The inherited characteristics of an individual are determined by about 50 000 gene pairs, arranged on 23 pairs of chromosomes, one of each pair coming from the father and one from the mother. Genotype diversity is introduced by random selection and recombination during meiosis, as well as by occasional mutation. These genetic variants may at one extreme be incompatible with life, or at the other produce biochemical differences detectable only by special techniques, if at all. Between the two extremes there are many variations that produce functional abnormalities or inborn errors of metabolism (IEM). Incidences of IEM range from about 1 in 100 to 1 in 200 000, depending on the disorder and the population involved.