Malformations can be caused by many factors, both genetic and environmental. Conceptually, it has been easier to imagine a dominant or recessive gene or an extra chromosome or a drug taken by the mother as a cause of a malformation than it has been to conceptualize the process known as multifactorial inheritance. In this process of multifactorial inheritance both genetic and environmental factors combine to cause a structural malformation. Although the process may not be as easy to conceptualize, malformations due to multifactorial inheritance are much more common than those attributed to other recognized etiologies.
In the process of multifactorial inheritance both mutant genes (one or several) and environmental factors (virtually all of which are unknown) combine to produce an abnormality. This term, however cumbersome, is preferred to polygenic inheritance, a term that suggests correctly that several genes may be involved, but ignores the role of nongenetic factors in the pathogenesis of these malformations. There is no evidence that multiple environmental factors can simulate multifactorial inheritance.
Among newborn infants, the prevalence of major malformations is about 2.5%, with the apparent etiologies as shown in Table 1.
CHARACTERISTICS OF MULTIFACTORIAL INHERITANCE
The high frequency of malformations attributed to these complex etiologies makes it crucial that pediatricians be familiar with the patterns of occurrence and recurrence and be prepared to discuss these with parents.