Genetic Q! Please help! >_<?

Question

In humans, the ABO blood type is under the control of autosomal multiple alleles. Colour blindness is a recessive X-linked trait. If two parents who are both type A and have normal vision produce a son who is colour blind and is type O, what is the probability that their next child will be female who has normal vision and is type O? As with all questions, please show all working.

Answer 1

Essentially, this is a dihybrid cross. The parents have type A blood, but they produce a son with type O blood, therefore both must be genotype AO. Color blindness is sex linked, that is, the trait is carried on the X chromosome. Since females have 2 X chromosomes, they are much less likely to show recessive sex linked traits, while males, who are XY, show the traits much more often. In any case, since the father has normal vision, he must be XY. The mother also has normal vision, but their son is color blind, so she must have one copy of the gene on one of her X chromosomes, so she would be XXc. Then, you create a Punnett square with the cross being between AOXY and AOXXc. When you make the dihybrid cross, you will see that the probability that another child will be a female who has normal vision and is type O blood is 2 out of 16.