Multifactorial inheritance means that "many factors" (multifactorial) are involved in causing a birth defect. The factors are usually both genetic and environmental, where a combination of genes from both parents, in addition to unknown environmental factors, produce the trait or condition. Often one gender (either males or females) is affected more frequently than the other in multifactorial traits. There appears to be a different "threshold of expression", which means that one gender is more likely to show the problem over the other gender. For example, hip dysplasia is nine times more common in females than males.
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Multifactorial inheritance is responsible for the greatest number of individuals that will need special care or hospitalization because of genetic diseases. Up to 10% of newborn children will express a multifactorial disease at some time in their life. Atopic reactions, diabetes, cancer, spina bifida/anencephaly, pyloric stenosis, cleft lip, cleft palate, congenital hip dysplasia, club foot, and a host of other diseases all result from multifactorial inheritance.