Abnormalities in chromosome number give rise to diseases of karyotype. How might these aberrations occur?

Under usual conditions of meiotic division, each tetrad separates into its constituent homologous chromosomes. One homologue migrates to one pole and the other homologue to the opposite pole during anaphase of the first meiotic division. If this separation does not occur, all the tetrads may move to one pole, while the opposite pole may receive no chromosomes at all. This would eventually produce diploid cells as gametes (in those organisms in which meiosis is involved in gamete formation). Should such a diploid gamete unite with a more typical haploid gamete, a zygote would be produced with three sets of chromosomes, a triploid individual. In
plants, the formation of triploid and even higher orders of polyploidy represents a mechanism for producing new species of the organism in the course of evolution. This alteration of ploidy is less common in animals.
More commonly, a single tetrad will fail to separate into its constituent chromosomes. This will eventually result in gametes that have a double dose of one chromosome and others that have no representative for that particular chromosome. In Down’s syndrome, a sperm or egg with two chromosomes 21 unites with a normal haploid sperm or egg to produce a zygote with three such chromosomes. The failure of tetrads to disjoin is called nondisjunction, and disorders arising from the phenomenon are known as diseases of nondisjunction. They include Down’s, Klinefelter’s (XXY male genotype), and Turner’s (XO, i.e., having only one X chromosome,female genotype) syndromes. Fragmentaton, deletion, and internal inversion of chromosomes or chromosome parts may also produce diseases of karyotype.