Figure 1.

Illustration of zygosity and clinical interpretations. Black stars represent variants; boxes represent alleles. A wild-type patient carries no variants, resulting in normal-activity alleles (green). A heterozygous patient carries one variant, resulting in one reduced or inactive allele (red) and one active allele (green). A partly reduced enzyme activity is expected, since there is still one active allele left. For homozygous patients, both variants result in a reduced or inactive allele (red). Depending on the effect of the variants on the protein, a reduced or absent enzyme activity is expected. Compound heterozygous patients can carry variants on different alleles (in trans) or on one allele (in cis), resulting in differences in enzyme function, either like that of a heterozygous patient or a homozygous patient.