Autosomal Recessive

Autosomal recessive is a mode of inheritance of genetic traits located on the autosomal chromosomes. In order to be affected by an autosomal recessive disorder, an individual must inherit two copies of the mutant gene, one from each parent. If both parents are carriers of a mutant gene, they have a 25% chance with each child to pass on the autosomal recessive condition.

Overview

Autosomal refers to the non-sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Recessive means that two copies of the gene are required to have the trait or disorder. One inherited gene copy is from the mother (maternal allele) and one from the father (paternal allele).

Genetic Mechanism

In autosomal recessive inheritance, both parents must carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. When two carriers of the same autosomal recessive gene have children, the possibilities for their offspring are:

• A 25% chance of inheriting two mutant genes (affected),
• A 50% chance of inheriting one mutant gene and one normal gene (carrier),
• A 25% chance of inheriting two normal genes (unaffected).

The carriers can pass the mutant gene to their children, but they are usually not affected by the condition. This is because autosomal recessive diseases require both copies of the gene to be mutated for the phenotype to be expressed.

Examples of Autosomal Recessive Disorders

Some examples of autosomal recessive disorders include:

• Cystic fibrosis: A condition affecting the respiratory and digestive systems.
• Sickle cell anemia: A disorder that leads to misshapen red blood cells.
• Tay-Sachs disease: A rare disorder that destroys nerve cells in the brain and spinal cord.
• Phenylketonuria (PKU): A condition in which the body cannot break down the amino acid phenylalanine, leading to harmful levels in the body.

Diagnosis and Carrier Testing

Diagnosis of an autosomal recessive disorder often involves genetic testing to identify mutations in both copies of a gene. Carrier testing can be performed to identify individuals who carry one copy of a mutated gene associated with an autosomal recessive disorder. This is particularly important for people with a family history of an autosomal recessive condition or those in populations with a high prevalence of specific genetic disorders.

Implications for Genetic Counseling

Genetic counseling is recommended for individuals or couples who are carriers of autosomal recessive disorders, especially if they are considering having children. Genetic counselors can provide information about the risk of passing the condition to offspring and discuss the options available, including prenatal testing.