Question

4(a). Complete the following Punnett squares of a dihybrid cross between two rats. One has black fur (BB) and short tail (tt), the other has brown fur (bb) and long tail (TT). Give the parental gametes and the F1 offspring grid.
4(b). How many of the offspring will have: (i) black fur and short tail; (ii) brown fur and long tail; (iii) black fur and long tail; (iv) brown fur and short tail?
4(c). If there were twenty (20) chromosomes in the leaf cell of a plant, how many chromosomes would be in each of the following cells of the plant: (i) pollen grain; (ii) guard cell; (iii) ovule; (iv) root cell?

Accepted Answer

(a) Parental gametes: horizontal (PGH): Bt; vertical (PGV): bT. All F1 offspring genotype: BbTt (black fur, short tail — since B and T are the given dominant alleles based on the cross set up).

(b) Offspring with: (i) black fur and short tail — all 16/16 (since all offspring are BbTt, heterozygous for both traits, showing the dominant phenotypes black fur and short tail); (ii) brown fur and long tail — 0; (iii) black fur and long tail — 0; (iv) brown fur and short tail — 0. (All 16 offspring show the same phenotype: black fur and short tail, since this is a cross between two pure-breeding/homozygous parents for different traits.)

(c) Number of chromosomes: (i) Pollen grain — 10 (haploid, since it's a gamete); (ii) Guard cell — 20 (diploid, a somatic cell); (iii) Ovule — 10 (haploid, gamete-forming structure); (iv) Root cell — 20 (diploid, a somatic cell).

WAEC Biology 2023 Theory — Question 7

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