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Question

A certain form of muscular dystrophy is inherited as a sex-linked,

recessive gene. Jack has muscular dystrophy. (Neither of his parents

has this disease.) Jane, Jack’s wife, does not have muscular

dystrophy, but her father does. What fraction of their daughters

would you expect to have muscular dystrophy? What fraction of

their sons would you expect to have muscular dystrophy?

Answer 1

To solve problems like these, you first need to extract information from the question about the parents. Sometimes, this will include information from grandparents or siblings. Here, the first information is that the trait is X-linked recessive. In other words, alleles will be X^a (the mutant allele, which causes muscular dystrophy), X^A (the wild type allele which causes a person to be healthy) and Y (which causes male-ness, but does not have much/any other genetic information). From there, worth through step-by-step to see what you can conclude about each parent's contribution to a child.

Since the father has MD, he must have an X^a allele, and since he is biologically male, his other allele must be Y... so we know everything about him--he is X^aY. We are told that Jack's wife is healthy, so she must have an X^A allele... but we are told her FATHER had MD... this means he must have been X^aY, AND that he must give his single X to any daughter (if he had given a Y, she would've been his son!). So Jack's wife must be X^AX^a.

So Jack x wife = X^aY x X^AX^a

For SONS, Jack MUST give Y, and there is a 50% chance his wife will give X^a, so 50% chance sons will have MD

For DAUGHTERS, Jack MUST give Xa, and if the daughter is to have MD, she must be X^aX^a, so mother must give X^a, which is 50% chance