Question

. AABB and aabb individuals were crossed to each other, and the F1 generation was backcrossed to the aabb parent. 850 AaBb, 950 aabb, 105 Aabb, and 95 aaBb offspring resulted.

a. How far apart are the a and b loci?

b. What progeny and in what frequencies would you expect to result from testcrossing the F1 generation from a AAbb x aaBB crosss to aabb?

c. In a typical meiosis, how many crossovers occur between genes A and B?

d. Assume that the A and B loci are on the same chromosome, but the offspring from the testcross described above were 496 AaBb, 504 aabb, 502Aabb, and 498 aaBb. How would your answer to part (c) change..

Answer 1

a).

Hint: Always recombinant progeny are less numbered in linkage problems. The recombinant progeny in the question are Aabb and aaBb.

Recombination frequency = (no. of recombinants / total progeny)100

= (200/2000)100 = 10%

Recombination frequency (%) = Distance between genes (mu)

Distance between genes = 10 mu

b).

AAbb x aaBB ---Parents

AaBb ------------F1

If the two genes are assorted independently, the following are progeny in testcross.

AaBb (F1) x aabb ----Testcross

	Ab
AB	AaBb (25%)
Ab	Aabb (25%)
aB	aaBb (25%)
ab	aabb (25%)

If the two genes are linked in 10 m.u, then the progeny as follows..

Ab/Ab x aB/aB ---Parents

Ab/aB ------------F1

Ab/aB (F1) x ab/ab ----Testcross

	Ab
AB	AB/ab (5%)
Ab	Ab/ab (45%)
aB	aB/ab (45%)
ab	ab/ab (5%)

c). There would occurs one crossover between gene A and B as they are closely arranged.

d).

Based on the information all the progeny of the testcross ratio is similar to 1:1:1:1.

This ratio is obtained when the two genes are assorted independently and the two genes are not linked.