Chapter 4 Solutions — Principles of Inheritance and Variation

Mendel selected garden pea (Pisum sativum) for his hybridisation experiments due to several advantageous features:

1. Pea plants have many distinguishable contrasting characters (7 pairs of traits such as tall/dwarf, round/wrinkled seeds, yellow/green seeds, etc.) that are easy to observe and score.

2. They are naturally self-pollinating, which ensures true-breeding (homozygous) lines are easy to maintain.

3. Cross-pollination can be performed easily by hand, allowing controlled hybridisation experiments.

4. They have a short generation time and produce a large number of offspring, enabling statistically significant data.

5. Plants are easy and inexpensive to grow and maintain in large numbers.

6. Traits are distinct with no intermediate forms (discontinuous variation), making scoring unambiguous.

For a diploid organism heterozygous at a single locus (e.g., Aa), two types of gametes are produced: A and a.

For each additional heterozygous locus, the number of gamete types doubles (assuming independent assortment).

The formula is: Number of gamete types = 2ⁿ, where n = number of heterozygous loci.

Here, n = 4.

Number of gamete types = 2⁴ = 16.

Law of Dominance states that: (i) characters are controlled by discrete units called factors (genes); (ii) factors occur in pairs; (iii) in a pair of dissimilar factors, one member (dominant) dominates the other (recessive) and is expressed.

Illustration using a monohybrid cross (tall × dwarf pea):

Parents: Tall (TT) × Dwarf (tt)

Gametes from tall parent: T only

Gametes from dwarf parent: t only

F₁ generation: All progeny are Tt (genotype) → All plants are Tall (phenotype).

Here, only the dominant character (tallness, T) is expressed; the recessive character (dwarfness, t) is not seen, even though the t allele is present.

F₁ × F₁ (Tt × Tt):

Gametes: T and t (from each parent)

F₂ genotypes: 1 TT : 2 Tt : 1 tt

F₂ phenotypes: 3 Tall : 1 Dwarf (phenotypic ratio 3:1)

The dominant character (tallness) is expressed in both TT and Tt plants; the recessive character (dwarfness) is expressed only in tt plants. The Law of Dominance explains why only one parental character is seen in F₁ and why both appear in F₂ in a 3:1 ratio.