The appearance of twins often leads to questions about genetic inheritance and the likelihood of a multiple birth. The rate of twin births has risen significantly, but whether twins are inherited depends entirely on the biological mechanism that produces them. Understanding the difference between the two main types of twins is the first step in determining the role genetics plays.
Identical Versus Fraternal Twins
The world of twins is divided into two distinct biological categories: identical (monozygotic) and fraternal (dizygotic).
Monzygotic twins occur when a single fertilized egg spontaneously splits into two separate embryos early in development. Because they originate from the same zygote, identical twins share virtually all of their DNA and are always the same sex. This splitting event is considered a random biological accident and has no strong genetic link that passes through a family.
Dizygotic, or fraternal, twins develop from two separate eggs, each fertilized by its own unique sperm, resulting in two distinct embryos. These twins are genetically no more alike than any other siblings, sharing about fifty percent of their variable DNA. Fraternal twins can be the same or different sexes. The crucial difference is that the tendency to have dizygotic twins is the only type of twinning known to be heritable and runs in families.
Maternal Inheritance of Dizygotic Twins
The heritability of fraternal twins is almost exclusively tied to the mother’s reproductive biology, specifically her genetic predisposition to hyperovulation. Hyperovulation is the biological event where a woman’s ovaries release two or more eggs during a single menstrual cycle, rather than the typical single egg. If both of these eggs are successfully fertilized by separate sperm, a set of dizygotic twins is conceived. This trait is passed down through the female line, as it is the mother’s physiology that controls the number of eggs released.
This ability to hyperovulate is influenced by specific genetic variations that affect the regulation of reproductive hormones. Researchers have identified gene variants that contribute to this trait, including those related to Follicle-Stimulating Hormone (FSH) signaling. One such variant is found in the FSHB gene, which is involved in the production and secretion of FSH, a hormone that stimulates the growth of ovarian follicles and the maturation of eggs. A variation in this gene can lead to higher circulating levels of FSH, increasing the likelihood that multiple follicles mature simultaneously.
Another gene variant associated with fraternal twinning is found near the SMAD3 gene, which influences how the ovaries respond to the presence of FSH. The combination of these genetic factors can result in a woman having a heightened ovarian response, causing the ovaries to release two eggs in one cycle. Having just one copy of the gene variants in both the FSHB and SMAD3 locations can increase a woman’s chance of having fraternal twins by as much as twenty-nine percent compared to the general population. This genetic tendency can be inherited by a daughter from her mother or father, but it is only expressed in the body of a person who ovulates.
Paternal Influence and Non-Genetic Factors
While the father’s genes do not directly influence his partner’s ability to hyperovulate, he plays an important role in passing the hyperovulation trait to the next generation. A man can inherit the gene variants for hyperovulation from his own mother and pass them on to his daughter. If his daughter inherits this genetic tendency, she will then have an increased chance of releasing multiple eggs and conceiving fraternal twins herself. In this scenario, it may appear that the trait skipped a generation, as the father did not have twins, but he was a carrier who transmitted the gene to his child.
Non-Genetic Factors
Several non-genetic factors significantly increase the odds of a dizygotic twin pregnancy. A woman’s age is a strong factor, as the likelihood of hyperovulation increases as maternal age approaches the late thirties and early forties. This is related to naturally higher levels of FSH as the reproductive window begins to close. The use of assisted reproductive technologies (ART), such as ovulation-inducing drugs or the transfer of multiple embryos during In Vitro Fertilization (IVF), has also contributed substantially to the increase in twin births globally. Other influences include:
- Ethnicity
- Higher body mass index
- Having had several previous pregnancies