(a) (i) Define the law of independent assortment. (ii) State the findings from Mendel's experiment involving white-flowered and red-flowered pea plants. (ii...

Answer Details

(a)(i) Law of independent assortment

Mendel's law of independent assortment states that when two or more pairs of contrasting characters (alleles) are considered together, each pair separates and is inherited independently of the other during the formation of gametes.

(a)(ii) Findings from the red-flowered and white-flowered pea cross

When a pure-breeding red-flowered pea plant was crossed with a pure-breeding white-flowered pea plant, all the offspring of the first filial generation (F₁) were red-flowered. When these F₁ plants were self-crossed, the second generation (F₂) contained both red-flowered and white-flowered plants in the ratio of about 3 red : 1 white. This showed that red is dominant to white, and that the white character had only been hidden (masked) in the F₁, reappearing in the F₂.

(a)(iii) Illustration by diagrams

Let R = allele for red flower (dominant), r = allele for white flower (recessive).

Parents: RR (red) × rr (white)

Gametes: R and r

F₁: all Rr (red-flowered)

F₁ cross: Rr × Rr

F₂ ratio: 3 red : 1 white.

(b) Why A and B can be heterozygous or homozygous but O only homozygous

Blood groups in man are controlled by three alleles: IA and Ib, which are both dominant, and io, which is recessive. Because IA and Ib are dominant over io, group A can be produced by either IAIA (homozygous) or IAio (heterozygous), and group B can be produced by either IbIb (homozygous) or Ibio (heterozygous); in the heterozygous forms the dominant allele still expresses A or B and masks the recessive io. Group O, on the other hand, is caused only by the recessive allele io, which is expressed only when no dominant allele is present. Therefore group O can arise only from the homozygous recessive genotype ioio and cannot exist in a heterozygous state.