People with nail­patella syndrome have poorly developed or absent kneecaps and nails. Individuals w

36.People with nail­patella syndrome have poorly developed or absent kneecaps and nails. Individuals with alkaptonuria have arthritis as well as urine that darkens when exposed to air. Both nail­patella syndrome and alkaptonuria are rare phenotypes. In the following pedigree, vertical red lines indicate individuals with nail­patella syndrome, while horizontal green lines denote individuals with alkaptonuria.
a. What are the most likely modes of inheritance of nail­patella syndrome and alkaptonuria? What genotypes can you ascribe to each of the individuals in the pedigree for both of these phenotypes?
b. In a mating between IV­2 and IV­5, what is the chance that the child produced would have both nail­patella syndrome and alkaptonuria? Nail­patella syndrome alone? Alkaptonuria alone? Neither defect?

First, I got a). IV­2 has genotype of nnA_ and IV­5 has Nnaa.
For b), the textbook manual says the chance of their child having both nail-patella and alkaptonuria is: (1/2) chance of IV­2 receiving Aa x  (1/2) chance of IV2 donating n a gamete x (1/2) chance of IV5 donating n a gamete
= 1/8.

But isn't the chance of IV2 having Aa alleles dependent on his father, III2? Since III2 also has genotype nnA_ and III1 has nnAA, his son can only be a carrier of Alkaptonuria when his father is a carrier. So should I have to multiply by another 1/2, chance of his father being a carrier then multiply the rest (1/8)= 1/16?

I know how to do the rest of question. Any help would be appreciated.