Abstract
Abstract The majority of human neural tube defect (NTD) cases are nonsyndromic, meaning that the NTD is the only defect, not just one feature of an identified syndrome such as trisomy 13 or Meckel-Gruber syndrome. The etiology of nonsyndromic NTDs is considered to be complex or multifactorial, with both environmental and genetic factors implicated. In fact, it is likely that multiple genes and environmental factors interact in order for complex disorders such as NTDs to occur. The genetic factors may make the fetus susceptible, with the environmental factors providing the trigger or catalyst for the development of the NTD. A multifactorial threshold model, in which the liability to this complex disorder follows a normal distribution pattern, is often used to illustrate the concept of complex disease inheritance (Edwards, 1969). In this threshold model, when the additive effects of several genetic and environmental factors cumulate to increase the liability to disease, the phenotype is expressed (Fig. 14.1).
Published Version
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