What evidence suggests that Down syndrome is more often the result of nondisjunction during oogenesis rather than during spermatogenesis?

Key Concepts

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to gametes with an abnormal number of chromosomes. In the context of Down syndrome, which is caused by an extra copy of chromosome 21 (trisomy 21), nondisjunction can occur during meiosis in either oogenesis or spermatogenesis, but the implications differ based on the parent of origin.

Oogenesis vs. Spermatogenesis

Oogenesis is the process of egg (ovum) formation in females, which occurs in a finite number of oocytes that are arrested in development until ovulation. In contrast, spermatogenesis is the continuous production of sperm in males. Research indicates that maternal age significantly affects the risk of nondisjunction during oogenesis, suggesting that most cases of Down syndrome arise from errors in egg formation rather than sperm.

Maternal Age Effect

The maternal age effect refers to the increased risk of chromosomal abnormalities, including Down syndrome, associated with advanced maternal age. As women age, the likelihood of nondisjunction during oogenesis rises, particularly after age 35. This phenomenon is less pronounced in males, where sperm production remains relatively constant, indicating that the majority of Down syndrome cases are linked to maternal rather than paternal nondisjunction.