Question

"DMD is a recessive sex-linked disorder. A woman and man who are both free of the disorder, have two children. Their eldest son develops DMD, while their younger son is free of the disorder. Determine the genotypes of the parents and children." If both parents are free, and the dad obviously can't be a carrier, how can one of their children have the disorder if it is recessive??

Answer 1

Definitely the mother is the carrier. It must be a X linked disorder.

Let the dominant allele be X and the recessive be x( the one that carries DMD).
And the father is not a carrier - so the genotype is XY.
Mother - Xx
So, the genotype of the eldest son is xY.
And the genotype of the young son is XY.