Consider a genetic crossing experiment with yeast involving allelic markers a and A. Why, on first consideration, would you expect that the number of cells yielding one a type and three A type spores...

Consider a genetic crossing experiment with yeast involving allelic markers a and A. Why, on first consideration, would you expect that the number of cells yielding one a type and three A type spores would equal the number yielding three a type and one A type? What mechanism might be responsible for a serious biasing of the numbers in one direction?