If A/A ⋅ B/B is crossed with a/a ⋅ b/b and the F1 is testcrossed, what percentage of the testcross progeny will be a/a ⋅ b/b if the two genes are (a) unlinked; (b) completely linked (no crossing over at all); (c) 10 m. u. apart; (d) 24 m. u. apart?

a) 25%; b) 50%; c) 45%; d) 38%

Explanation:

a) Unlinked genes

A/A B/B X a/a b/b

F1: A/a B/b

Testcross A/a B/b X a/a b/b

The homozygous recessive individual only produces ab gametes.

The F1 produces four types of gametes (each of them with a frequency of 1/4) : AB, Ab, aB and ab.

25% of the progeny will be a/a b/b.

b) Completely linked genes

AB/AB X ab/ab

F1: AB/ab

Testcross AB/ab X ab/ab

The F1 produces only two types of gametes, the parentals (each of them with a frequency of 1/2) : AB and ab.

50% of the progeny will be ab/ab.

c) 10 m. u. apart

AB/AB X ab/ab

F1: AB/ab

Testcross AB/ab X ab/ab

The F1 produces four types of gametes, the parentals AB and ab and the recombinants Ab and aB.

Since the genes are 10mu apart, 10% of the produced gametes will be recombinant and 90% will be parentals. Since there are two types of parental gametes, each of them has a frequency of 45%.

45% of the progeny will be ab/ab.

d) 24 m. u. apart

This is very similar to c).

Since the genes are 24mu apart, 24% of the produced gametes will be recombinant and 76% will be parentals. Since there are two types of parental gametes, each of them has a frequency of 38%.

38% of the progeny will be ab/ab.