Chromosomal Aberrations

Chromosomal abnormalities can result from either a variation in the chromosome number or from structural changes. These events may occur spontaneously or can be induced by environmental agents such as chemicals, radiation, and ultraviolet light. However, mutations are most likely due to mistakes that occur when the genes are copied as the cells are dividing to produce new cells. These abnormalities may involve the autosomes, sex chromosomes, or both. The disruption of the DNA sequence or an excess or deficiency of the genes carried on the affected chromosomes results in a mutation. Such a change may or may not alter the protein coded by a gene. Often, however, a mutation results in the disruption of gene functionality. The resulting altered or missing protein can disrupt the way a gene is meant to function and can lead to clinical disease. Only mutations occurring to the DNA in the gametes will potentially pass on to the offspring.

Mutations appear in gametes in one of two ways. A mutation may be inherited from one of an individual's parents. However, a mutation may also occur for the first time in a single gamete, or during the process of fertilization between an egg cell and a sperm cell. In this case the mutation or change is often called a de novo mutation. The parents are not affected by the condition and are not "carriers" of the mutation. The affected individual will have this mutation in all of his or her cells and may be able to pass the mutation on to any offspring. Some common abnormalities and their resulting phenotypes are discussed below.