The idea of a “twin gene” often sparks curiosity, leading many to wonder if having twins is simply a matter of family inheritance. This prompts questions about the underlying biological mechanisms and whether genetics truly play a role in multiple births. Understanding the science behind twinning can clarify these assumptions.
The Science of Twinning
Twinning occurs in two distinct ways: identical and fraternal. Identical twins, monozygotic twins, originate from a single fertilized egg. This egg, after being fertilized by one sperm, spontaneously divides into two separate embryos early in development. These twins share nearly identical genetic material and are almost always of the same sex.
Fraternal twins, dizygotic twins, form through a different biological process. They result when two separate eggs are released during a single menstrual cycle and are each fertilized by different sperm. Unlike identical twins, fraternal twins are genetically no more alike than any other siblings born at different times, sharing approximately 50% of their genes. They can be of the same sex or different sexes and may not have similar appearances. The formation of identical twins is generally considered a random event, occurring in about 3 to 4 out of every 1,000 births globally and is not typically influenced by family history. Fraternal twinning, however, is more likely to have a genetic component.
Hyperovulation and Fraternal Twins
The “twin gene” concept primarily refers to a genetic predisposition for hyperovulation, not a single gene. Hyperovulation is the tendency for a woman’s ovaries to release more than one egg during a single menstrual cycle, instead of the usual single egg. If both eggs are fertilized by separate sperm, fraternal twins result. This mechanism is responsible for naturally occurring fraternal twin pregnancies.
Scientists have identified specific gene variants linked to hyperovulation and an increased chance of fraternal twins. For example, variants near the FSHB gene and within the SMAD3 gene have been associated with this trait. The FSHB gene influences the production of follicle-stimulating hormone (FSH), which stimulates egg maturation in the ovaries. Higher or prolonged levels of FSH can lead to multiple follicles developing and releasing eggs. The SMAD3 gene regulates how the ovaries respond to FSH, with certain variants potentially increasing ovarian responsiveness. Women carrying both of these gene variants may have up to a 29% increased chance of conceiving fraternal twins.
Genetic Inheritance Patterns
The genetic predisposition for hyperovulation, leading to fraternal twins, is passed down through families. Since only women ovulate, the mother’s genetics directly determine her chances of having fraternal twins. If a woman has a close female relative, such as a mother or sister, who has had fraternal twins, her likelihood of also having fraternal twins is approximately doubled.
Twins are often thought to “skip a generation,” but this can be explained by the inheritance pattern. A father can carry the genes for hyperovulation and pass them on to his daughters, even though he cannot have twins himself because he does not ovulate. If his daughter inherits these genes, her chances of hyperovulation and subsequently having fraternal twins increase. This creates the appearance of twins skipping a generation, as the genetic trait is inherited through the male line but expressed only in the female descendants.
Other Influences on Twinning Rates
Other factors can influence the likelihood of having fraternal twins. Maternal age is a significant contributor; women over 30, especially over 35, have higher rates of hyperovulation, increasing their chance of conceiving fraternal twins. This is partly due to rising follicle-stimulating hormone (FSH) levels as a woman ages.
Ethnicity plays a role in twinning rates. Certain populations, such as those of African descent, particularly the Yoruba people of Nigeria, exhibit higher rates of fraternal twinning, with some regions seeing rates as high as 45-50 twin sets per 1,000 live births. Conversely, populations in South America, South Asia, and Southeast Asia tend to have lower rates, around 6 to 9 twin sets per 1,000 live births.
Fertility treatments have increased twinning rates globally. Procedures like in vitro fertilization (IVF), especially when multiple embryos are transferred, and ovulation-inducing drugs can raise the probability of multiple births. Fertility drugs, for instance, can increase the chance of twins to as high as 30%.