Because a female has two X chromosomes, she has twice as many X-chromosome genes as does a male. This would seem to result in an overdose of some genes. The inactivation of the X chromosome explains certain observations. In contrast, an additional autosomal chromosome can be fatal during early fetal development. Gene Abnormalities Abnormalities of one or more genes, particularly recessive genes, are fairly common.
Every human being carries six to eight abnormal recessive genes.
However, these genes don't cause cells to function abnormally unless two similar recessive genes are present. In the general population, the chance of a person's having two similar recessive genes is very small, but in children of close relatives, the chances are higher. Chances are also high among groups that intermarry, such as the Amish or Mennonites. A person's genetic makeup is called a genotype. However, abnormal characteristics expressed by an abnormal gene may cause a hereditary disease.