Assume that a mutation occurs in the gene responsible for the production of hexosaminidase a, such that only about 50% of the enzyme activity is found in the heterozygote compared with a homozygous normal individual. if heterozygotes are phenotypically normal, we would say that the mutant allele is recessive to its normal allele.

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In the case of autosomal dominant disease, the person is either homozygous (which is very rare) or heterozygous. On the molecular level, either the mutation produced a new deleterious protein for the organism, or the mutation affected an existing protein in the physiological state and that a 50% activity is not enough to compensate for the needs. of the body.

In case of autosomal recessive disease, the sick person is always homozygous. If the two loci each have a different mutant allele, it is called a "composite heterozygote". People with autosomal recessive inheritance disorder have "mandatory heterozygote" parents.

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