Most diseases result from a combination of genetic susceptibility and environmental exposure. The occurrence of “genetic” diseases depends more on the exact coding of DNA with little to no influence from the environment. In genetic diseases, a change in the DNA code of a gene alters the function of the protein it encodes, resulting in disease.
Definition
Genes lie on units of tightly wound DNA called chromosomes. Each person inherits one pair of sex chromosomes which determine if the person is male or female and 22 pairs of autosomal chromosomes. One chromosome from each pair comes from the mother and the other from the father. Patients with an autosomal recessive disease inherit a faulty copy, or allele, of an autosomal gene from both the mother and the father, whereas dominant traits or diseases require inheritance of only one faulty allele.
Inheritance
In general, each allele in a gene pair has a 50 percent chance of ending up in an egg or sperm for transmission to the next generation. The parents of a child with an autosomal recessive disease must either have the disease themselves, or be free of the disease but have one faulty allele. People who have one faulty allele of a recessive gene are called carriers of the disease.
Risk
An affected person will always transmit a faulty allele to their children, and 50 percent of the time a carrier will transmit the faulty allele to their children.
If both parents have an autosomal recessive disease, they will both transmit the faulty allele to their children, all of whom will have the disease. If both parents are carriers, each of their children has a 25 percent chance of having the disease, a 50 percent chance of becoming a carrier and a 25 percent chance of having two normal alleles. If one parent has the disease and the other is a carrier, each of their children has a 50 percent chance of having the disease and a 50 percent chance of being a carrier.
Most recessive disorders are rare and few people in a population carry a faulty allele for the disorder. Children whose parents are closely related have an increased risk for an autosomal recessive disease, because the parents are more likely to have inherited a faulty allele from a common ancestor.
Mechanism
A faulty allele can reduce or stop the production of the protein it encodes, or produce a protein that functions abnormally. For carriers of an autosomal recessive disease, the normal allele can compensate for the faulty allele and produce enough normal protein to prevent the disease.
Examples
Examples of autosomal recessive disease include cystic fibrosis; sickle cell anemia; and phenylketonuria, or PKU. Because these diseases have serious consequences, potential carriers might opt for genetic testing to assess the risk of having an affected child or passing along the faulty allele.
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