Twins, and the reasons behind why some people have them and others don't, has fascinated scientists for centuries.
They are relatively common - occurring once in 100 pregnancies - and while the cause of non-identical twins has been linked to genetics, the genes involved had not been identified until now.
Researchers have pinpointed two gene varians related to twinning, and one can play a major role in the age of a girl's first period, age at menopause and infertility.
The breakthrough was made by Dr Hamdi Mbarek and Professor Dorret Boomsma from the Vrije Universiteit in Amsterdam.
The first gene variant is close to the gene coding for the secretion of follicle stimulating hormone, called the FSHB gene. This hormone is released by the pituitary, the small brain region that governs almost all major hormonal events in the body.
The second variant relates to the so-called SMAD3 gene and is likely to be involved in the way the ovaries respond to follicle stimulating hormone.
Both variants result in the multiple follicle growth needed for the development of non-identical twins.
Non-identical twins, medically known as dizygotic, develop from two separate eggs from separate follicles fertilised by separate sperm cells. Whereas identical twins develop from one embryo that splits in half, dizygotic twinning thus starts with a multiple ovulation.
Discovery of the main genes behind the mystery of spontaneous dizygotic twinning was long awaited, and is of great scientific interest and clinical importance,' said Professor Boomsma.
However, there are still more genes to be found that influence spontaneous dizygotic twinning.