Tim and Stephanie are devastated when they find out their newborn son has hemophilia- sex-linked recessive disease. Tim is shocked because he doesn’t have hemophilia, and figured if his son has it he would have gotten it from him. Is Tim correct in his thinking? Explain. Also explain what their genotypes must be in order for neither parents to have the disease but their son does. Create a Punnett square, and indicate the mother's and father's genotype.

First of all, the thinking of Tim is wrong. Hemophilia is X-linked and a male (XY) child always inherits his X chromosome from his mother. Hence, the newborn son of Tim must have inherited the allele for the disease from Stephanie.

Now, neither Tim nor Stephanie showed any symptom of hemophilia. It could only mean that Tim is totally free of the hemophilic allele while Stephanie is a carrier for it. Because a man has just one X chromosome, he can either be totally free of the abnormality or affected by it, no in-between. A woman has two X chromosomes, hence, she can be totally free, a carrier, or affected for the disease. Assuming the allele for hemophilia to be h, Tim's genotype would, therefore, be $X^HY$ while that of his wife would be $X^HX^h$ .