Females have two of the same kind of sex chromosomes (XX), while males have two distinct sex chromosomes (XY). An oocyte (female gamete) has a single X chromosome. A sperm cell (male gamete) has either an X chromosome or a Y chromosome. If a Y-bearing sperm cell fertilizes an oocyte, the zygote is a male (XY). If an X-bearing sperm cell fertilizes an oocyte, then the zygote is a female (XX). The male (XY) and female (XX) offspring are in a 50-50 ratio with an equal number of boys and girls. Therefore, the chance of having a boy is 1/2 or 50% and the chance of a girl is also 1/2 or 50%. This ratio can be demonstrated by tossing a coin many times and keeping track of the number of heads and tails. If enough tosses are made, the number of heads and tails should be very close to 50-50
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