Additive genetic effects

The failure to find non additive genetic effects in the large twin registry sample remains a puzzle and will be resolved only when we are able to test this sample with instruments comparable to those used in the other studies. As an aside it is worth mentioning that the two instruments used to derive the factor scales make use of quite different methods of measurement (paired comparisons vs. a Like, Indifferent, Dislike format) and utilize different content. Each instrument, however, yielded the same results. 

Commonly, we wish to know the heritability of a trait (b2) or the proportion of variation attributable to genetic effects. Strictly speaking heritability is defined as the proportion of variation explained by additive genetic effects (T a). Hence... 

Inherited together, G1691A (factor V) and G20210A (factor II, prothrombin) convey at least a twentyfold increased risk for a venous thromboembolic event (VTE). They are commonly seen together in thrombophilia patients thus supporting the additive genetic effect associated with complex diseases. ... 

On the other hand, the severity model assigns the different presentation of the core symptoms observed in autism to a continuous severity gradient with an additive genetic effect in terms of susceptibility - i.e. the more susceptibility genes a person has, the more severe the phenotype will be (Spiker et al., 2002 Constantino et al., 2004 Ring et al., 2008). 

In addition to the frequency considerations attendant to the examination of polymorphisms or haplotypes, one must also consider the impact of possible differences in the magnitude of effects of any putative loci. The magnitude of any effects is denoted as "scale" effects based on the notion from quantitative genetics that there will be a displacement from the overall population mean for a trait that is dependent on genotype. To illustrate the effects of scale and frequency, consider two well-known examples of genetic effects. These are the effect of the apolipoprotein E (apo E) polymorphism on cholesterol levels and the impact of the familial hypercholesterolemia polymorphism on cholesterol levels. 

Table 3. Median proportions of variance attributed to additive genetic, nonadditive genetic, shared environmental, and nonshared environmental effects plus error and broad heritability, derived from eight kinships, for seventeen brief strong vocational interest blank scales...

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