Assume that in a series of experiments, plants with round seeds were crossed with plants with wrinkled seeds and the following offspring were obtained: 220 round and 180 wrinkled. (6 points total, 2 points each) (a) what is the most probable genotype of each parent? (b) what genotypic and phenotypic ratios are expected? (c) based on the information provided in part (b), what are the expected numbers of progeny (400 total) of each phenotypic class?

a) Rr and rr

b) Phenotypic ratio: 1/2 round, 1/2 wrinkled, Genotypic ratio: 1 (Rr) : 1 (rr)

c) 200 round, 200 wrinkled

Explanation:

Mendel performed experiments where he considered the genes coding for seed shape in them. The allele for round shape (R) is dominant over the allele for wrinkled shape (r). For question a, the parent exhibiting the dominant trait can either be homozygous (RR) or heterozygous (Rr). Since it was crossed with a parent with recessive trait (wrinkled seed shape), it will produce only offsprings with round seed shape if it is in a homozygous state(RR).

But some of the offsprings possessed wrinkled shape (recessive trait), hence, the probable genotype is heterozygous (Rr).

Question b: Based on the probable genotype of the parents in (a) above i.e. Rr and rr, a cross between them will yield offsprings with two phenotypes and genotypes.

The phenotypes will be half (1/2) of round-seeded plants, and half (1/2) of wrinkled seeded plants.

The genotype of the round-seeded offsprings will be Rr while that of the wrinked-seeded offsprings will be rr, in the ratio 2:2 or 1:1.

Question c: Based on the probable phenotypic ratio 1:1 in question b above, out of 400 progenies, 200 will be round-seeded while 200 will be wrinkled-seeded.

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