What genetic occurrence can lead to chromosome disorders?

Nondisjunction is the correct choice because it refers to an error in cell division during meiosis or mitosis in which chromosomes fail to separate properly. This failure can result in gametes that have an abnormal number of chromosomes—either too many or too few. When such gametes fuse during fertilization, they can lead to offspring with chromosome disorders, such as Down syndrome, which is caused by an extra copy of chromosome 21.

Crossing over, while a crucial process in genetic diversity, typically does not lead to chromosome disorders, as it involves the exchange of genetic material between homologous chromosomes without changing the overall number of chromosomes. Mutation refers to changes in the DNA sequence that can affect genes but do not inherently lead to chromosome disorders unless they coincide with chromosomal alterations. Deletion can result in loss of genetic material from a chromosome, which is also significant but is a different mechanism from nondisjunction, which specifically deals with the failure of chromosome separation.