If you missed it in school, chromosomes are the very, very basic stuff of life. These microscopic chromosomes contain the instructions for proteins to multiply and combine in such ways as to make up your brain, your heart, everything in your body.

Humans have 46 paired chromosomes, 23 from your father and 23 from your mother, the former contained in the sperm and the latter in the mother’s egg. When the sperm penetrates the egg, the two sets combine to produce your own chromosome, which you pass on to your children. (Different species have different numbers: cows have 30 pairs, chicken 39, flies 4, and bananas have 22.)