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Male sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They contain either an X chromosome or a Y chromosome. Female egg cells however, contain only the X sex chromosome and are therfore homogametic. The sperm cell determines the sex of an individual. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female.
In the case of humans, this means that parent cells have two chromosomes and gametes have one. During fertilization, gametes from the sperm combine with gametes from the egg to form a zygote. The zygote contains two sets of 23 chromosomes, for the required 46. All of the gametes in the motherâs eggs possess X chromosomes. The fatherâs sperm contains about half X and half Y chromosomes. The sperm are the variable factor in determining the sex of the baby.