Sex-linked diseases are inherited through one of the "sex chromosomes" -- the X or Y chromosomes. Autosomally inherited diseases are inherited through the non-sex chromosomes (autosomes), pairs 1 through 22.

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 matching genes must be abnormal to produce disease. If only one gene in the pair is abnormal, the disease is not manifest or is only mildly manifest. Someone who has one abnormal gene (but no symptoms) is called a carrier. A carrier can pass this abnormal gene onto his or her children.

In general, the term "sex-linked recessive" usually refers to the more specific case of X-linked recessive.

Related terms and topics:

• Gene
• Chromosome
• Inheritance
• Heredity and disease
• Genetic counseling and prenatal diagnosis
• Sex-linked dominant
• Autosomal dominant
• Autosomal recessive

X-linked diseases usually occur in males. Males have only one X chromosome, so a single recessive gene on that X chromosome will cause the disease. Although the Y chromosome is the other half of the XY gene pair in the male, the Y chromosome doesn't contain most of the genes of the X chromosome and therefore doesn't protect the male. This is seen in diseases such as hemophilia and Duchenne muscular dystrophy.

TYPICAL SCENARIOS

For a given birth, if the mother is a carrier (only one abnormal X) and the father is normal:

• 25% chance of a normal boy
• 25% chance of a boy with disease
• 25% chance of a normal girl
• 25% chance of a carrier girl without disease

• 100% chance of a normal boy
• 100% chance of a carrier girl without disease

X-LINKED RECESSIVE DISORDERS IN FEMALES

Females can get an x-linked recessive disorder, although it would be very rare. An abnormal gene on the X chromosome from each parent would be required, since a female has 2 X chromosomes. This could occur in the two scenarios below.

For a given birth, if the mother is a carrier and the father has the disease:

• 25% chance of a healthy boy
• 25% chance of a boy with the disease
• 25% chance of a carrier female
• 25% chance of a girl with the disease

If the mother has the disease and the father has the disease:

• 100% chance of the child having the disease, whether boy or girl.

The odds of either of these two scenarios are so low that x-linked recessive diseases are sometimes referred to as “male only” diseases, although this is not technically correct.