Answer:
10%
Step-by-step explanation:
Given:
man AB/ab
woman ab/ab
gene distance 20 cM
Genetic distances within a given linkage group are measured in centiMorgans.
Children will be AB/ab, Ab/ab, aB/ab and ab/ab
Here , as given gene distance is 20 cM, so the recombination frequency is 20%.
So the frequency of recombinant progeny is 20 %.
The the probability that their first child will be Ab/ab is 10%.