Callie Acebo
Callie Acebo voted up John McCann's answer
A test cross is done with an organism that is a dihybrid for two unlinked autosomal genes. What proportion of the progeny will be true-breeding for both genes? 0%, 25%, 50%, 75%, 100%

25% of course!


Do your cross by Punnet square or branching line and see that only four true breeders for the original cross will be your outcome.

Looks something like this.

Aa X Bb

--------------The rest will be mixes of the above and not true breeders.