Early in their marriage, in their mid-20s, Charles and Leona had two normal children, a boy and girl. Later, when Charles and Leona were both 42 years old, Leona unexpectedly became pregnant again, despite the birth control methods the couple had practiced for many years. Both parents were in good physical health and there was no history of genetic disorders in either of their families. Still,

Question

they wondered about the risks of a genetic or chromosomal disorder in their child-to-be. (a) What might a genetic counselor tell the parents about their chances of having a child with Down syndrome? (b) What methods might be employed to determine if the fetus is normal prior to birth? (c) What other kinds of defects might these methods detect?

What will be an ideal response?

Answer 1

(a) Statistically, the risk of a child with Down syndrome is 1 out of 65 for a mother in this age range, as compared to only 1 out of 1,000 when the earlier children were born. Thus, the risk has increased substantially. (b) Amniocentesis or chorionic villus sampling (CVS) would allow karyotyping, which would reveal the extra chromosome on the 21st pair in the case of Down syndrome. (c) All of the sex-chromosome abnormalities, with any sample of fetal cells; some recessive defects such as sickle-cell anemia, Tay-Sachs disease, cystic fibrosis, and hemophilia.