How Genetic Disorders Are Inherited

Dominant and recessive patterns

Genetic disorders are caused by the mutation of a gene. This genetic mutation can be passed from parents to a child. Whether the child will develop the genetic disorder depends on the pattern of inheritance for the disorder.

Patterns of inheritance

There are two rules that are important for understanding the inheritance of genetic disorders. For any disorder, you need to know:

• whether the disorder occurs with one copy of the mutated gene or two, and
• whether the mutation is on one of the sex chromsomes (X or Y), or on one of the autosomes (the other 22 pairs of chromosomes).

These rules combine to make patterns of autosomal dominant, autosomal recessive, and X-linked recessive inheritance.

Autosomal dominant disorders

In autosomal dominant disorders, the mutated gene occurs on an autosome and the person only needs one copy of it to have the disorder. Males and females are equally likely to inherit the genetic mutation and be affected by it. Children of a person with an autosomal dominant disorder have a 50% chance of inheriting the disorder.

Autosomal recessive disorders

In autosomal recessive disorders, the mutated gene occurs on an autosome but the person needs two copies of the gene, one from each parent, in order to have the disorder. A person with only only copy of the muated gene is called a carrier. Carriers do not have the disorder and are not affected by the mutated gene in any way. However, the carrier can pass on the genetic mutation to his or her children.

If the parents of a child both carry the genetic mutation for an autosomal recessive disorder, the child has a 25% chance of inheriting both mutations and having the disorder, a 50% chance of inheriting only one copy of the mutated gene (thus becoming a carrier), and a 25% chance of not inheriting the mutation at all. Each child the couple has will have the same chances of inheriting the mutated gene.

X-linked recessive disorders

In X-linked recessive disorders, the mutated gene occurs on the X (female) chromosome. Males have one X chromosome and one Y chromosome, so a mutated gene on the X chromosome is enough to cause a disorder. Females have two X chromosomes, so a mutated gene on one X chromosome usually has less effect on a female because of the non-mutated copy of the same gene on the other X chromosome.

A female with a gene mutation on one X chromosome would be a carrier. On average, 50% of her sons will inherit the mutation and develop the disorder, and 50% of her daughters will inherit the mutation and become a carrier.

_Sources:

"Understanding Modes of Inheritance." Genetics Tools. 14 Feb 2005. GeneTests. 29 Jan 2008

"How Recessive Conditions Are Inherited." Modes of Inheritance. National Tay-Sachs & Allied Diseases Association. 29 Jan 2008