Question

Assume the California wheat breeder is certain the stripe rust resistant F2 plants in their nursery are either RR or Rr and the F1 parent of these F2s was the genotype Rr. If they harvest F3 seeds from all of the resistant F2s, what fraction of the F3 would be susceptible to stripe rust?

Answer 1

Answer:

1/4 or 1/2 depending on mating partner of F2

Explanation:

Assuming that the F1 parent Rr mated with a plant with the RR, Rr or rr genotype the F2 offspring would have the following genotypes:

Rr x RR= 1/2 RR, 1/2 Rr

Rr x Rr = 1/4 RR, 1/2 Rr, 1/4 rr

Rr x rr = 1/2 Rr, 1/2 rr

The F2 generation with the RR genotype could not be the parents of the F3 generation because to have individuals with the rr genotype the F2 generation would have to mate with a plant that is either Rr or rr in which case the fraction of the population with the stripe rust phenotype will either be 1/4 or 1/2, respectively.