Each gene adds to or subtracts from the trait and acts in an additive manner independent of other genes. Few persons are at the extremes of the distribution and many are in the middle, because a person is unlikely to inherit many factors acting in the same direction. Environmental factors, each adding to or subtracting from the final result, will also produce a normal distribution. The affected person is predisposed to the condition, representing the sum of genetic and environmental influences. The risk in more distant relatives, who are likely to inherit only a few high-liability genes, is much lower.
Environmental factors play a role in multifactorial inheritance.
1 mo before conception to 3 mo after conception. Although maternal folic acid deficiency plays a major role, it is not the only environmental cause of NTDs. Thus, compared with a male, a female requires more potent liability genes to develop the condition, has more affected siblings, and has a greater risk of having affected offspring. Many specific genes are being found. Genetically determined predisposing factors, including a family history and biochemical and molecular parameters, can identify persons at risk who would most likely benefit from preventive measures.