Over 10 000 human diseases are caused by defects in single genes. These single gene disorders, which are also described as unifactorial or monogenic diseases, are individually very rare but they affect about 1 per cent of the population as a whole.
Since only a single gene is involved in each case, these diseases generally have simple inheritance patterns in family pedigrees. This means they can be traced through families and their occurrence in later generations can be predicted (see Genetic counselling ). The defective version of the gene responsible for the disease is known as a mutant allele or a disease allele.
Single gene disorders can be divided into a number of different categories according to how they are transmitted from generation to generation.
Some are described as dominant diseases because only one mutant allele is required, and such diseases tend to crop up in every generation.
Other diseases are described as recessive because both copies of the gene must be defective in order for the disease to occur. These recessive diseases often skip generations because mutant alleles can be carried without any effect if a normal allele is also present.
Many single gene disorders affect both sexes equally. However, where the relevant gene is present on the X-chromosome, the associated disease tends to be more common in males (see X-linked diseases ).