How to use an ordinal or discrete phenotype in plink?

Question

[noahconnally]

I want to run an association using plink. I know that you can use data that is continuous and data that is categorical. However, my data is ordinal. Each individual has a phenotype that is 0, 1, 2, or 3. So it's not continuous, but it's also not simply categorical - 2 is closer to 3 than 0, for example. Basically, my data are discrete and I can't figure out how to get plink to work with that. Thank you!