Chromosomal abnormalities (numerical or structural alterations) occur from time to time in human beings. They arise in various ways:
By an error in nuclear division called “non-disjunction” involves a pair of chromosomes which may fail to separate and are carried to one pole. The resulting daughter cells contain an unequal number of chromosomes, 45 or 47. This numerical abnormality in which the chromosome number is not an exact multiple of the haploid number is called aneuploidy.
If a particular pair of chromosomes has three chromosomes instead of two, it is called “trisomy”; if there is only one chromosome instead of two in any given pair of chromosomes, it is called “monosomy”. Non-disjunction may occur during gametogenesis or during mitosis. It is more common in sex chromosomes.
Sometimes during nuclear division, a portion of one chromosome breaks away and becomes attached to another which is not homologous to the first. This is called translocation.
A piece of a chromosome may become detached and lost from the karyotype resulting in the loss of one or more genes. If the loss is severe, it may be incompatible with live birth.
Some genes may appear twice in the same chromosome. This is called duplication.
Sometimes a chromosomal segment becomes inverted and then the order of sequence of genes is altered.
These are special class of structurally abnormal chromosomes arising because of misdivision, i.e., transverse division instead of the normal longitudinal division.
The cells of the body are compounded of cells of two or more genetically different chromosomal types. This can result by mutation or non-disjunction either during embryo or later life.