Inheritance of a specific disease, condition, or trait depends on the type of chromosome affected (autosomal or sex chromosome). It also depends on whether the trait is dominant or recessive. Sex-linked diseases are inherited through one of the sex chromosomes (the X or Y chromosome).
Dominant inheritance occurs when an abnormal gene from one parent is capable of causing disease, even though a matching gene from the other parent is normal. The abnormal gene dominates the gene pair.
For an X-linked dominant disorder: If the father carries the abnormal X gene, all of his daughters will inherit the disease and none of his sons will have the disease. That is because daughters always inherit their father's X chromosome. If the mother carries the abnormal X gene, half of all their children (daughters and sons) will inherit the disease tendency.
In other words, if there are four children (two males and two females) and the mother is affected (one abnormal X, she has the disease) but the father is not, the statistical expectation is for:
- Two children (one girl and one boy) with the disease
- Two children (one girl and one boy) without the disease
If there are four children (two males and two females) and the father is affected (abnormal X, he has the disease) but the mother is not, the statistical expectation is for:
- Two girls with the disease
- Two boys without the disease
This does not mean that the children will necessarily be affected.