Final answer:
In yeast mating type switching, the a and alpha mating type genes that are not expressed are found in the nucleus.
Step-by-step explanation:
In yeast mating type switching, the a and alpha mating type genes that are not expressed are found in the nucleus. The a and alpha mating type genes are regulated by a repressor protein called alpha2. This repressor binds to specific sequences in the DNA, preventing the expression of the opposite mating type genes.
Therefore, the genes are present in the nucleus but are not transcribed and translated into proteins.