Propose a hypothesis to explain these results

Imagine you have two pure-breeding lines of canaries, one with yellow feathers and the other with brown feathers. In crosses between these two strains, yellow female X brown male gives only brown sons and daughters, while brown female X yellow male gives only brown sons and yellow daughters. Propose a hypothesis to explain these results. (Please don’t tell me that the brown trait is dominant to the yellow because that is not the correct answer. It has something to do with sex-linked traits.)