Hemophilia is a sex-linked recessive trait. A phenotypically normal woman has phenotypically normal parents. However, she has a hemophiliac brother. What are the genotypes of her parents? What are her chances of being a carrier for hemophilia? If she is a carrier and marries a male with hemophilia, what is the chance of a child being a hemophiliac?

A. parents: Hh and Hh

B. chances of being a carrier: 50%

C. 50%

Explanation:

A. Since hemophilia is a recessive trait, both parents must carry the recessive alelle for it to show through in the child. However, we know both of them are phenotypically normal, meaning a dominant trait (not hemophiliac, H) is masking the recessive trait (hemophiliac, h)

B. The options the woman has when you create a punnet square with her parents genotypes are HH (doesn't have any hemophilia, because hemophilia is recessive, so represented with a lower case h) Hh (there is twice the likeliness she is an Hh since it occurs two times when you combine her parents genotypes. Hh means she is a carrier, but does not have the trait) and hh (meaning she definitely has it). So there is four options, two of which mean carrier, meaning she has a 50% chance of being a carrier.

C. genotypes of the parents:

mom: Hh

dad: hh

when you create their punnet square, you will get two occurrences of Hh and two occurrences of hh. Remember that hh means hemophilia. This means the child has a 50% chance of having hemophilia.

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