How do twins develop?
Human life emerges from cells. Identical (monozygotic) twins emerge from one single fertilized egg cell, which divides into two parts at an early stage of development. Thus, looked at in a genetic way, identical twins are alike and one could describe them as natural clones. If two egg cells are fertilized by one semen each at the same time, fraternal (dizygotic) twins grow in the womb. Their genetic disposition differs to the same degree as that of normal siblings, so that they share an average 50% of their genes. This implies that a specific pair of siblings can share more or less 50% of its genes. But not only genes have an impact on development – we become the person that we are also due to environmental influences. Thus, twins, especially identical ones, are strikingly similar, but they are not identical as such.
How frequent are twin births?
In Germany the frequency of twin births is about 1.2% (1:85). One third of the twins are identical and thus of same sex, one third are fraternal and of same sex and one third are fraternal and of opposite sex. Furthermore, the tendency to produce more egg cells and thus deliver fraternal twins, can be inherited recessively by women. This means that if a mother has fraternal twins in her family, the possibility increases that she herself gives birth to multiples. However, this is not the case for identical twins. Moreover, the frequency of multiple births is increased by artificial insemination as several egg cells are inserted into the mother.