Question

Assuming independent assortment of each gene, in the cross female mouse: Aa Bb cc Dd ee FF x male mouse: Aa bb Cc Dd ee fY. Assume all of these genes have simple dominance/recessive relationships. The F/f genetic locus is on the X chromosome in mice.

(a). What is the expected ratio of F1 progeny that will be phenotypically identical to the female parent?

(b) What ratio of progeny will be genotypically identical to the female?

(c) . What ratio will be phenotypically A_B_C_?

(d) Now assume that genes A and B are so tightly linked that no recombination can occur between them, and the phase of the female mouse has A and B in coupling. Now what ratio of progeny will be A_B_C_? (C/c is still unlinked to A/B)

Answer 1

(a)

The phenotype of the Female mouse is ABcDeF.

The genotype of the two parents are: Aa Bb cc Dd ee FF x Aa bb Cc Dd ee fY

The proportion of progeny with the phenotype ABcDeF will be determined by multiplying together the proportion of progeny with each of the phenotypes. For example, given that both parents are heterozygous for the A locus, 3/4 of the progeny will be phenotypically A.

Therefore, the proportion of progeny that is ABcDeF
= 3/4 * 1/2 * 1/2 * 3/4 * 1 * 1 = 9/64 = 0.141

(b)

None of the progeny will be genotypically identical to the female parent. This is because the female parent has two F alleles on either X chromosome. However, the male parent has the genotype fY, and therefore, all the female progeny from this cross will have the genotype Ff.

(c)

The ratio of progeny that is phenotypically A_B_C_ is 3/4 * 1/2 * 1/2 = 3/16.

3/16 of the progeny from this cross will have the phenotype ABC.

(d)

Given that the A and B loci are tightly linked in coupling phase and not recombination can occur between the two loci, only two types of gametes can be produced from this locus: AB and ab. Since the C locus is unlinked to this locus, it will assort independently.

=> Proportion of progeny with phenotype ABC
   = Proportion of progeny inheriting the AB gamete * Proportion of progeny inheriting C gamete
   = 1/2 * 1/2 = 1/4

Thus is A and B are linked in the coupling phase, then 1/4th of all progeny from this cross will have the ABC phenotype.