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An autosomal dominant gene is one that occurs on an autosomal (non-sex determining) chromosome. As it is dominant, the phenotype it gives will be expressed even if the gene is heterozygous. This contrasts with recessive genes, which need to be homozygous to be expressed.
The chances of an autosomal dominant disorder being inherited are 50% if one parent is heterozygous for the mutant gene and the other is homozygous for the normal, or 'wild-type', gene. This is because the offspring will always inherit a normal gene from the parent carrying the wild-type genes, and will have a 50% chance of inheriting the mutant gene from the other parent. If the mutant gene is inherited, the offspring will be heterozygous for the mutant gene, and will suffer from the disorder. If the parent with the disorder is homozygous for the gene, the offspring produced from mating with an unaffected parent will always have the disorder. See Mendelian inheritance.
Some diseases inherit in this fashion but still affect less than 50% of the offspring; in this case the disease is termed of having incomplete penetrance.
Examples of dominant genes include the tumor supressor genes BRCA1 and BRCA2. Mutations in these genes lead to the development of breast cancer, as the tumour-suppressing functions of the proteins encoded by the genes have been disabled.
Another example of an autosomal dominant disorder is Huntington's disease, which is a neurological disorder resulting in impaired motor function. The mutant gene results in an abnormal protein, containing large repeats of amino acid glutamine. This defective protein is toxic to neural tissue, resulting in the characteristic symptoms of the disease.
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