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

Verified step by step guidance

1. Understand that Down syndrome is most commonly caused by trisomy 21, which results from nondisjunction, an error in chromosome separation during meiosis.
2. Recognize that oogenesis (egg formation) and spermatogenesis (sperm formation) are the two processes where nondisjunction can occur, leading to an abnormal number of chromosomes in the gametes.
3. Review cytogenetic studies and parental origin analyses of the extra chromosome 21 in individuals with Down syndrome, which often use molecular markers to determine whether the extra chromosome came from the mother or the father.
4. Note that a majority of these studies show the extra chromosome 21 is maternally derived, indicating nondisjunction occurred during oogenesis rather than spermatogenesis.
5. Consider biological factors such as the prolonged arrest of oocytes in meiosis I, which increases the chance of nondisjunction during egg formation, providing a mechanistic explanation for the maternal origin bias.

Key Concepts

Nondisjunction

Nondisjunction is the failure of chromosome pairs to separate properly during cell division, leading to gametes with abnormal chromosome numbers. In the context of Down syndrome, nondisjunction results in an extra copy of chromosome 21, causing trisomy 21.

Oogenesis vs. Spermatogenesis

Oogenesis is the process of egg formation in females, characterized by long arrest phases and fewer cell divisions, while spermatogenesis is sperm formation in males, involving continuous and rapid cell divisions. These differences influence the likelihood of nondisjunction events.

Parental Origin of Trisomy 21

Studies using genetic markers show that most cases of trisomy 21 originate from errors in the maternal egg rather than the paternal sperm. This evidence includes higher rates of nondisjunction during oogenesis, especially with increasing maternal age.