Abstract
Many traits vary in a dichotomous manner, although the underlying genetic determination is polygenic. The genetic basis of such dimorphic traits can be analyzed using the threshold model, in which it is assumed that there is a continuously distributed underlying character and the phenotype is determined by whether the character is above or below a threshold. Threshold traits frequently vary with environmental variables such as photoperiod, temperature and density. This effect can be accounted for using a threshold model in which (1) there is a critical value of the environmental variable at which a genotype switches to the alternate morph, and (2) switch (threshold) points are normally distributed in the population. I term this the environmental threshold (ET) model. I show that the ET model predicts that across environments differing in only one factor the genetic correlation will be 1. This prediction is supported by data from three wing dimorphic insects. Evidence is presented that the genetic correlation between environments differing in two components (temperature and photoperiod) is less than 1.