Nondisjunction

Homologous chromosomes are virtually identical chromosomes that occur in pairs, one member inherited from each parent. Humans have Nondisjunction occurs when homologous chromosomes fail to separate, creating some cells with too many, and others with too few, chromosomes. forty-six chromosomes, or twenty-three homologous pairs. In normal meiosis, homologous chromosomes pair up and, by attachment to the spindle fibres, become aligned at the cell equator. Prior to the first meiotic division, the members of each homologous pair migrate to opposite poles of the cell by means of the pulling action of the spindle fibers. This ensures that, upon completion of meiosis, each gamete will contain one copy of every chromosome.

However, the segregation process is not error-free, and every so often it happens that two homologous chromosomes fail to separate (disjoin) and thus both migrate to the same pole. This gives rise to two types of gamete. One type possesses two copies of the chromosome, whereas the other type lacks that chromosome altogether. This condition, involving the loss or gain of a single chromosome, is referred to as aneuploidy. Fusion of an aneuploid gamete with a normal gamete gives rise to a zygote with an odd number of chromosomes.

A zygote which has one less than the normal diploid number of chromosomes (2n−1) is said to be monosomic, and such zygotes do not usually develop to term. Zygotes containing an extra chromosome (2n+1) are trisomic for the chromosome of interest, and these may develop, though usually with severe abnormalities.

Various complements of sex chromosomes that yield different phenotypes: two XX, normal female; one X, a Turner syndrome female; one X and one Y, normal male; two X and one Y, a Klinefelter male.