Inheritance Patterns

Autosomal recessive inheritance is due to recessive alleles carried on autosomes. An individual possessing only one recessive allele is known as a carrier. An individual must inherit two recessive alleles, one from each parent, in order to express the recessive trait. When two carrier parents have off-spring, each offspring has a 25 percent chance of inheriting two alleles and expressing the trait. The two recessive alleles need not be precisely identical, as long as each is nonfunctional. An individual possessing two different alleles with the same effect is known as a compound heterozygote. Compound heterozygotes account for some cases of the neurologic disorder known as Friedreich's ataxia.

Medical conditions due to autosomal recessive traits also number in the many hundreds. These include cystic fibrosis (affecting ion transport in the lungs and pancreas), Tay-Sachs disease (affecting lipid metabolism and storage, especially in the brain), and hemochromatosis (affecting iron metabolism and storage in a variety of organs).

The number of people with such conditions is actually much higher than that for autosomal dominant conditions. This is because inheritance of one harmful recessive allele does not produce symptoms, and so the individual can reproduce and pass the allele on to children easily. Thus, most harmful recessive alleles are not deleted from a population's gene pool as rapidly as most dominant ones, and the likelihood of inheriting two copies is consequently higher. Most humans harbor a small handful of known harmful alleles; it is only when they mate with another who has the same set that there is a chance of bearing children that express the disorder. Customs that warn against marrying close relations have the effect of minimizing the likelihood of offspring with homozygous recessive conditions.