Autosomal dominant inheritance: 50% risk and pedigree pattern
In autosomal dominant inheritance, one altered copy of a gene on a non-sex chromosome causes disease, and each child of an affected parent has a 50% chance of inheriting it.
Remember
Each child of a heterozygous parent (one altered and one normal copy) with an autosomal dominant condition has a 50% chance of inheriting it, whatever the sex, and the chance applies afresh to every pregnancy.
In an autosomal dominant condition the gene sits on an autosome, which is one of chromosomes 1 to 22 and not a sex chromosome. Dominant means that the altered allele (a version of a gene) shows its effect when the other copy is normal, so one altered copy is enough for a person to be affected.
An affected person is usually heterozygous: they carry one altered and one normal copy. Half of the eggs or sperm they make carry the altered copy, so the chance that any one child inherits the altered gene is 50%, whatever the child's sex. The chance applies afresh to every pregnancy, so an unaffected earlier child does not make the next child safe. If the other parent is unaffected and so passes on a normal copy, each child has a 50% chance.
A pedigree (a family tree that tracks a condition) shows the pattern. In a pedigree, the condition usually affects successive generations, and males and females are affected equally. A father can pass the condition to his son, since the gene lies on an autosome and not on a sex chromosome.
Two features can make the pattern less clear. Reduced penetrance means that some people with the altered gene do not show the condition, so a generation can appear to be skipped. A new mutation in an egg or sperm can also leave a child affected when neither parent is.
Huntington's disease is the standard example. It brings progressive problems with movement, thinking and mood. The disease usually begins when a person is between 30 and 50 years old, so people often have children before they know they are affected.
Key facts
- Risk to each child
- an affected heterozygous parent passes on the altered gene in half of pregnancies, so the risk is 50%, or 1 in 2, for each child.
- Sex
- sons and daughters are at the same risk, since the gene lies on an autosome rather than a sex chromosome.
- Pedigree
- the condition usually affects successive generations, with males and females affected equally.
- Father to son
- possible, because the gene is not on the X or Y chromosome.
- Exceptions
- two unaffected parents rarely have an affected child in a dominant condition, for example after a new mutation or reduced penetrance.
- Two altered copies
- homozygous people (two altered copies) are rare in dominant disease.
- Example
- Huntington's disease, where symptoms usually start between 30 and 50 years of age.
Exam traps
- A dominant condition does not commonly skip generations, because it usually appears in each generation of a family. Commonly skipping generations is the recessive pattern, and an apparent skip is occasional and can happen when the gene has reduced penetrance.
- Males being affected more often than females is not a feature of an autosomal dominant condition, because the gene lies on an autosome and so both sexes are affected equally.
- A 25% risk is the affected child risk for two carriers (people with one altered copy and no symptoms) of a recessive condition, whereas a heterozygous affected parent with a dominant condition has a 50% chance for each child.
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