1.

4 APPLY KNOWLEDGE F HEREDITARY TO SOLVE PROBLEMS

Cell → Nucleus → Chromosome → DNA → Gene → DNA Bases (A, T, C, G)

The relationship between DNA, genes, and chromosomes can be
understood as follows:

1. DNA (Deoxyribonucleic Acid)

o DNA is a molecule that carries the genetic instructions for

life.

o It has a double-helix structure made of nucleotides

(Adenine, Thymine, Cytosine, Guanine).

o DNA is found in the nucleus of eukaryotic cells and in the

cytoplasm of prokaryotic cells.

2. Genes

o A gene is a specific section of DNA that codes for a protein

or trait.

o Genes determine characteristics like eye color, height, and

blood type.

o Humans have around 20,000–25,000 genes.

3. Chromosomes

o A chromosome is a long, coiled structure of DNA that

contains many genes.

o Humans have 46 chromosomes (23 pairs) in the nucleus of

each cell.

o During cell division (mitosis and meiosis), chromosomes

ensure DNA is passed to new cells.

How They Are Related:

🔹 DNA is the fundamental genetic material →

🔹 Genes are sections of DNA that provide instructions →
🔹 Chromosomes are organized structures containing many genes

Example:
Imagine a book (chromosome) → The book is made of sentences
(genes) → The sentences are made of letters (DNA bases: A, T, C, G).

1.4.2

1. Dominant Allele
A dominant allele is one that expresses its trait even when only one copy
is present in a heterozygous individual. It is represented by a capital
letter (e.g., B for brown eyes).

 Dominant traits mask recessive traits in heterozygous individuals.

 Example: If a person has BB (homozygous dominant) or Bb

(heterozygous), they will have brown eyes.

Mendelian Explanation:

Gregor Mendel discovered that dominant alleles always express their

traits in the F1 generation when paired with a recessive allele.

2. Recessive Allele

A recessive allele is only expressed when an individual has two copies

(homozygous recessive). It is represented by a lowercase letter (e.g., b
for blue eyes).

 If a dominant allele is present (Bb), the recessive trait is not

expressed.

 Example: A person will only have blue eyes if their genotype is bb.

Why Are Some Alleles Recessive?

Recessive alleles often code for non-functional or less active proteins,

meaning they don’t produce enough of a trait (e.g., no melanin for blue
eyes).

3. Co-Dominance

In co-dominance, both alleles in a heterozygous individual are fully

expressed without blending.

 Example: Blood Type AB (IAIB) → Both A and B alleles are

expressed, leading to a mix of both traits.

 Another example: Roan cattle → If a cow has one red hair allele (R)
and one white hair allele (W), it will have both red and white
hairs (not a blend).

4. Homozygous

An individual is homozygous for a trait if they have two identical

alleles for a gene.
  Homozygous Dominant (BB) → Both alleles are dominant, and
the dominant trait is expressed.

 Homozygous Recessive (bb) → Both alleles are recessive, and the

recessive trait is expressed.

Example of Homozygosity:

 A person with BB for eye color has brown eyes.

 A person with bb has blue eyes.

5. Heterozygous

An individual is heterozygous when they have two different alleles for

a gene (Bb).

 The dominant allele determines the phenotype.

 Example: A person with Bb will have brown eyes, because B

(brown) is dominant over b (blue).

Heterozygosity in Disease Carriers:

Some genetic disorders are recessive (e.g., cystic fibrosis, sickle cell
anemia). A heterozygous person (Ss) may be a carrier but not show
symptoms.

6. Genotype

The genotype is the genetic makeup of an organism—it refers to the

specific alleles an individual has for a trait.

Examples of Genotypes:

 BB (homozygous dominant) → Brown eyes

 Bb (heterozygous) → Brown eyes

 bb (homozygous recessive) → Blue eyes

Genotype vs. DNA:

A genotype is a small part of an organism’s entire DNA sequence that
codes for a specific trait.

7. Phenotype
The phenotype is the physical expression of a trait—what we see as a
result of the genotype.

Examples of Phenotypes:

 Brown eyes (BB or Bb)

 Blue eyes (bb)

 Tall plant vs. short plant in pea plants

Environmental Influence on Phenotype:

While genotype determines phenotype, environment can also play a
role. For example:

 Siamese cats have a gene for fur color that is temperature-

sensitive, leading to darker fur in cooler body areas.

 Height is influenced by both genes and nutrition.

Quick Recap with Punnett Square Example:

If B = Brown Eyes (dominant) and b = Blue Eyes (recessive):

Parent 1 Parent 1
(B) (b)

B (Parent BB
2) (Brown)

b (Parent Bb
2) (Brown)

 75% chance of brown eyes (BB or Bb)

 25% chance of blue eyes (bb)

Final Summary

Term Definition Example

Allele that expresses its trait even when

Dominant B (brown eyes)
only one copy is present.

Allele that only expresses when two

Recessive b (blue eyes)
copies are present.
Term Definition Example

Co- Both alleles are expressed equally in a

Blood Type AB
Dominant heterozygote.

Homozygo BB (brown eyes), bb

Two identical alleles (BB or bb).
us (blue eyes)

Heterozyg
Two different alleles (Bb). Bb (brown eyes)
ous

Genotype The genetic makeup (BB, Bb, or bb). Bb (brown eyes)

The physical expression of the

Phenotype Brown eyes
genotype.

1.4.3

1. 3:1 Ratio – Monohybrid Cross (Heterozygous × Heterozygous)

A 3:1 ratio occurs in a monohybrid cross between two heterozygous

individuals (Aa × Aa) for a single gene with complete dominance.

Example: Pea Plant Height (T = Tall, t = Short)

 T = Dominant (Tall plant)

 t = Recessive (Short plant)

 Cross: Tt × Tt (Heterozygous × Heterozygous)

Punnett Square:

T T

TT
T Tt (Tall)
(Tall)

Tt tt
t
(Tall) (Short)

Genotypic Ratio:

 1 TT (Homozygous Dominant)

 2 Tt (Heterozygous)

 1 tt (Homozygous Recessive)
1:2:1

Phenotypic Ratio:
  3 Tall (TT + Tt)

 1 Short (tt)
3:1✅

What Happens?

 75% of offspring will be Tall (TT or Tt)

 25% of offspring will be Short (tt)
 The dominant trait (Tall) appears in most offspring, but the recessive trait (Short) can
still appear in homozygous recessive (tt) individuals.


2. 1:1 Ratio – Test Cross (Heterozygous × Homozygous Recessive)

A 1:1 ratio occurs when a heterozygous individual (Aa) is crossed with

a homozygous recessive individual (aa).

Example: Pea Flower Color (P = Purple, p = White)

 P = Dominant (Purple)

 p = Recessive (White)

 Cross: Pp × pp (Heterozygous × Homozygous Recessive)

Punnett Square:

P p

Pp pp
p
(Purple) (White)

Pp pp
p
(Purple) (White)

Genotypic Ratio:

 2 Pp (Heterozygous)

 2 pp (Homozygous Recessive)
1:1

Phenotypic Ratio:

 2 Purple (Pp)

 2 White (pp)
1:1✅
  ✅ All offspring (100%) will be Tall (Tt).
Phenotypic ratio → 1 Tall : 0 Short (100% Tall)

 (B) If the Tall plant is Heterozygous (Tt × tt)

  T  t

 Tt  tt
 t
(Tall) (Short)

 Tt  tt
 t
(Tall) (Short)

 ✅ 50% of the offspring will be Tall (Tt), and 50% will be Short
(tt).
Phenotypic ratio → 1 Tall : 1 Short (1:1)

 What Happens in a Test Cross?

 If all offspring are Tall → The unknown parent is homozygous

dominant (TT).

 If half the offspring are Tall and half are Short → The unknown
parent is heterozygous (Tt).

 This method helps determine the genotype of an individual

with a dominant trait.

Summary of Expected Ratios

Genotypic Phenotypic
Cross Type Example
Ratio Ratio

Heterozygous × 3:1 Ratio 1 TT : 2 Tt : 3 Tall : 1

Heterozygous (Tt × Tt) (Tall:Short) 1 tt Short

Heterozygous ×
1:1 Ratio 1 Purple : 1
Homozygous Recessive 1 Pp : 1 pp
(Purple:White) White
(Pp × pp)
These ratios follow Mendel’s laws and occur when traits follow simple
dominance inheritance.

Problem 1: Pea Plant Height (Dominant & Recessive Trait)

Question:

In pea plants, tall (T) is dominant over short (t). A heterozygous tall plant
(Tt) is crossed with another heterozygous tall plant (Tt).
What are the possible genotypes and phenotypes of the offspring, and
their ratios?

Step 1: Define Parent Genotypes

 Parent 1: Tt (Heterozygous tall)

 Parent 2: Tt (Heterozygous tall)

Step 2: Draw a Punnett Square

T t

T T
T
T t

T
t tt
t

Step 3: Analyze the Ratios

Genotypic Ratio (Genetic Makeup)

 1 TT (Homozygous dominant)

 2 Tt (Heterozygous)

 1 tt (Homozygous recessive)
Ratio: 1 TT : 2 Tt : 1 tt

Phenotypic Ratio (Physical Traits)

 3 Tall plants (TT and Tt)

 1 Short plant (tt)

Ratio: 3 Tall : 1 Short
Answer:

✅ Genotypic Ratio = 1 : 2 : 1
✅ Phenotypic Ratio = 3 : 1 (Tall : Short)

Problem 2: Test Cross to Determine Genotype

Question:

A farmer has a tall pea plant but does not know if it is homozygous
dominant (TT) or heterozygous (Tt).
To determine its genotype, the farmer crosses it with a homozygous
recessive (tt) short plant.
What are the possible outcomes?

Step 1: Define Parent Genotypes

 Unknown Tall Plant: Could be TT or Tt

 Short Plant: tt

Case 1: If the Tall Plant is TT

T T

T T
t
t t

T T
t
t t

Result:

 100% Tall plants (Tt)

 No short plants (tt)

✅ If all offspring are tall, the unknown parent was TT.

Case 2: If the Tall Plant is Tt

T t

T t
t
t t
 T t

T t
t
t t

Result:

 50% Tall (Tt)

 50% Short (tt)

✅ If half the offspring are short, the unknown parent was Tt.

Answer:

 If all offspring are tall → Parent was TT (Homozygous)

 If half are tall and half are short → Parent was Tt

(Heterozygous)

Problem 3: Human Eye Color (Brown vs. Blue)

Question:

In humans, brown eyes (B) are dominant over blue eyes (b).
A heterozygous brown-eyed person (Bb) has a child with a blue-
eyed person (bb).
What are the possible genotypes and phenotypes of the child?

Step 1: Define Parent Genotypes

 Brown-eyed Parent: Bb (Heterozygous)

 Blue-eyed Parent: bb (Homozygous recessive)

Step 2: Draw a Punnett Square

B b

Bb bb
b
(Brown) (Blue)

Bb bb
b
(Brown) (Blue)
Step 3: Analyze the Ratios

Genotypic Ratio

 2 Bb (Heterozygous brown)

 2 bb (Homozygous blue)

 Ratio: 1 Bb : 1 bb

Phenotypic Ratio

 2 Brown-eyed (Bb)

 2 Blue-eyed (bb)

 Ratio: 1 Brown : 1 Blue

Answer:

✅ Genotypic Ratio = 1 Bb : 1 bb
✅ Phenotypic Ratio = 1 Brown : 1 Blue

This means the child has a 50% chance of having brown eyes and a
50% chance of having blue eyes.

Final Summary of Solved Problems

Problem Cross Genotypic Ratio Phenotypic Ratio

Monohybrid Tall ×
1 TT : 2 Tt : 1 tt 3 Tall : 1 Short
Cross (Tt × Tt) Tall

If TT: 100% Tt
Test Cross (T? × Tall × If TT: 100% Tall; If
(Tall); If Tt: 1 Tt : 1
tt) Short Tt: 1 Tall : 1 Short
tt

Eye Color (Bb × Brown ×

1 Bb : 1 bb 1 Brown : 1 Blue
bb) Blue

1.4.5

What Are Mutations?

Mutations are changes in the DNA sequence of an organism. They can

happen naturally or be caused by external factors. Mutations can affect
how genes work and may lead to changes in traits.
Factors That Cause Mutations

1. Natural (Spontaneous) Mutations

These happen on their own during normal cell processes:

 DNA Replication Errors: When cells divide, the enzyme that

copies DNA (DNA polymerase) can make mistakes, like adding the
wrong base (A, T, C, or G).

 Chemical Changes in DNA:

o Depurination: A base (like adenine or guanine) falls off the

DNA strand, leaving a gap.

o Deamination: A base loses a chemical group (like cytosine

turning into uracil), which can cause errors in base pairing.

 Repetitive Sequences: Sometimes, DNA polymerase "slips" when

copying repetitive sequences, adding or removing extra bases.

2. External Factors (Induced Mutations)

These are caused by things in the environment that damage DNA:

 Chemical Mutagens:

o Base Analogs: Chemicals that look like normal DNA bases

but pair incorrectly (e.g., 5-bromouracil acts like thymine but
pairs with guanine).

o Alkylating Agents: Chemicals that add small groups to DNA

bases, messing up their pairing (e.g., ethyl
methanesulfonate).

o Intercalating Agents: Chemicals that squeeze into the DNA

strand, causing extra bases to be added or removed during
replication.

 Radiation:

o UV Light: Causes thymine bases to stick together, creating

"thymine dimers" that block DNA replication.

o Ionizing Radiation: X-rays or gamma rays can break DNA

strands or damage bases.

Why Do Mutations Matter?

Mutations can:

 Be harmless (no effect).

 Cause genetic disorders or diseases (e.g., cancer).

 Provide variation, which can help organisms adapt to their

environment (important for evolution).

1.4.6

What Are Mutations?

Mutations are changes in the DNA sequence of an organism. They can

occur in somatic cells (body cells) or germ cells (sperm and egg cells).
Only mutations in germ cells can be passed on to offspring and affect
inheritance.

How Do Mutations Cause Genetic Disorders?

Genetic disorders are diseases or conditions caused by changes

(mutations) in genes or chromosomes. These mutations can:

1. Alter Protein Function: Genes provide instructions for making

proteins. If a mutation changes the gene, it may produce a faulty
protein or no protein at all, leading to a disorder.

2. Change Chromosome Structure or Number: Mutations can

affect entire chromosomes, leading to disorders caused by extra,
missing, or rearranged chromosomes.

Examples of Genetic Disorders

1. Albinism

 Cause: Albinism is caused by a mutation in a single gene that

affects the production of melanin, the pigment responsible for skin,
hair, and eye color.

 Inheritance Pattern:

o Albinism is an autosomal recessive disorder. This means a

person must inherit two copies of the mutated gene (one
from each parent) to have the disorder.

o If a person inherits only one mutated gene, they are

a carrier but do not show symptoms.

 Significance of Mutation:
 o The mutation disrupts the production of melanin, leading to
little or no pigment in the skin, hair, and eyes.

o This results in sensitivity to sunlight, vision problems, and an

increased risk of skin cancer.

2. Down’s Syndrome

 Cause: Down’s syndrome is caused by a chromosomal

mutation called trisomy 21, where a person has three copies of
chromosome 21 instead of the usual two.

 Inheritance Pattern:

o Down’s syndrome is not typically inherited from parents.

Instead, it usually occurs due to a random error during the
formation of egg or sperm cells (nondisjunction).

o In rare cases, it can be inherited if a parent carries

a translocation involving chromosome 21.

 Significance of Mutation:

o The extra chromosome disrupts normal development, leading

to intellectual disabilities, distinct facial features, and an
increased risk of heart defects and other health problems.

Why Are Mutations Significant in Inheritance?

1. Source of Genetic Disorders: Mutations can directly cause

genetic disorders like albinism and Down’s syndrome.

2. Carrier Status: In recessive disorders like albinism, carriers (people

with one mutated gene) can pass the mutation to their children
without showing symptoms themselves.

3. Genetic Counseling: Understanding mutations helps families

assess the risk of passing on genetic disorders and make informed
decisions.

4. Evolution and Diversity: While some mutations cause disorders,

others contribute to genetic diversity, which is important for
evolution and adaptation.

1.4.7
What Is Variation?

Variation refers to the differences in characteristics (traits) between

individuals of the same species. These differences can be caused
by genetic factors, environmental factors, or a combination of both.

Types of Variation

1. Continuous Variation

 Definition: Traits that show a range of values within a population.

These traits are usually controlled by many genes (polygenic
inheritance) and influenced by the environment.

 Characteristics:

o The traits vary gradually from one extreme to another.

o They often follow a bell-shaped curve (normal distribution)

when graphed.

o Examples: Height, weight, skin color, and shoe size.

 Causes:

o Genetic Factors: Multiple genes contribute to the trait.

o Environmental Factors: Nutrition, climate, and lifestyle can

influence the trait.

 Example: Human height is a continuous trait because people range

from very short to very tall, with most people falling somewhere in
the middle.

2. Discontinuous Variation

 Definition: Traits that fall into distinct categories with no

intermediate forms. These traits are usually controlled by one or a
few genes and are less influenced by the environment.

 Characteristics:

o The traits are qualitative (described in categories, not

measured).

o They do not show a smooth range of variation.

o Examples: Blood type, tongue rolling, and presence of

dimples.
  Causes:

o Genetic Factors: A single gene or a small number of genes

determine the trait.

o Environmental Factors: Usually have little or no effect on

the trait.

 Example: Blood type is a discontinuous trait because people can

only be type A, B, AB, or O—there are no intermediate types.

Why Are These Variations Important?

 Continuous Variation:

o Helps populations adapt to changing environments (e.g.,

variation in height or weight).

o Provides raw material for natural selection and evolution.

 Discontinuous Variation:

o Useful for studying inheritance patterns (e.g., Mendelian

genetics).

o Helps identify genetic disorders or traits controlled by single

genes.

Example to Summarize

 Continuous Variation: Imagine a group of students lined up by

height. You’ll see a smooth range from the shortest to the tallest
student, with most students being of average height.

 Discontinuous Variation: Imagine the same group of students

grouped by blood type. They will fall into distinct categories (A, B,
AB, or O), with no in-between types.

1.4.8

Competition occurs when organisms struggle for limited resources

like food, water, shelter, or mates. Since resources are limited, not
all individuals can survive and reproduce.

How Variation and Competition Lead to Differential Survival

1. Variation in Traits
 What is variation? Variation refers to differences in traits (e.g.,
size, color, speed, resistance to disease) among individuals of the
same species.

 Sources of variation:

o Genetic variation: Caused by mutations, genetic

recombination during meiosis, and sexual reproduction.

o Environmental factors: Influences like diet, climate, or

exposure to diseases.

 Example: In a population of rabbits, some may have thicker fur,

while others have thinner fur.

2. Competition for Resources

 What is competition? Organisms compete for limited resources

like food, water, shelter, and mates.

 Why does competition happen? Resources are limited, and not

all individuals can survive and reproduce.

 Example: In a forest, rabbits compete for grass, and predators like

foxes compete for rabbits.

3. Differential Survival

 What is differential survival? Individuals with traits that give

them an advantage in their environment are more likely to survive
and reproduce.

 How does it work?

o Advantageous traits: Traits that help organisms survive and

reproduce better in a given environment.

o Disadvantageous traits: Traits that make survival and

reproduction harder.

 Example: In a cold environment, rabbits with thicker fur are more

likely to survive and reproduce than those with thinner fur.

4. Natural Selection
 What is natural selection? The process by which advantageous
traits become more common in a population over time, while
disadvantageous traits become less common.

 How does it relate to variation and competition?

o Variation provides the raw material (different traits).

o Competition determines which traits are advantageous.

o Differential survival ensures that advantageous traits are

passed on to the next generation.

 Example: If a disease spreads through a rabbit population, rabbits

with genetic resistance to the disease are more likely to survive and
pass on their resistance genes.

Why Is This Important?

 Evolution: Variation and competition drive natural selection, which

leads to evolution over time.

 Adaptation: Populations become better suited to their environment

as advantageous traits become more common.

 Biodiversity: Differential survival contributes to the diversity of life

on Earth.

1.4.9

Role of Natural Selection in Evolution

1. Drives Adaptation:

o Natural selection helps populations become better suited to

their environment by favoring traits that improve survival and
reproduction.

o Example: Giraffes with longer necks were better able to reach

food, so over time, the population evolved longer necks.

2. Creates Biodiversity:

o Natural selection leads to the development of new species as

populations adapt to different environments.

o Example: Darwin’s finches evolved different beak shapes to

eat different types of food on the Galápagos Islands.

3. Explains Evolutionary Changes:

 o Natural selection explains how species change over time and
how new species arise from common ancestors.

o Example: Humans and chimpanzees share a common

ancestor, but natural selection led to the evolution of distinct
traits in each species.

1.4.10

What Is Artificial Selection?

Artificial selection (also called selective breeding) is the process

by which humans intentionally breed plants or animals with
desirable traits to produce offspring with those traits. This is
different from natural selection, which occurs without human
intervention.

How Does Artificial Selection Work?

1. Identify Desirable Traits: Farmers or scientists select organisms

with traits they want (e.g., higher yield, disease resistance, better
taste).

2. Breed Selected Organisms: The selected individuals are bred

together to produce offspring.

3. Repeat Over Generations: This process is repeated over many

generations to strengthen the desired traits.

Benefits of Artificial Selection in Agriculture

1. Improved Crop Yields

 Artificial selection has been used to develop crops that produce

more food per plant.

 Example: Modern wheat and rice varieties have been bred to

produce higher yields, helping to feed a growing global population.

2. Enhanced Nutritional Value

 Crops can be bred to have higher levels of essential nutrients.

 Example: Golden rice has been genetically modified (through

selective breeding and biotechnology) to contain higher levels of
vitamin A, helping to combat malnutrition.

3. Disease and Pest Resistance

 Plants and animals can be bred to resist diseases or pests, reducing
the need for chemical pesticides.

 Example: Some corn varieties have been bred to resist pests like the
corn borer, reducing crop losses.

4. Drought and Climate Tolerance

 Artificial selection can create crops that grow well in harsh

conditions, such as drought or extreme temperatures.

 Example: Drought-resistant maize varieties have been developed to

thrive in dry climates.

5. Better Taste and Quality

 Crops can be bred for improved taste, texture, or appearance,

making them more appealing to consumers.

 Example: Seedless watermelons and sweeter tomatoes are results

of artificial selection.

6. Faster Growth and Shorter Life Cycles

 Plants and animals can be bred to grow faster or mature more

quickly, increasing productivity.

 Example: Broiler chickens have been selectively bred to grow larger

and faster for meat production.

7. Adaptation to Specific Environments

 Crops can be tailored to grow in specific regions or soil types.

 Example: Certain rice varieties have been bred to grow well in

flooded fields, making them ideal for regions with heavy rainfall.

8. Economic Benefits

 Higher yields and better-quality products increase profits for

farmers.

 Example: High-yield soybean varieties have boosted the income of

farmers in many countries.

Real-Life Examples of Artificial Selection in Agriculture

1. Corn (Maize):
 o Modern corn has been selectively bred from a wild grass called
teosinte. Over thousands of years, humans have transformed
it into the large, sweet, and high-yield crop we know today.

2. Dairy Cattle:

o Dairy cows have been bred to produce more milk. For

example, Holstein cows are known for their high milk
production.

3. Bananas:

o The bananas we eat today are seedless and have been

selectively bred from wild bananas, which are small and filled
with hard seeds.

Challenges of Artificial Selection

While artificial selection has many benefits, it also has some

drawbacks:

 Reduced Genetic Diversity: Focusing on specific traits can reduce

the genetic variety in a population, making crops or livestock more
vulnerable to diseases or environmental changes.

 Ethical Concerns: Selective breeding in animals can sometimes

lead to health problems (e.g., breathing issues in bulldogs or joint
problems in heavily muscled livestock).