Plink 1.9: How to select major two allele from tri-allelic SNPs for PLINK analysis?

Question

[Hirofumi]

Hello, every one. How to select major two allele from tri-allelic SNPs for PLINK analysis?

For example, since A and T are the major alleles,
I want to test only A and T from "A>T,G" SNV.

Thank you for reading!