inherited disease

Disease resulting from the effect of having defective genes. These diseases are inherited because genetic material is accurately copied before being passed onto the offspring from a parent. However, they can also arise as a result of a sudden change in the DNA known as a mutation. Common examples of inherited diseases in humans are cystic fibrosis, sickle-cell anaemia, polydactyly, and Huntingdon's disease. Downs syndrome is an example of a disease arising from mutation, but which is not inherited.

Cystic fibrosis, sickle-cell anaemia, polydactyly, and Huntingdon's disease are all each the result of a single defective gene. Down's syndrome, however, is usually the result of having an extra chromosome containing thousands of genes. In polydactyly (extra fingers) and Huntingdon's disease (physical and mental degeneration) the diseases show dominance - this means that inheriting a single defective gene will result in the disease. Cystic fibrosis (viscous secretions, especially in the lungs) is recessive, which means that two defective genes have to be inherited for the disease to occur.

In sickle-cell anaemia people who inherit a single defective gene are normal most of the time, but can show anaemia from time to time. However, if a person inherits two defective genes, they show severe illness. This defective gene exhibits incomplete dominance. The gene for sickle-cell anaemia is more common than would be expected considering the anaemia it causes. It is more common than expected because inheritance of a single defective gene makes the person partly immune to malaria, so natural selection favours these individuals where malaria is common.