Genetic Correlation
Standard Definition
Like heritability, genetic correlation is typically defined in the context of an additive genetic model. Let \(Y_A\) and \(Y_B\) denote two phenotypes of interest. Let \(G_A,G_B\), and \(E_A, E_B\) denote the respective genetic and environmental components of the phenotypes.
The model is
In the context of this model, genetic correlation is defined as the Pearson correlation of the genetic components:
When Are Traits Genetically Correlated?
What does it mean biologically when two traits are genetically correlated? The most straightforward case is that of pleiotropy: an overlapping set of genetic variants drives the two phenotypes. This corresponds to the causal graph \(A \leftarrow g\rightarrow B\), where \(g\) denotes the shared variants. Mediated pleiotropy is also possible, in which variants cause one trait, which causes the other: \(g\rightarrow A\rightarrow B\).
Besides these straightforward cases, there are more exotic possible causes of genetic correlation, as discussed here. Briefly,
- Two traits can be genetically correlated because genetics affects the behavior of a parent, which affects the phenotype of their child.
- Two traits can be genetically correlated because individuals with these traits tend to mate at a higher rate than would be expected under random mating.