Are you a twin?
2% of pregnancies produces twins. About a third of these are identical twins, with identical genes, making them genetic clones. Non-identical twins share around half their genes, so they are no more alike than ordinary brothers and sisters. In twin studies, scientists assume that both sorts of twins usually share the same environment: upbringing, diet and so on. But this is not necessarily the case.
Are twins always the same?
Twins are born when two babies grow in their mother's womb at the same time. During fertilisation, a single sperm usually fertilises a single egg, which then starts to grow into an embryo, by dividing into two cells, then four, and so on. Occasionally, one embryo splits into two that then grow into genetically identical individuals – clones. Non-identical twins develop if two eggs are fertilised at the same time, by two separate sperm. They have no more genetic similarity than brothers and sisters.
Can twins be different heights?
Scientists can study twins to see how our genes and environment affect our appearance, health and behaviour. They have compared the height and weight of identical twins with those of non-identical twins, for example. Both depend on diet and genes, but to different extents. Your genes do affect your adult weight, but the amount you eat is more important. Your adult height depends on how tall your parents are, although it is also affected by your diet.