A 20-year-old woman has a robesonian translocation involving chromosome 21 and a second acrocentric chromosome. What is the theoretic likelihood of a functional trisomy 21 if one of her ova is feilized by a normal sperm?

Theoretically, a person who carries a robesonian translocation with chromosome 21 and a second acrocentric chromosome has a 1 in 3 chance of having a child with trisomy 21; However, the risk of a live bih of a child with Down syndrome is actually much less, presumably because of a high incidence of spontaneous aboion of such fetuses. The impoant point is that a robesonian translocation predisposes to a heritable form of Down syndrome. The risk is not related to maternal age and is much higher than the risk in the general population, which is 1 in 1500 for women 20 years of age and younger, increasing to 1 in 25 in women older than 45 years of age.