An adjective to describe the genetic state when the pair of genes under consideration consists of two alleles that are different. Disease or disability can follow if one of the alleles is at once seriously abnormal and dominant (see dominant inheritance) over the other allele, or if the alleles are different but both are deleterious (a state called compound heterozygosity).

Because genes come in pairs, one on each of the chromosomes that make up a chromosome pair, the two genes of the pair are not always identical. The manydifferent forms that a particular gene can take (and still function, for better or for worse, as a gene within that gene's job description) are referred to as its alleles. If the alleles are identical, you are homozygous for that gene; if they are not, you are heterozygous. Abnormal alleles cause genetic disease or disability: if one allele is enough to cause abnormality then the gene is dominant (inherited with dominant inheritance) and the abnormality is present in the heterozygous and the homozygous state; if two alleles are needed to cause abnormality, then the gene is recessive (inherited with recessive inheritance) and the abnormality is present in the homozygous state and in a state where the two alleles are different but both are harmful (called compound heterozygosity. In the case of alleles found on X-chromosomes (see sex chromosome) but not on Y-chromosomes, which are smaller, a recessive gene will be unopposed in males (and so will act as a dominant gene), whereas female carriers of the allele will be unaffected except in the extremely unlikely event that they inherit (or gain by mutation) a second abnormal allele; this mode of inheritance is called sex-linked recessive inheritance.

A heterozygous state in which the inherited alleles of the particular gene are different and both harmful, with clinical effects similar to a homozygous state.

A serious genetic disease (with a recessive inheritance pattern) characterised by a major disturbance of the body's mucus secretions, and thus a cause of incapacitating disease of the lungs. Important in our context because the heterozygous (or carrier) state in men can manifest with azoospermia due to congenital absence of the vasa deferentia -- the infertility of which is nowadays able to be overcome with testicular sperm extraction and ICSI, thus risking inadvertent transmission.

The commonest of the alleles to result in cystic fibrosis (when two delta F508 alleles are present) or (can occur when one F508 allele is present in a man). A woman heterozygous for this gene (with one F508 allele) will be a carrier for cystic fibrosis (and for CBAVD) but will herself be otherwise normal. There are however many other faulty alleles that can also contribute to these clinical conditions. The most common alleles can be looked for at preimplantation genetic diagnosis (PGD).