Fly trap.
Puzzle: Assume we’re talking about a complete dominance, non-pleiotrophic, non-epistatic, single gene. Let’s say you have two boxes of flies. One box has 100% red males. The other has 100% blue females. You mate the two boxes. They produce children. All the male children are 100% blue. All the female children are 100% red. You now mate all the children. Now the males are 50% blue and 50% red. The females are also 50% blue and 50% red. What the hell?
Solution: Red is a sex-linked dominant allele. (Remember that males are XY and females are XX.) In the original population, all the males were X(r)Y and all the females are X(b)X(b) where X(_) means _ is a linked to the X chromosome. When you cross X(r)Y and X(b)X(b), you get four equally possible choices: X(r)X(b) twice and X(b)Y twice. Hence, all the females are red because red is dominant and all the males are blue because it only takes one recessive allele on the X chromosome to express the recessive phenotype. That’s because the Y chromosome is completely neutral with regards to this whole fracas.
When you mate X(r)X(b) and X(b)Y, you get four equally possible choices: X(r)X(b), X(r)Y, X(b)X(b), and X(b)Y. Therefore, 50% red and 50% blue for males and females.