Genetic Disorders

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Genetic Disorders

Genetic disorders are conditions that have some origin in a person's genetic makeup. Genetic disorders are more severe than simple abnormalities and usually result in some type of medical problem. A genetic disorder is not the same thing as a disease.

Genetic disorders are generally one of two types: those that are inherited (and are governed by the same rules that determine all our traits), and those that are the result of some type of mutation or change that took place while the embryo was developing. Sometimes this mutation is caused by environmental factors. Within this first set of inherited disorders, a disorder that is transmitted by genes inherited from only one parent is called an "autosomal dominant disorder." The term "autosomal" refers to any of the twenty-two sets of chromosomes (forty-four individual chromosomes) common to both males and females that determine all of the traits except a person's gender. (The single pair of chromosomes that determine sex is called the sex chromosome and is different between men and women.)

In an autosomal dominant disorder (ADD), an individual inherits a dominant gene from one parent that causes the disorder. Since it is dominant, the gene has a fifty percent chance of being expressed. Sometimes an individual will inherit a gene for a genetic disorder that is not a dominant gene. In that case, it is a recessive gene that is "covered" by the normal gene (since genes are always inherited in pairs), and the person will not have the disorder. There are approximately 2,000 ADDs, ranging from the inconvenient (like an extra toe) to eventual death (as in Huntington's disorder). Although certain disorders reduce a person's chance of surviving (and therefore of passing the gene on to offspring), many ADDs do not affect reproduction or are diagnosed fairly late in life.

Autosomal recessive disorders, also called recessive genetic disorders (RGD), are the result of both parents supplying a recessive gene to their offspring. In this case, the offspring has no chance to avoid the disorder since it does not have any normal gene to possibly "cover" for the recessive one. Instead, the individual has two recessive genes for the same trait. There are about 1,000 RGDs, and the odds for getting two of the same recessive genes are as high as 25 percent. Some of the better-known recessive genetic disorders are sickle-cell anemia, cystic fibrosis, and Tay-Sachs disease. Galactosemia is an example of what is called a metabolic RGD. In this case, a person with this disorder lacks a certain enzyme needed to break down the sugar found in milk. Another disorder, adenosine deaminase deficiency, is, like galactosemia, one of the few treatable genetic diseases. This immune disease is sometimes cured by a bone marrow transplant.

Certain other traits are not received from the set of twenty-two pairs of autosomal chromosomes but are instead found on a single pair of sex

chromosomes. Since females have two X chromosomes and males have one X and one Y, males are more susceptible to X-related gene defects. Since females have two X chromosomes, any defect on one is no more likely to be expressed than a gene defect on any other chromosome (unless they inherited two X's with defects). However, since males have only a single X chromosome, any defective gene on that chromosome will be expressed. The bleeding condition Hemophilia A is a sex-linked or X-linked recessive disorder, as is color blindness.

Finally, there are genetic disorders that are caused not by gene mutations but by things going wrong right from the beginning of fertilization and cell splitting. Sometimes an abnormal number of chromosomes are formed (either too few or too many), resulting in such conditions as Down syndrome in which children are born with some degree of mental retardation and sometimes heart defects. Although genetic disorders are not common, it is estimated that of all newborns, as many as one percent will have some form of chromosome-related condition.