A cross of ab X ++ using Chlamydomonas yields three possible patterns of unordered tetrads:
Parental ditypes (P) produce two ++ cells and two ab cells.
Nonparental ditypes (NP) produce two a+ cells and two +b cells.
Tetratypes (T) produce one cell of each of the four possible genotypes.

The data indicates independent assortment of the two genes, suggesting they are not linked, while the small number of tetratypes (T) is useful for mapping the centromere.