What is non-disjunction?

• A. Chromosomes fail to replicate before division.
• B. Each chromosome from a pair moves to a different cell, as normal.
• C. Chromatids separate during mitosis.
• D. Both chromosomes from one pair go into one cell and no chromosomes for that pair go into the other cell.

Answer: (D)

Non-disjunction is when chromosomes do not separate properly during cell division, resulting in gametes with abnormal chromosome numbers. In this case, both chromosomes from a pair go into one daughter cell while the other daughter cell receives none, creating aneuploid cells. This can happen during meiosis I (homologs failing to separate) or meiosis II (sister chromatids failing to separate). The other statements describe normal processes or different errors: failing to replicate is not about mis-segregation, normal disjunction means each daughter cell gets one chromosome from the pair, and chromatids separating during mitosis is the normal, not an error, separation.