How are single-gene disorders inherited?

Most of our characteristics are encoded by a pair of alleles. These are different forms of the same gene, one inherited from the father and one from the mother. Inheriting a mutated allele can lead to disease in one of two ways. In 'dominant' diseases, such as Huntington's chorea, only one mutant allele need be inherited for the disease to develop. In 'recessive' diseases, such as cystic fibrosis, a person must inherit two mutant alleles, one from each parent, to be affected. Genetic counsellors commonly use Punnett squares to determine the probability that a given pregnancy will result in a child with a single-gene disorder. This involves drawing up all the possible combinations of the parents' alleles, and calculating the percentage or ratio of children which would be affected. A subset of single-gene disorders are said to be X-linked, as the alleles responsible are encoded on the X-chromosome. An interesting consequence of this is that although an X-linked disease may be classed as recessive, a boy can inherit only one mutant allele and still develop disease, as he has no second X-chromosome containing a healthy allele to offset the effects of the mutant one.