Two parents consult a genetic counselor. They do not understand why all their sons are colorblind (an X-linked trait) but none of their daughters are colorblind. Both of the parents appear normal. How would you explain this to the parents?

There is 50% probability that a son of carrier mother (X^cX) and normal father (XY) is affected with colorblindness. Since father is normal, all the daughters will be normal.

Explanation:

Being X linked recessive trait, males can be affected with colorblindness if their mother is either colorblind or carrier for the disease. Since both the parents are normal, the mother is carrier for the disease and is heterozygous dominant with genotype X^c X. She would deliver the "X^C" to 50% of her sons making them colorblind.

Daughters can get affected with X linked disorder such as colorblindness if their father is affected with the disease. In the given question, father is normal (XY). So, none of the daughters can be colorblind.