The genes behind non-identical twins

An international research collaboration has identified the gene variants that increase a woman’s chance of giving birth to non-identical (dizygotic) twins by almost a third.

The study was conducted by researchers from the Queensland University of Technology (QUT), in collaboration with colleagues from Iceland, the Netherlands and the US. The results have been published in the American Journal of Human Genetics.

Unlike identical twins, which share the same genetic make-up because they are the result of one embryo splitting in half, non-identical twins develop from two separate eggs that have emerged from separate ovarian follicles at the same time and been fertilised by separate sperm cells. Two thirds of twins are non-identical pairs and are as genetically alike as their other siblings.

With spontaneous dizygotic (DZ) twinning occurring in only 1–4% of women, exactly what makes some women more susceptible than others? According to Associate Professor Dale Nyholt, from the QUT Institute of Health and Biomedical Innovation (IHBI), it’s all in the genes.

“We found one variant is close to the gene coding for the secretion of the hormone that stimulates ovarian follicles to release an egg and the second variant is in a location likely to be involved in the ovaries’ response to follicle stimulating hormone,” he said.

“When both variants are present, a woman has a 29% greater chance of having non-identical twins.”

Professor Nyholt noted that one of the gene variants also had significant effects on other fertility measures, including the age of a girl’s first period, age at menopause, number of children and the age at first and last child.

“It also affects the genes behind polycystic ovary syndrome, which is a major cause of female infertility,” he said.

“This discovery will help research on the response to hormone stimulation for assisted reproduction such as IVF.”

Professor Nyholt believes that in future, a simple gene test could be developed to identify women at risk of a strong response to hormonal treatment. This could prevent the serious complication of ovarian hyperstimulation syndrome.