Final answer:
Nondisjunction is the improper segregation of chromosomes during meiosis, leading to gametes with abnormal numbers of chromosomes and potentially resulting in genetic disorders.
Step-by-step explanation:
The failure of chromosomes to segregate properly during mitosis or meiosis is called nondisjunction. This occurs when homologous chromosomes or sister chromatids fail to separate during cell division. In the specific context of meiosis, nondisjunction can happen during the first division, where synapsed homologs do not separate, leading to one of the resulting cells having both homologs and the other lacking them. This is referred to as meiosis I nondisjunction. Alternatively, nondisjunction can occur during meiosis II when replicated chromosomes (sister chromatids) fail to separate. This results in some gametes with an extra chromosome or a missing chromosome, known as aneuploidy, and is referred to as meiosis II nondisjunction.
Such chromosomal aberrations can lead to various genetic disorders, depending on which chromosomes are involved and in what number. For instance, an individual may end up with a trisomy, such as in the case of Down syndrome, where there are three copies of chromosome 21 instead of the usual two.