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 able to cause disease even though the matching gene from the other parent is normal. The abnormal gene dominates.
Recessive inheritance occurs when both matching genes must be abnormal to produce disease. If only one gene in the pair is abnormal, the disease does not show up or is mild. Someone who has one abnormal gene (but no symptoms) is called a carrier. A carrier can pass this abnormal gene to his or her children.
The term "sex-linked recessive" usually refers to X-linked recessive.
X-linked diseases usually occur in males. Males have only one X chromosome. A single recessive gene on that X chromosome will cause the disease.
The Y chromosome is the other half of the XY gene pair in the male. However, the Y chromosome doesn't contain most of the genes of the X chromosome. It therefore doesn't protect the male. This is seen in diseases such as hemophilia and Duchenne muscular dystrophy .
For a given birth, if the mother is a carrier (only one abnormal X chromosome) 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
If the father has the disease and the mother is normal:
- 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, but this is very rare. An abnormal gene on the X chromosome from each parent would be required, since a female has two 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 girl
- 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. However, this is not technically correct.
Female carriers can have a normal X chromosome that is abnormally inactivated. This is called "skewed X-inactivation." These females may have symptoms similar to those of males.