Question

Which one of the following CANNOT lead to trisomy 21?

a) maternal non-disjunction in meiosis I
b) maternal non-disjunction in meiosis II
c) paternal non-disjunction in meiosis I
d) paternal non-disjunction in meiosis II
e) choose this answer if all of the above can lead to trisomy 21

Answer 1

All options listed (maternal and paternal nondisjunction in either meiosis I or II) can lead to trisomy 21, which is the genetic basis for Down syndrome.

Step-by-step explanation:

The correct answer to the question of which one of the following CANNOT lead to trisomy 21 is e) choose this answer if all of the above can lead to trisomy 21. Nondisjunction can occur during both meiosis I and meiosis II and can involve either the maternal or paternal gametes. In meiosis I, nondisjunction is the failure of homologous chromosomes to separate, while in meiosis II, it is the failure of sister chromatids to separate. Both scenarios can result in gametes with an extra chromosome (n+1) that, when fertilized, contribute to the presence of three copies of a chromosome in the zygote, leading to trisomy 21, more commonly known as Down syndrome.