Inheritance of Autosomal Recessive Genetic Diseases

These genetic diseases are diseases caused by an error in a single DNA gene. Autosomal means the errors occurs on chromosome 1..22 rather than on the 23rd sex-linked X chromosome. Recessive means that disease only occurs when a person has two copies of the bad gene. Usually this means they must inherit the disease from both parents.

Some examples of autosomal recessive diseases are Cystic Fibrosis, Phenylketonuria, Sickle Cell Anemia, Tay Sachs, Albinism, and galactosemia. See autosomal recessive diseases for a full list.

Recessive diseases often occur in genes that produce an enzyme. In a carrier, who has only one bad copy, there is often no disease, because the second gene can pull up the slack, and maintain health. In some recessive diseases, a carrier gets a mild form of the disease.

Autosomal recessive diseases are relatively rare, because to get the disease a person must inherit a bad gene from each parent, not just one. So both parents must have a bad gene. However, parents can be carriers without the disease, since they typically only have one bad gene themselves.

Inheritance patterns: There are various clear patterns common to any autosomal recessive genetic disease:

• Children of an affected person will typically not have the disease (except in the rare case that they too marry someone who is also affected or a carrier of exactly the same disease), but the odds are 100% the child will be a carrier. The affected parent has two bad copies of the gene, so the child gets a bad gene from that parent, but usually a good second copy from the other unaffected parent.
• If only one parent is a carrier (and the other unaffected), the child cannot get the disease, but might still be a carrier (typically 50% chance of being a carrier).
• If both parents are a carrier, there is a 25% chance that their child will have the disease. There is also a 50% chance the child will be a carrier, and only 25% chance the child will be neither diseased nor carrier. The situation where both parents are carriers is the most likely way that children with the disease are born.
• If one parent has the disease, and the other is a carrier, a child has a 50% chance of getting the disease, and 50% chance of being a carrier. The child definitely gets one bad gene from the diseased parent, and has a 50% chance of getting a second one from the carrier parent.
• Other children: If parents have one affected child, the odds of a second are usually 25%. If parents have a child with the disease, this almost always means that they are both carriers. The chances a second child will also have the disease are the same as above for two parent carriers: 25% chance of disease, 50% chance the second child is a carrier, and 25% chance of neither disease nor carrier. Note that genetic testing can often detect the rarer case where a child gets a genetic disease without both parents being carriers (perhaps only one is a carrier).
• Gender bias: Male or females get the disease equally, because an autosomal error is unrelated to the sex chromosomes.
• Inheritance patterns tend to be "horizontal", which a generation being affected (i.e. many siblings of the same parents), but not their parents nor their own children. Parents and next-generation children will usually be carriers.

Sporadic cases: A genetic disease that occurs when neither parent has any genetic defect is called a sporadic genetic disease. These cases arise via random genetic mutations in the DNA. A sporadic genetic mutation is not likely as a cause of an autosomal recessive disease, because it would require two identical random mutations (one in each copy of the gene) at the same time.

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