Monozygotic and dizygotic twins
On average in Western Europe every 13 to 14 per 1,000 deliveries are twins. In the Netherlands the number is slightly higher: 18 to 19 twins per 1,000 births. In 1999 3556 twins, 71 triplets and 4 quadruplets were born in the Netherlands.
Dizygotic or fraternal twins: of all twins born approximately 65% are dizygotic. These twins develop from two seperate eggs that are fertilized simultaneously by two different sperm. The resultant twins are genetically like normal brothers and sisters. Their genetic predisposition is 50% the same and they can be both of equal or opposite sex.
Monozygotic or identical twins: only 3 or 4 of every 10 twin pairs are monozygotic. They develop from one zygote that splits and forms two embryos very soon after fertilization. Identical twins have exactly the same genetic material. They are always two boys or two girls.
Identical or fraternal twins?
Without information about zygosity nothing can be said about the influence of heredity and environment on the factors that are researched. With large numbers of twins we will initially try to get an answer from our questionnaires. Questions on similarities in appearance play an important role. Are the children of the same sex and similar enough in terms of physical characteristics (eye colour, hair colour, skin colour and face shape) that they often get confused for each other by outsiders, there is a strong indication for monozygosity. Blood group or DNA research is then needed for further determination.
Blood group research provides more certainty: as soon as at least one difference is found in the blood groups of twins it is certain that the two are fraternal twins. If no difference is found then the twins are almost but still not 100% sure monozygotic. DNA testing is the most reliable method to determine zygosity. This research is based on the occurrence of DNA elements that show the same base sequence and specific patterns. Identical twins show the same pattern, fraternal twins show a different pattern.