Video Transcript
The pedigree chart provided shows the inheritance of hemophilia within a family. A and B are a couple planning to have a child. What is the probability, in percent, that the child would be a male with
hemophilia? (A) 50 percent, (B) 25 percent, (C) 20 percent, (D) zero percent.
Hemophilia is a blood clotting disease. Those affected by it are almost exclusively male and have an issue forming blood
clots, resulting in uncontrollable or excessive bleeding. This recessive trait is carried on the X sex chromosome. We can see from the pedigree that the couple here, A and B, are a carrier female and
unaffected male, respectively.
In order to find out what their probability of having a child with hemophilia is, we
will have to construct a Punnett square. But first, we have to establish the genotypes of the parents. You may recall that a genotype is the alleles of an organism. For this question, our alleles are uppercase H, representing the gene for healthy
blood clotting, and lowercase h, representing the gene for hemophilia.
Because this trait is located on the X chromosome, we will put the alleles as a
superscript on the X chromosome, like so. So, the genotype for the carrier female would be X uppercase H X lowercase h. And the genotype for the unaffected male would be X uppercase H Y.
Now, to find probability, we will conduct a Punnett square between our couple. We fill in the sex chromosomes from each parent from the top and left side to
determine all the offspring possibilities. Then, we determine the phenotype of the offspring. We can see that there is one out of the four children possibilities that is a male
with hemophilia.
Therefore, the probability that the child is a male with hemophilia is one out of
four. So, our correct answer choice is (B), 25 percent.
