Question

Hemophilia is a blood clotting disorder that is a sex-linked trait carried on the X chromosome. If a woman with hemophilia and a man without the disorder have two sons and two daughters, how many of their children would have hemophilia

None

One

Two

Four

Answer 1

Do a Punnett square. The first step is come up with names for your alleles.

The woman has two X chromosomes and the man has one X and one Y chromosome.

Because the woman has hemophilia and hemophilia is recessive, both her X chromosomes must have the hemophilia allele for her to have the disease. So you can label that particular allele Xh for X (the chromosome it is on) and h (the hemophilia allele). Her genotype is Xh Xh.

The Dad does not have hemophilia, so he must have a normal X allele. His genotype is XY.

These are the genotypes you should use in your Punnet square. Helpful?

Answer 2

What is a Punnett square, I fear I may have not learned that yet, or do not remember. Should I just google it?

Answer 3

Wikipedia would be better than Google. The Punnett Square for this cross looks like this:




The Mom's alleles are on the top, the Dad's alleles are on the side and the offspring's alleles will be in the squares.