An abnormal gene on the X chromosome may cause no noticeable defect in a girl because she also has a normal matching gene on her other X chromosome. As noted, they affect boys almost exclusively. Incidence Dominant genetic diseases are collectively the most common, occurring in about 7 in 1,000 live births. Recessive diseases are next, affecting about 2.5 in 1,000. Least common are X-linked disorders, occurring in about 4 in 10,000 live births.
Several factors affect the incidence of particular genetic diseases.
Some diseases are concentrated in specific ethnic groups. Gaucher's and Tay-Sachs diseases, for example, are part of a group of genetic diseases that occur most often in Ashkenazi Jews from Eastern Europe. This raises the risk of a recessive disorder in their children. While some genetic diseases are caused by single dominant genes, by single pairs of recessive genes, or by single genes on the X chromosome, others result from interactions between more than one pair of genes. And many apparently result from the interaction of genes with outside, environmental factors.