Both men and women have sex chromosomes. Men usually have one X and one Y chromosome, while women have two X's.
When an egg or sperm is made, it only gets one of the sex chromosomes from the parent. This means that women can only make eggs with an X chromosome. But men can make either X or Y sperm.
During fertilization, the sperm cells race toward the mother-to-be's egg cell. If a sperm with a Y beats all others, then the fetus will be XY. The pregnancy will result in a boy.
However, if a sperm with an X wins the race to the egg, then the fetus will be XX. The parents will have a baby girl.
Nearly everyone's chances are around 50% for having a boy and 50% for having a girl. And yet, we all know families that are all boys or all girls.
DNA, or deoxyribonucleic acid, is a long chain molecule that plays a central role in life on earth. The information encoded in strands of DNA controls the genetic makeup of organisms. The DNA molecule has a backbone of sugars and phosphate groups off of which hang simpler units called nucleotide bases.
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