Any disorder caused, wholly or partly, by one or more faults in a person’s DNA. Genetic disorders may be congenital or may become apparent later in life. Many of them are familial. However, a child may be born with a genetic disorder when there is no previous family history. A genetic disorder can occur in 2 ways: one or both parents have a defect in their own genetic material which is then inherited, or a mutation occurs during the formation of the egg or sperm cell. Genetic disorders fall into three broad categories: chromosomal abnormalities, unifactorial defects, and multifactorial defects. In the first, a child is born with an abnormal number of whole chromosomes (as in Down’s syndrome), or extra or missing bits of chromosomes. Unifactorial disorders are rare, and are caused by a single defective gene or pair of genes. They may be sex-linked (with the defective gene carried on one of the sex chromosomes) or autosomal (with the defective gene carried on one of the other 44 chromosomes). X-linked recessive disorders are the most common type of sex-linked disorder. In these conditions, the defective gene is on the X chromosome. Women have 2 X chromosomes in their cells; men have only 1, inherited from their mothers. When a woman inherits 1 defective gene, its effect is masked by the normal gene on her other X chromosome and she has no outward abnormality. She is still capable of passing the gene onto her children, and is called a carrier. Carrier females transmit the defective gene on average to half their sons, who are affected, and to half their daughters, who become carriers in turn. When a male inherits the defective gene from his mother, there is no normal gene on a second X chromosome to mask it, and he displays the abnormality. Affected males therefore far outnumber affected females. They pass the defective gene to none of their sons but to all of their daughters, who become carriers. Haemophilia is a disorder of this type. Multifactorial disorders such as asthma are caused by the additive effects of several genes, along with environmental factors; the pattern of inheritance is less straightforward.