The enzyme defects are caused by abnormal genes. in most cases, the child must inherit an abnormal gene from each parent in order to have the problem. Defects have been observed in almost all bio logical reactions, and the responsible genes are scattered among the chromosomes. The enzyme defects that lead to the inborn errors of metabolism are caused by genetic abnormalities present from the time of conception.

parents who have had one child with an inborn error of metabolism are at risk to have other affected children. One in four pregnancies of such parents can be expected to result in an affected child.

Some inborn errors of metabolism are more often found in certain racial and ethnic groups. For example, sickle-cell anemia occurs among those of African descent, Tay-Sachs occurs more often among Ashkenazi jewish populations, and those of northern European heritage are more likely to pass on defective genes for cystic fibrosis.

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