Each cell in the human body contains about 25,000 to 35,000 genes. Genes carry the information that determines your traits, and how your body grows and develops. Genes are made up of DNA, which has its own alphabet made of just 4 letters — A, T,C, and G (not 26 like in our alphabet!). These four letters are put together into different combinations and make up each of our genes.
Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than one percent of the total number of genes) have slight differences between people. These small differences contribute to each person's unique physical features.
As you can see in this picture above, all of our genetic information is found inside the cells of our bodies. Genetic information is stored in a protective compartment in the cell called the nucleus. All of the strands of DNA are tightly wound up into structures called chromosomes.
Every person has 23 pairs of chromosomes, which makes 46 in total.
Chromosomes numbered 1-22 are the same in both males and females. The last pair, the 23rd pair, is the pair that determines a person's gender. If a person has XX, or two X chromosomes, she will be a female. If a person has XY, or one X chromsome and one Y chromosome, he will be a male.
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