A single, abnormal gene on one of the autosomal chromosomes (one of the first 22 "non-sex" chromosomes) from either parent can cause certain diseases. One of the parents will usually have the disease (since it is dominant) in this mode of inheritance. Only one parent must have an abnormal gene in order for the child to inherit the disease.

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 trait 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 pairs of 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 does not occur or is only mildly present. However, a person with a single defective gene is called a carrier, meaning the disease can be passed on to children.

CHANCES OF INHERITING A TRAIT

For an autosomal dominant disorder: If one parent has an abnormal gene and the other parent a normal gene, there is a 50% chance each child will inherit the abnormal gene, and therefore the dominant trait.

In other words, if it is assumed that 4 children are produced from a couple in which one parent has an abnormal gene for a dominant disease, the STATISTICAL expectation is for:

• 2 children normal
• 2 children with the disease

This does not mean that children WILL necessarily be affected. It does mean that EACH child has a 50:50 chance of inheriting the disorder. Children who do not inherit the abnormal gene will not develop or pass on the disease.

See also:

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

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