Most of us know at least one set of twins. That's hardly surprising, since about 1 in 80 human deliveries produces twins. There are two main kinds: fraternal, or dizygotic (DZ), and identical, or monozygotic (MZ). Fraternal twins occur when two eggs are fertilized by two sperm. They are like ordinary brothers and sisters, except for the fact that they are born together. Although no one knows why, the tendency to have fraternal twins seems to run in families; if they occur in one generation, they have a better-than-average chance of showing up in the next.
Unlike fraternals, identical twins occur randomly, at the rate of about four sets for every 1,000 births. They are created when one egg is fertilized by one sperm and then splits in half. The result: two foetuses with the same genetic makeup. How identical are identical twins? Well, if one set married another set, their children would all be brothers and sisters - genetically speaking.