On fertilisation the chromosomes combine to give a total of 46. Normally a female has an XX pair and a male an XY pair. A chromosomal abnormality occurs when there is a defect in a chromosome or in the arrangement of the genetic material on the chromosome. Chromosomal abnormalities give rise to specific physical features, but it should be stressed that there may be wide variations in the severity of the symptoms in individuals with the same chromosome abnormality. Increases and decreases in chromosomal material interfere with normal body function and development.
Chromosomal abnormalities give rise to specific physical features.
There are two main types of chromosomal abnormality which may occur during meiosis and fertilisation. These are known as numerical aberrations and structural aberrations. A ring formation of the X chromosome Structural aberrations Where these occur there is a rearrangement in the location of, or a loss of, genetic material. When cultured under specific conditions fragile sites may be located on the X chromosome. This gives rise to Fragile X Syndrome where boys are worse affected but one third of affected girls also have some degree of learning difficulty.