Why are there males and females? Why are there two sexes instead of three, or twelve, or one? On this Moment of Science we'll learn why two is the magic number.

Let's start with why there is sex at all. When two organisms  51  sexually, sperm and egg combine, using DNA from both partners to make offspring. Combining DNA has a number of advantages, like correcting  52  in each parent's genes, so the offspring are healthier.

But why are there two sexes? This means, if you want to reproduce, you can only mate with fifty percent of the population, making the perfect mate that much harder to find. If there  53  only one sex you could reproduce with anyone, and it would be  54  to find a mate. You'd think this would be an advantage for survival.

As it turns  55  lots of other stuff can happen when two cells combine to reproduce. While the DNA in the cells' nuclei might be happy to join forces, other parts of the cell might not be so pleased with the merger. Specifically, a cell's mitochondria  56  violently resist merging with other mitochondria, and feuding mitochondria can turn the DNA's love feast into a microscopic battlefield.

If there were only one sex, then mitochondria in each reproductive cell would start out on  57  footing. They would need to fight it out every time. With a two-sex system, one sex can always  58  disarm, surrendering its mitochondria for  59  easier relations between the sexes. Male sperm cells are smaller than female eggs partly because they surrendered this mitochondrial battle long ago.  60  mitochondria always win.