Pedigree patterns do not permit the diagnosis of a multifactorial trait as the phenotype is dictated by the action of multiple genetic loci and the environment.
Autosomal or sex-linked single gene conditions generally produce distinct phenotypes, said to be discontinuous: the individual either has the trait or does not. Multifactorial traits, alternatively, may be discontinuous or continuous.
With multifactorial discontinuous traits, the risk in the affected family is elevated relative to the rest of the population, but falls with more distant relationship to the affected individual within the family.
Continuous multifactorial traits present as a spectrum of gradation of the trait within a population: this is how normal human characteristics are determined.
Twin concordance and family correlation studies are required if multifactorial inheritance is suspected.
A prime example of multifactorial inheritance is spina bifida. Geographical differences within the UK have suggested a genetic influence related to Celtic descent. Seasonal variation in incidence, and the greater incidence in lower social classes, suggest that an environmental influence is also acting.
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