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.
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 chromasomes.
- 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.
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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