Sex Determination
In humans, an ovum can carry only an X chromosome (of the X and Y chromosomes), whereas a sperm may carry either an X or a Y; 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.
Read more about this topic: Gamete