Case Study

1. Review the four principles of sex-linked inheritance listed just before the case study. Which individuals in figure 5.2 demonstrate principle 1?

Bill, Terry, Greg, and Jenny.

2. Which individuals in figure 5.2 represent principle 2?

Carl, Terry, and Greg.

3. Which individuals in figure 5.2 represent principle

Florence, Jessica, Terry, and Greg.

4. Why are none of the males in figure 5.2 shown as carriers of hemophilia?

Males cannot carry hemophilia because it is an X-linked disease and males only have a single X chromosome. If the male has an X, he has the disease.

5. What is the probability that Marcy and Tom will have any child with hemophilia (irrespective of sex)? Show how you calculate this.

The probability of Marcy and Tom having a child with hemophilia is 12.5%. This is because Marcy has a 50% chance of being a carrier. Marcy’s children in turn have a 50% chance of receiving a copy of the X chromosome. Thus, there is a 25% chance that any of the children would receive a copy of the recessive X allele from Marcy. Tom does not have hemophilia, so we know that the gene on his X chromosome is normal. Therefore, only Marcy and Tom’s sons could have hemophilia. The total probability of Marcy and Tom having a child with hemophilia is 12.5% which brakes down mathematically (0.5)(0.5)(0.5) = .125.

6. Suppose Marcy becomes pregnant and has an amniocentesis, a prenatal cell-sampling technique that can identify the sex of a child (among other things). She finds that she is going to have a baby girl. Should she be worried that the baby will have hemophilia? Why or why not? Suppose she decides to be sterilized after her baby’s birth, so she will have only this one child. Should she be concerned that her grandchildren will have hemophilia? Why or why not?

Marcy should not be worried that the baby girl will have hemophilia. Even if Marcy is a carrier she has a 50% probability, as well as Tom does not have hemophilia. Therefore, a baby girl would receive a copy of Tom’s X chromosome that does not have the allele for hemophilia.

7. Blood typing can help determine paternity. Suppose an unmarried woman gives birth, charges a certain man with being the father, and sues for child support. He demands blood tests, which show that the woman is type A, the baby is type A, and he is type B. Does this evidence rule him out as the father? Explain why or why not.

No, the evidence does not rule out the man as the father of the child. A person with blood type A may have both A and O, while a person with type B blood may have B and I. If both the woman and the man each have one O, the child could inherit an blood type A which would make the child blood Type A.

8. The drugs for cancer chemotherapy target rapidly dividing tumor cells, but also any other cells in the body that undergo rapid mitosis. In light of this, explain why these drugs often cause hair loss as a side effect.

Hair loss is often a side effect of chemotherapy because hair growth occurs by mitosis in the hair bulb. Since the drugs used for chemotherapy target rapidly dividing the drugs kill the mitotic cells of the hair bulb.

9. Based on the data in table 5.1, which type of cancer—colon or oral—do you think is more treatable (has the better prognosis)? Explain your reasoning.

According to table 5.1, oral cancer has a better prognosis because 27.3% of people with oral cancer die compared to almost 50% of people with colon cancer.