Chapter 8: Heredity

Q1. How do Mendel's experiments show that traits may be dominant or recessive?

Solution

Mendel crossed tall pea plants (TT) with dwarf pea plants (tt). In the F₁ generation, all plants were tall (Tt). This showed that:
• The tall trait is dominant (expressed even when one copy is present)
• The dwarf trait is recessive (expressed only when two copies are present)
When F₁ plants were self-crossed, F₂ generation showed both tall and dwarf in ratio 3:1.

Q2. How do Mendel's experiments show that traits are inherited independently?

Solution

Mendel's dihybrid cross showed independent inheritance:
• He crossed plants with two different traits (round-yellow vs wrinkled-green seeds)
• In F₂ generation, he got four types of offspring in ratio 9:3:3:1
• This showed that the inheritance of one trait (shape) is independent of another trait (colour)
• This is called the Law of Independent Assortment

Q3. What is a gene? What is its role?

Solution

A gene is a unit of heredity that is transferred from parent to offspring. It is a segment of DNA that codes for a specific trait.

Role:
• Determines physical characteristics
• Controls enzyme production
• Responsible for inherited traits
• Genes exist in pairs (alleles) on chromosomes
• Each parent contributes one allele for each trait

Q4. What are the different types of variations?

Solution

Types of variation:
• Somatic variation: Occurs in body cells, not inherited (e.g., scars, muscles)
• Germinal/Gametic variation: Occurs in reproductive cells, inherited
• Continuous variation: Gradual differences (e.g., height, skin colour)
• Discontinuous variation: Distinct categories (e.g., blood group, eye colour)
• Mutation: Sudden, heritable changes in DNA

Q5. How is the sex of a child determined in humans?

Solution

Sex determination in humans:
• Humans have 23 pairs of chromosomes (46 total)
• 22 pairs are autosomes, 1 pair is sex chromosomes
• Females have XX chromosomes
• Males have XY chromosomes
• The father determines the sex of the child
• If X sperm fertilizes egg → Girl (XX)
• If Y sperm fertilizes egg → Boy (XY)
• Probability is 50:50 for each child

Q6. What are Mendel's laws of heredity?

Solution

Mendel's laws:
• Law of Dominance: In a pair of contrasting traits, one is dominant and the other recessive
• Law of Segregation: Two alleles of a gene separate during gamete formation, each gamete gets only one allele
• Law of Independent Assortment: Genes of different traits are inherited independently of each other

Q7. What is a Punnett square? How is it used?

Solution

A Punnett square is a diagram used to predict the outcome of a genetic cross. It shows all possible combinations of alleles from two parents.

For example, Tt x Tt cross:

        T      t
T    TT    Tt
t    Tt    tt

It shows the genotypic ratio (1:2:1) and phenotypic ratio (3:1).

Q8. What is the difference between genotype and phenotype?

Solution

Genotype: The genetic makeup of an organism (the alleles it carries). Example: TT, Tt, tt.

Phenotype: The observable physical characteristics of an organism. Example: Tall, Dwarf.

Genotype determines phenotype, but environment can also influence phenotype.

Punnett square showing F₁ monohybrid cross

Key Concepts:
• Gene: Unit of heredity
• Allele: Alternative forms of a gene
• Dominant: Trait expressed in heterozygous state
• Recessive: Trait expressed only in homozygous state
• Monohybrid cross ratio: 3:1 (phenotypic), 1:2:1 (genotypic)
• Sex determination: XX = Female, XY = Male

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