An abnormal gene on one of the autosomal chromosomes (one of the first 22 "non-sex" chromosomes) from each parent is required to cause the disease. People with only one abnormal gene in the gene pair are called carriers, but since the gene is recessive they do not exhibit the disease.

In other words, the normal gene of the pair can supply the function of the gene so that the abnormal gene is described as acting in a recessive manner. BOTH parents must be carriers in order for a child to have symptoms of the disease; a child who inherits the gene from one parent will be a carrier.

The inheritance of genetic diseases, abnormalities, or traits is described by both the type of chromosome on which the abnormal gene resides (autosomal or sex chromosome), and by whether the gene itself is dominant or recessive. This is due to whether a single defective gene from one parent (dominant inheritance) or both copies of the gene (one from each parent) are defective (recessive inheritance).

Autosomal diseases are inherited through the non-sex chromosomes, (pairs 1 through 22). Sex-linked diseases are inherited through one of the sex chromosomes, the X chromosome (diseases are not inherited through the Y chromosome).

Dominant inheritance occurs when an abnormal gene from ONE parent is capable of causing disease even though the matching gene from the other parent is normal. The abnormal gene dominates the outcome of the gene pair.

Recessive inheritance occurs when BOTH genes of a pair must be abnormal to produce disease. If only one gene in the pair is abnormal, the disease is not manifested or is only mildly manifested. However the abnormal gene can be passed on to the children.

CHANCES OF INHERITING A TRAIT

For an autosomal recessive disorder: When both parents are carriers of an autosomal recessive trait, there is a 25% chance of a child inheriting abnormal genes from both parents, and therefore of developing the disease. There is a 50% chance of each child inheriting one abnormal gene (being a carrier).

In other words, if it is assumed that 4 children are produced, and both parents are carriers (neither exhibits any disease), the STATISTICAL expectation is for:

• 1 child with 2 normal chromosomes (normal)
• 2 children with 1 normal and 1 abnormal chromosome (carriers, without disease)
• 1 child with 2 abnormal chromosomes (has the disease)

This does not mean that children WILL necessarily be affected. It does mean that EACH child has a one in four chance of inheriting the disorder and a 50:50 chance of being a carrier.

RELATED TOPICS

Autosomal dominant
Genetic counseling and prenatal diagnosis
Sex-linked dominant
Sex-linked recessive

For detailed information, see heredity and disease (genetics).