. In some rare instances such as erythroblastosis fetalis in humans, selection acts against the heterozygote and instead, favors both homozygotes (opposite of selection for the heterozygote). How would you model this form of selection?

The given case is an illustration of the disruptive selection phenomenon, which is one of the components of natural selection. In the process, both the extreme homozygotes are preferred or are considered more standard in comparison to the middle heterozygote.  

The disruptive selection is also called diversifying selection, and in this form of selection, the extreme values for a specific trait are considered over the transitional values. In this kind of selection, the graph obtained would be demonstrating the bell curve that exhibits two peaks.