 A non-normal ovum can end up carrying two or no X chromosomes, as a result of an irregularity at either of the two stages of meiosis, while a non-normal sperm cell can end up carrying either no sex-defining chromosomes, an XY pair, or an XX pair; thus the male sperm determines the sex of any resulting zygote.If the zygote has two X chromosomes it will develop into a female, if it has an X and a Y chromosome, it will develop into a male.For birds, the female ovum determines the sex of the offspring, through the ZW sex-determination system.