Each cell in the human body includes a complete set of 23 chromosomes: one inherited from our mother and the other from our father. One pair of chromosomes determines if we are male or female. The other 22 sets are unrelated to our sex determining chromosomes, and they are referred to as autosomal chromosomes.
Each chromosome is composed of genes. Some genes are dominant, meaning a child only has to inherit the gene from one parent to express that characteristic. Other genes are recessive, meaning that the child would need to inherit the gene from both parents to express that characteristic. Gene mutations can occur in a variety of ways. If a mutation occurs in a recessive autosomal gene, it can cause an autosomal recessive genetic disorder. Some common autosomal recessive disorders are:
- Cystic fibrosis
- Sickle Cell Anemia
- Tay Sachs Disease
Autosomal Recessive Inheritance
Autosomal recessive genetic disorders occur when there are mutations in a pair of genes. This means that genes from both the mother and the father must have the mutation in order for the child to inherit a copy of each mutated gene. When this occurs, the child then has the possibility of having an autosomal recessive disorder.
When a parent is a carrier of a recessive gene, but does not have an autosomal genetic disorder, they have one good copy of the gene and one mutated copy. Many times, parents are healthy individuals who may not even be aware that they are carriers of a mutated gene. If two people with the same mutated gene reproduce, they have a 25% chance per pregnancy that their offspring will have an autosomal recessive genetic disorder. There is also a 50% chance that the child will inherit only one copy of the gene with the mutation, and therefore, they would be healthy carriers just like both their mother and father.
Scientists and physicians believe that all people contain somewhere around 20 mutated genes each that could result in a recessive gene disorder.
Testing for Autosomal Recessive Inheritance
It is possible to test if a person is a carrier of some mutated recessive genes. Some couples opt for testing prior to pregnancy if they have a history of genetic disorders in the family or simply if they are concerned about the likelihood of their offspring inheriting a genetic disorder.
Often an autosomal recessive genetic disorder will not be diagnosed until after a child is born due to the fact that the parents are unaware that they are carriers of a mutated gene.
Autosomal Recessive Disorders and Ethnicity
There are certain types of autosomal recessive disorders that disproportionately affect specific populations. Sickle cell anemia disproportionately affects African Americans. It is estimated that one in twelve African Americans carry the mutated gene for this disorder. This disorder affects the red blood cells and their ability to carry oxygen. Cystic fibrosis is more common among people of European descent, and it is estimated that one in 29 Caucasians are carriers of this mutated gene.