In humans alcohol shows a strong dose-response effect with more severely affected phenotypes at increasing doses. An example of fetal genotype influencing susceptibility to a teratogen is phenytoin. More than one half of fetuses exposed to phenytoin do not develop the fetal hydantoin syndrome. In fetuses that develop the syndrome, the severity of the syndrome is associated with decreased activity of the enzyme epoxide hydrolase in the fetus. Maternal genotype Genetic defects in the mother may result in metabolite production that is deleterious to the developing fetus.
Examples are maternal PKU and diabetes.