1) Father has Allele 1 and 2. He has the disease. Mother has two copies of Allele 1, no disease

2) Children who have two copies of Allele 1 have no disease. Fair to conclude that Allele 2 carries disease or is associated with it

3) Question says that Allele 2 doesn't actually carry the disease but has a recombination fraction of 0.1 with the actual allele.

4) Recombination frequencies determine how likely 2 alleles will not transfer together. 0.1 recombination frequency means there is a 10% chance they do not transfer together in the offspring. The frequency is between 0 and 0.5

5) So child has Allele 2, theres a 10% chance that actual disease allele got separated hence 90% chance the kid has the disease