Genetics
Humans have over 21,000 different genes. Boys have around 80 genes that girls don't have. These genes are critical for turning a fertilized human egg into a boy. This isn't the only difference between boys and girls, though. Boys and girls also use many of the other 21,000 or so genes they share in different ways. It is this combination of extra genes and how we use some of the rest of our genes that makes boys different from girls. All people have a pair of what are called the sex chromosomes. There are two kinds of these chromosomes: the X chromosome and the Y Chromosome. Girls have two X Chromosomes while boys have one X Chromosome and one Y Chromosome. So girls are XX and boys are XY.
(http://genetics.thetech.org/ask/ask273) |