this may be a silly question but I'm a bit confused with the information I can find regarding the difference between additive and multiplicative genetic models. I understand that for a bi-allelic locus, and a binary outcome, the risk of heterozygotes (e.g. AG) in relation to the reference genotype (say AA, withr risk=1) in both models is for example k, and that in the additive model the risk of the homozygote GG is 2k and in a multiplicative model this is k², like this:
I also found out that the allelic association test (G vs.A) assumes (and tests for ?) the multiplicative model and that additive assosiation can be achieved with a trend test.
I have been trying to find out what is the difference between these if I want to run them in a logistic regression, but I only find instructions for coding the both models like this: AA=0, AG=1, GG=2, and all the sources only discuss either one of these models, never both. So which model this coding actually represents (multiplicative?), and how do I code the other?
How about running these in PLINK, I understood that the basic logistic regression (with the --logistic option) offered there is a multiplicative model, how do I then get an additive model?