Unfortunately, no DNA analysis had been conducted on the woman’s brother to determine what type of mutation in the DMD gene he had. The woman was investigated by molecular analysis and found to be heterozygous (A1/A2) for a microsatellite marker closely linked to the DMD gene. No relatives except the parents of the woman were available for analysis.

a. Can the phase of the mutation in the woman be determined from analysis of the available individuals?

b. Can this information be used to diagnose her pregnancy?

c. What other molecular analysis could be performed on the fetus?

7. Discuss the relative advantages and disadvantages of the following diagnostic procedures, and cite types of disorders for which they are indicated or not indicated: amniocentesis, CVS, first-semester maternal serum screening, second trimester screening, noninvasive screening of cell-free fetal DNA (noninvasive prenatal screening [NIPS]).

8. Suppose the frequency of Down syndrome is 1 in 600 in pregnancies in women younger than 35 years. Consider the following two strategies for prenatal detection of the disorder:

• All pregnant women younger than 35 years are offered CVS or amniocentesis.

• All pregnant women undergo a sequential screening strategy, as follows: All participate in first-trimester screening with pregnancy-associated plasma protein A (PAPP-A), human chorionic gonadotropin (hCG), and nuchal translucency. Sensitivity is 84% with a false-positive rate of 5%. Those who score positive are offered CVS, and all use it. Those who score negative are screened during the second trimester with a quadruple maternal serum screening, which has 81% sensitivity and a 5% false-positive rate. Those who score positive are offered amniocentesis, and all use it.

Assuming that a population of 600,000 women younger than 35 years are pregnant:

a. How many CVS procedures or amniocenteses are done overall, given these two strategies?

b. What fraction of the total expected number of affected fetuses is detected under the two strategies? What fraction is missed?

c. How many CVS or amniocentesis procedures would need to be done to detect one fetus with Down syndrome under these two strategies?

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