Allele: This is an allele that only shows its effect if the dominant allele is
not present. It's like the quieter version of the trait.
• Homozygous pair: This means having two identical alleles for a particular gene.
For example, if you have two alleles for brown eyes, you're homozygous for that trait.
• Heterozygous pair: This means having two different alleles for a particular gene.
For instance, if you have one allele for brown eyes and one for blue eyes, you're
heterozygous for that trait.
• Phenotype: This is the physical expression of a trait. It's what you actually see in an
organism, like someone having blue eyes or brown hair.
• Genotype: This is the genetic makeup of an organism, the combination of alleles it
has for a particular trait. It's like the genetic code that determines what traits you have,
even if they're not visibly expressed.
• Monohybrid Cross: This is a breeding experiment between two organisms that
have different versions of one particular trait. For example, crossing tall and dwarf pea
plants to see how their offspring inherit height.
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 • Monohybrid Ratio: This is the pattern of traits seen in the offspring (or progeny)
of the rst generation (F1) when you cross two organisms that have different versions of
one trait. It shows how the traits are inherited.
• Dihybrid Cross: This is a breeding experiment between two organisms that have
different versions of two different traits. For instance, crossing tall, purple- owered pea
plants with dwarf, white- owered pea plants to study inheritance of both height and ower
color.
• Dihybrid Ratio: This is the pattern of traits seen in the offspring of the rst
generation (F1) when you cross two organisms that have different versions of two traits. It
shows how these traits are inherited together.
• Homologous Chromosomes: These are pairs of chromosomes that are similar in
size and shape, with one chromosome from each parent. They carry genes for the same
traits, although they might have different versions of those genes.

Genetics
Genetics is the study of how characteristics pass from parents to offspring and the rules that
govern this process. Gregor Mendel, often called the Father of Genetics, made signi cant
contributions to this eld through his experiments with garden peas.

Two Modern Applications of Genetics:

1. Genetic Engineering: This involves altering the genetic makeup of an organism by
introducing new genes into its chromosomes. This process creates genetically modi ed
organisms (GMOs) that can produce desired products. For example, bacteria have been
modi ed to produce insulin.
2. Genetic Counselling: This practice advises newly married couples about the likelihood of
passing on undesirable traits to their children. By understanding the genetic makeup of both
parents, certain genetic diseases can be prevented or minimized in their offspring.

Inheritance in Humans — Heredity, or the transmission of genetically based traits from

parents to offspring, is evident in humans. Various physical characteristics, such as eye color,
hair shape, and skin color, are inherited from parents. Despite inheriting traits from parents,
offspring still exhibit variations.

Variation in Population
Humans, as a species, share many characteristics, but individuals within a population exhibit
differences. Even within the same race or tribe, individuals vary in their physical features.
These differences among individuals are termed variations.

Character and Traits — Any inheritable feature is considered a character, while the
different forms of a character are called traits. For example, hair color is a character, and
black hair and blonde hair are traits of that character.
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Chromosomes: Carriers of Heredity
Chromosomes — Chromosomes are structures within cells that contain genetic
information. They become visible during cell division, and their arrangement and
characteristics can be studied through high-powered microscopes. Each species has a speci c
number of chromosomes, and humans have 46 chromosomes arranged in 23 pairs. These
pairs are called homologous chromosomes because they are similar in size and shape and
come from each parent.

Autosomes and Sex Chromosomes — Among the 23 pairs of chromosomes, 22 pairs

are autosomes, which are identical chromosomes responsible for most inherited traits. The
23rd pair consists of sex chromosomes, designated as X and Y. Females have two X
chromosomes (XX), while males have one X and one Y chromosome (XY).

Chromosomes in Homologous Pairs — Chromosomes occur in pairs in organisms,

resulting in an even number of chromosomes. This pairing ensures that each offspring
receives one chromosome from each parent, contributing to genetic diversity.

Sex Determination - Son or Daughter:

The sex of offspring is determined by which type of sperm fertilizes the egg. The egg always
carries an X chromosome, while sperm can carry either an X or a Y chromosome. If an X-
bearing sperm fertilizes the egg, the offspring will be female (XX), while if a Y-bearing
sperm fertilizes the egg, the offspring will be male (XY).
Genes and Genome:

Genes — Genes are speci c segments of DNA located on chromosomes that determine
hereditary characteristics. Each gene carries instructions for producing a particular trait or
protein. Humans have approximately 30,000 genes, with chromosome 1 containing the
largest number and the Y chromosome containing the fewest.

Genome —The genome refers to the entire complement of DNA in an organism,

including all genes and non-coding regions. It represents the complete set of genetic
instructions necessary for the development and functioning of an organism.

Genes and Their Alleles

Genes and Alleles — Genes are units of heredity that come in pairs, with each gene
having two alternative forms called alleles. These alleles produce different effects on a
particular trait. For example, in the trait of tongue rolling, there are two alleles: one for
rolling the tongue (dominant) and the other for not rolling (recessive).
Dominant and Recessive Alleles — Among the two alleles of a gene, one is dominant,
meaning it exerts its effect over the other allele, which is recessive. In the case of tongue
rolling, the ability to roll the tongue (dominant allele, represented by 'R') dominates over the
inability to roll (recessive allele, represented by 'r').
• Genotype: This refers to the genetic makeup of an organism, representing the
combination of alleles it possesses. For example, a homozygous dominant genotype
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 (RR) means having two dominant alleles, while a heterozygous dominant genotype (Rr)
means having one dominant and one recessive allele.
• Phenotype: This refers to the observable characteristics or traits of an organism,
which are determined by its genotype. For example, the ability to roll the tongue is a
phenotype controlled by the presence of at least one dominant allele.

Chromosomes and Genes

Chromosomes and Genes — Genes are located on chromosomes, the structures within
cells that contain genetic information. Each chromosome carries multiple genes, which
determine various hereditary characteristics of an organism.
Genes and Inheritance:
• Inheritance of Traits: Genes are passed from parents to offspring during
reproduction. Offspring inherit one copy of each gene from each parent, contributing
to genetic diversity.
• Punnett Square: The inheritance of traits can be predicted using a Punnett square,
which shows the possible combinations of alleles that offspring can inherit from their
parents.

Sex-Linked Inheritance
X-Linked Inheritance — Certain genetic disorders, like hemophilia and color-blindness,
are more common in males than in females because the genes responsible for these traits are
located on the X chromosome. Hemophilia, for instance, is a condition where blood fails to
clot properly, leading to excessive bleeding. While females can be carriers (heterozygous),
males with a single recessive allele on their X chromosome are more prone to the disorder.
Criss-Cross Inheritance — This term refers to the inheritance pattern of X-linked
genes from mothers to sons and from fathers to daughters. For example, if a mother is a
carrier of a recessive X-linked trait like color-blindness and the father is color-blind, their
daughters have a chance of inheriting the disorder. Similarly, sons inherit their X
chromosome from their mother, making them more susceptible to X-linked traits carried by
their mothers.
Y-Linked Inheritance — Traits linked to the Y chromosome are exclusively found in
males. Examples include hypertrichosis of ears (excessive hair growth from ears) and pattern
baldness. Since the Y chromosome is present only in males, traits determined by genes on
the Y chromosome are expressed only in males.

Mendel's Experiments on Inheritance

Mendel's Selection of Garden Pea:
Gregor Mendel, an Austrian monk, conducted groundbreaking experiments on garden pea
plants in the mid-19th century. He chose garden peas because they exhibited distinct traits
that were easy to manipulate and observe, such as ower color, seed shape, and pod color.
Monohybrid Crosses — Mendel crossed pure-breeding pea plants with different traits
for a single characteristic, such as ower color. He observed that the offspring (F1
generation) exhibited one dominant trait and one recessive trait in a 3:1 ratio. This led to the
formulation of the law of dominance and the law of segregation.
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 Dihybrid Crosses — Mendel also performed crosses involving two different traits, such
as seed shape and seed color. He observed that the traits segregated independently during
gamete formation, leading to a phenotypic ratio of 9:3:3:1 in the offspring (F2 generation).
This supported the law of independent assortment.
Mendel's Laws of Inheritance:
• Law of Dominance — In a pair of contrasting traits, one trait (the dominant trait)
masks the expression of the other (the recessive trait) when present together in an
individual. Only when the individual carries two recessive alleles does the recessive trait
manifest.
• Law of Segregation — During gamete formation, the alleles of a gene segregate or
separate from each other, ensuring that each gamete carries only one allele for each
gene. This segregation of alleles leads to genetic diversity in offspring.
• Law of Independent Assortment — Genes for different traits segregate independently
of one another during gamete formation. This means that the inheritance of one trait
does not in uence the inheritance of another trait, as long as the genes are located on
different chromosomes or are far apart on the same chromosome.
Application of Mendel's Laws:
• Understanding Mendel's principles helps predict the inheritance patterns of traits and
the frequency of certain combinations in offspring.
• Plant and animal breeders can use this knowledge to selectively breed individuals with
desirable traits, leading to the development of improved breeds.
• Hybridization techniques can be employed to create new varieties of plants with
bene cial traits by combining genes from different parental lines.

Mutation
De nition: Mutation refers to sudden changes in one or more genes or in the structure or
number of chromosomes. These alterations in the genetic material can result in changes in
certain traits or characteristics of an organism.
Examples: Sickle cell anemia is caused by a mutation in the gene responsible for hemoglobin
production, leading to the formation of abnormal, sickle-shaped red blood cells.
Additionally, exposure to radioactive radiation, such as from atomic explosions, can induce
mutations in genes, which may have effects on subsequent generations, potentially leading to
genetic disorders or abnormalities.
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