What is non-disjunction?

Non-disjunction refers specifically to the failure of homologous chromosomes or sister chromatids to separate properly during cell division, particularly during meiosis or mitosis. This error can occur during the first or second phase of meiosis, leading to gametes with an abnormal number of chromosomes. When these gametes participate in fertilization, they can result in offspring with chromosomal disorders, such as Down syndrome, which is caused by the presence of an extra chromosome 21.

The other choices present concepts related to genetics but do not capture the essence of non-disjunction. The separation of chromosomes during meiosis is a normal process, not a mistake. Random assortment refers to the process by which alleles are distributed into gametes independently, which also does not involve errors in chromosome separation. Finally, excessive cell division typically relates to uncontrolled cell proliferation rather than chromosome separation errors. Thus, the definition of non-disjunction is specifically about errors in the separation process.