Autosomal Dominant Inheritance

Autosomal is merely the name given to the non-sex chromosomes in a cell - and there are 44 autosomes in each of our cells. There are two copies of every gene in our cells and they send signals to create a protein. If one of those is faulty and the protein is not made properly or at all, then the mutation is said to be dominant.

Mendel also introduced the upper and lower case way of writing these traits. A dominant trait is written in upper case and recessive in lower case.

Dominant traits can manifest both in heterozygotes (Aa) and by homozygotes (AA). But almost all human beings who manifest autosomal dominant traits turn out to be heterozygote. The mutant gene is said to dominate the pair of genes.

About 450 human traits are known to have their genetic basis in dominant genes located on autosomes. Homozygotes for dominant traits (where both genes for a particular trait are mutant) are at such a disadvantage that most of them do not survive before birth and die as embryos.

A person with one faulty gene and one correct gene for a trait is said to be a carrier and will either have the condition from birth, or it could appear in later life or it may only appear if certain environmental factors trigger it; this is what is known as having a genetic predisposition for a condition.

If a parent is a carrier, there is a 50% chance that the offspring will inherit the gene. That doesn't necessarily mean that if the parent has four children, two of them will be affected. They could all be affected or none could be affected; each child has a fifty-fifty chance of inheriting the trait.

Another feature of autosomal dominant traits is that they sometimes appear unexpectedly among the offspring of unaffected parents. In general, the sudden unexpected appearance of an autosomal dominant trait in a lineage from which it was not previously known is the result of a mutation.

A good example of autosomal dominant inheritance is provided by the rather benign trait known as “woolly hair”, whose distribution has been well documented in Norwegian families. Affected persons have hair that is tightly kinked and very brittle, so that it breaks off before growing very long. People manifesting this dominant trait are heterozygous, and their genotype can be symbolized Ww (W for woolly). If an affected person (Ww) and an unaffected person (ww) produce offspring, we would expect about half of them to have normal hair and half to have woolly hair.

Autosomal dominant traits vary in their severity and affect boys and girls equally. Examples of medical conditions brought on by dominant traits are Huntington's disease, achondroplasia and some predispositions to cancers. Genetic testing is usually only considered if there is a family history of a condition.