Some common "Inborn Errors of Metabolism" in Humans

    All but one of these metabolic disorders are due to insufficiency of the enzyme activity involved, in individuals homozygous for the allele: they are therefore recessive conditions. The exception is that alteration of PRPP synthetase increases enzyme activity: this still occurs only in homozygotes for the modified allele, so the condition is still described as recessive.

Figure © 2000 by Griffiths et al. ; All text material © 2012 by Steven M. Carr