Question

why is that a diploid yeast cell always has one chromosome three (III) with a MATa and another with MAT alpha?

Answer 1

The diploid yeast cell always has one chromosome three (III) with a MATa and another with MAT alpha,

Because, a stable diploid yeast cell is only formed from the mating of two haploid yeast cell of which one consists of the a type chromosome and other is the slpha type cell. Both are attracted to each other by the mating factors, a pheromone and alpha pheromone.

In a diploid cell, the genes from a type produce the a factor that form the cell surface receptor that is turn binds to the alpha factor thus triggering the signal transductioin within the cell.