IX. Genetics of CAH
21-hydroxylase is an enzyme made by a gene located on the short arm of chromosome 6. This gene is located in an area of the chromosome that contains many other important genes whose products control immune function.
Various mutations of the 21-hydroxylase gene result in various degrees of CAH (e.g., salt-losing form, simple-virilizing form, nonclassic form). The frequency of 21-hydroxylase mutations in the general population is about 1 in 60 people. In other words, among a group of 60 women, one is likely to have a 21-hydroxylase mutation on one of her two copies of chromosome 6. Similarly, among a group of 60 men, one is likely to have a mutation on one of his two copies of chromosome 6 as well. People with a mutation on one of their two copies of chromosomes are called "carriers" (they are heterozygous for the mutation) but have no problems with their adrenal function.
When two carriers have children, there is a 25% chance that each will contribute their mutant chromosome 6 to the child. There are four possible outcomes for each pregnancy resulting from two carrier parents:
| Paternal normal 6 + Maternal normal 6 = Normal Child Paternal mutant 6 + Maternal normal 6 = Carrier Child Paternal normal 6 + Maternal mutant 6 = Carrier Child Paternal mutant 6 + Maternal mutant 6 = CAH Child |
Only the child who receives two copies of mutant chromosome 6 (one from the mother and one from the father) will be affected by CAH (these individuals are homozygous for the mutation).
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| Figure 5: Click to enlarge |
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