Participants:
Write your answers on the back of this sheet

 

In order to develop an understanding of the basic genetics in this case, work with your group to find answers to the following questions.

1. CF was known to be a recessive disorder long before the advent of the current techniques of molecular genetics. What kind of evidence could have supported such a single-recessive-gene interpretation? What kind of evidence would have made this interpretation unreasonable?

2. In a childless couple, both people are diagnosed as carriers of a CF mutation.

  1. What are the chances their first child will be normal?
  2. What are the chances they will have two CF children?
  3. If their first child is born with CF, what are the chances the second will also have CF?

3. A screening program is implemented which tests for common CF mutations.

  1. If they test for the most common mutation and will thus detect 71% of CF mutations, what fraction of "at-risk" (double-carrier) couples will be detected?
  2. If they test for the 4 most common mutations and will thus detect 85% of CF mutations, what fraction of "at-risk" (double-carrier) couples will be detected?
  3. In general, how does increasing test sensitivity affect the uncertainty facing a couple that undergoes testing?
  4. Examine the answer to question 3a. What would happen if you re-tested everyone whose initial test was negative? How many additional carriers would you find on re-testing?

4. In each of the following situations, is it possible the couple to have a CF child? Explain.

  1. Both test negative for CF in the screen for four mutations described above.
  2. The husband has two copies of the normal allele and the wife is a carrier.

5. About 4% (1 out of 25) of white Americans are carriers of CF genes. (These alleles are less common in other ethnic groups.)

  1. How many Mercerians would you expect to be carriers?
  2. About how many people in the US would you expect to be carriers?
  3. What is the likelihood that randomly chosen parents will both be CF carriers?
  4. What is the overall likelihood that a randomly chosen baby will have CF?