A 42-year-old woman becomes pregnant. Her physician recommends amniocentesis (a test for fetal chromosomal disorders). When the results are analyzed by the laboratory, it is determined that the fetus has 47 chromosomes. Name and fully explain what caused
this disorder. When the woman and her husband consult a genetic counselor, they are told that this problem is likely to have occurred in the woman's egg cells, not in her 50-year-old husband's sperm cells. Explain why.
What will be an ideal response?
The fetus has a trisomy. This resulted when one of the parents' gametes had a nondisjunction and was carrying an extra chromosome. During nondisjunction, chromosomes fail to separate properly in meiosis. Although this can happen in male or female gametes, the likelihood in this case was that the egg cell was the culprit. The risk of nondisjunction increases with age in women because the eggs are the same age as the female, and older cells are susceptible to damage.
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