What is a key consequence of nondisjunction during meiosis?

• A. Normal segregation with no aneuploidy.
• B. DNA replication occurs.
• C. RNA processing increases.
• D. Failure of homologous chromosomes or sister chromatids to separate during meiosis, leading to aneuploidies such as trisomies or monosomies.

Answer: (D)

Nondisjunction during meiosis is the failure of separation of homologous chromosomes or sister chromatids, resulting in gametes with an abnormal chromosome number. When such a gamete fertilizes a normal one, the zygote often becomes aneuploid, carrying an extra chromosome (trisomy) or missing one (monosomy). This direct outcome explains why conditions like trisomy 21 or monosomy X arise. The other ideas—DNA replication happening or RNA processing increasing—aren’t consequences of mis-segregation in meiosis, and normal segregation would avoid producing aneuploid gametes.