Twin births make up 1-3% of all births and in the Swedish Twin Registry there are over 140,000 twins. Twins may be identical twins or fraternal twins. Identical twins are formed from the same fertilized egg cell and have the same DNA arrangement, while fraternal twins develop when two eggs are fertilized on the same occasion. Their DNA arrangement may vary, just like between ordinary siblings.
DNA is the inherited traits that we all have in most of the body's cells and which determine how the body will appear and function. A child inherits half of the mother's and half of the father's DNA arrangement. The DNA arrangements in the two halves may be combined in many different ways, and siblings can be extremely unlike each other, but also extremely alike.