Twins

Non-identical twins are called dizygotic twins. Two separate eggs are fertilised by two separate sperm cells. They may share the same prenatal environment but are just brothers or sisters that happen to be the same age. Identical twins are called monozygotic twins. A single egg is fertilised by the same sperm cell and after the egg has been fertilised it splits into two sepearate embryos. Some monozygotic twins share almost exactly the same D.N.A and the same prenatal environment and can be thought of as natures clones. This is why psychologists are interested in monozygotic twins: they can help psychologists to measure the influence of genes on certain types of behaviour, to estimate how likely a particular behaviour is inherited. However, some monozygotic twins share the same placenta others do not. Some monozygoic twins share the same amniotic sac some do not. These differences may have a significant effect on the development of the two embryos: specific prenatal environmental differences may affect the expression of genes, they may influence the switching on or off of various genes and lead to variation in behaviour. They may be identical but the specific prenatal environment may be different for each twin. Maybe some monozygotic twins are not so identical after all.




You can see more Victorian twins here....but I think the video below is even creepier.