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.

Hemophilia is a X-linked recessive disease. Tim is not correct in his thinking because he can not pass the condition to his son. Only mother can pass it to sons since they get X chromosome from the mother. Father gives Y chromosome to sons which does not carry the hemophilic gene.

Tim and Stephanie's son can be hemophilic even when neither of the parents are hemophilic. In this case, Stephanie must be a carrier i.e. heterozygous for the trait.

Tim : XHY (normal)

Stephanie : XHXh (normal but carrier)

Their children:

        XH          Y

XH  XHXH   XHY

Xh   XHXh   XhY

As clear by the punnett square, all the daughters will be normal (half will be carriers like Stephanie). Half of the sons will be normal (XHY) and other half will have hemophilia (XhY).