Behavioural Genetic Interactive Modules
OverviewThis module is designed to illustrate the ways in which the different sources of variation in a trait (genetic and environmental) result in particular patterns of expected correlations between related individuals.
TutorialUpon loading the module you will see three main panels. The top two panels are both labelled Population Parameters. The left panel contains three sliders labelled A, D, C and E which correspond to additive genetic, dominance genetic, shared (common) environment and nonshared environment components of variance respectively. These four components of variance represent the relative balance of nature and nurture for a particular trait. This trait could be any continuous measure such as height, IQ or severity of anxiety symptoms. These sliders are used to control the relative balance of influences on the trait.
The panel to the right represents the percentage of total variance that each of these four components accounts for. We assume that the variation in the trait (including measurement error) is completely accounted for by these four latent variables and so these four percentages will always sum to 100%. Moving the sliders results in a corresponding change in these figures.
The panel at the bottom of the window contains a graphical and numerical display of the correlations that we would expect to observe for the trait, if the trait had the relative balance of genetic and environmental effects specified in the top panel (i.e. 100% heritable). The leftmost column is labelled Proband, the "correlation" between individuals and themselves. Of course, this correlation will always be a complete positive correlation of 1.0 (the maximum possible correlation). In the second column, the height of the green bar and the number below represents the correlation we would expect to observe between MZ twins. Likewise, the next three columns represent the correlations we would expect to see between DZ twins, full siblings and parent-offspring. The next two columns represent the expected correlations for MZ and DZ twins reared apart (i.e. no shared environment, but still shared genes). The correlation for half-siblings (half siblings raised in the same family) is next; finally, the expected correlation between the proband and any unrelated individual chosen at random is shown.
The only controls in this module are the four sliders in the top left panel. Moving the A slider up increases the relative proportion of trait variance attributable to additive genetic variation. Because all the other parameters are set to zero, this will suddenly jump to 100%. That is, the value for Heritability given in the top right panel will read 100%, indicating that the trait is completely determined by the additive effects of genes.
Looking at the expected correlations under this somewhat unrealistic scenario, we see that they represent the expected proportion of allele sharing for the different types of relatives. Because MZ twins share all their genes, then if the trait is completely determined by additive genetic effects, MZ twins will correlate completely for the trait. DZ twins, full siblings and parent-offspring pairs, all of whom share only half their alleles, are only expected to show a correlation of 0.50. Because this simple model states no environmental effects, twins reared apart are expected to be just as similar to each other as those reared together.
We would be very unlikely to ever see this pattern of correlations in families for any complex behavioral trait. For certain Mendelian traits with complete penetrance, we might observe a pattern of familial correlations close to this. Given that measurement error is incorporated into the nonshared environmental component of variance, it is extremely unlikely that this will ever be zero.
So try sliding the nonshared environment E control upwards. Note what happens: in the top right window we can see that the magnitude of the nonshared environmental component increases and all of the expected correlations between family members drop.
Note that the familial correlations remain in proportion with each other, however, when varying the E component. That is, the DZ twin correlation always remains half the expected MZ correlation, for example. Although the nonshared environmental is now contributing to the variance of the trait, it does not (by definition) contribute to the similarity between family members. Under this scenario, what makes related individuals similar is still completely the additive effect of genes, so we would still expect similarity to reflect exactly the expected proportion of allele sharing between relative pairs.
Now try increasing the C slider. Note how some correlations rise and some fall, all at different rates. It is the nature of this relationship, between the aetiological components of variance and the expected familial correlations that is the key to many quantitative genetic methods. Incorporating dominance (the D slider) further influences the pattern of expected correlations.
The expected correlations for each type of relative pair are entirely determined by the four population parameters. The Behavioral Genetics Appendix gives an account of how familial correlations can be expressed in terms of the sharing we would expect for variance components, which can be derived from basic biology (for the genetic components) or by definition (for the environmental components).