Recessive inheritance (not sex-linked)

Diseases caused by recessive inheritance can be observed only in homozygotes. In the presence of a normal gene (in heterozygote) abnormal phenotypic effects are not observed. Every child of a couple where one person is ill is a carrier. Usually recessive diseases are results of lack of functional enzymes and are thus called inborn errors of metabolism. Mutant genes causing these diseases are rather rare in the human gene pool and if parents are not related there is little chance that they are both heterozygous.