> Because upper vertebrates normally can't exert influence on the sex of the offspring before birth. Your DNA will have an X chromosom at least. In principle [1

> Because upper vertebrates normally can't exert influence on the sex of the offspring before birth. Your DNA will have an X chromosom at least. In principle [1], you can have either a second X or an Y. 50% probabilities. 1:1. That's not a sufficient explanation, as there could be a different ratio during the sperm cell generation process or selection during insemination or from the egg cells themselves. Y and X sperm cells could easily be handled differently at any point of the whole process.