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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 mammals, the sex of the offspring is determined by the sperm cell: a spermatozoon bearing a X chromosome will lead to a female (XX) offspring, while one bearing a Y chromosome will lead to a male (XY) offspring. Sperm cells were first observed by Anton van Leeuwenhoek in 1677.