The appearance of twins in a family often raises the question of whether the trait is genetic or simply chance. The reality is nuanced and depends entirely on the type of twins conceived. While a single pregnancy producing two babies is relatively rare, the mechanism for twin conception is not always random. Only one specific type of twinning is influenced by inherited genetics, which is key to understanding this unique form of multiple birth.
The Difference Between Identical and Fraternal Twins
Twinning is categorized into two biologically distinct types: identical (monozygotic or MZ) and fraternal (dizygotic or DZ).
Monzygotic twins result when a single egg is fertilized by one sperm, and the resulting embryo spontaneously splits into two separate embryos early in development. This creates two genetically identical individuals. This splitting event is considered random, and the worldwide incidence of identical twins remains constant at about three to four per 1,000 births, regardless of family history.
In contrast, dizygotic twins originate from two separate eggs, each fertilized by a different sperm cell during the same menstrual cycle. These twins are genetically no more similar than any other siblings. Dizygotic twinning is the type influenced by inherited factors, as it requires the mother to release more than one egg simultaneously. The incidence of fraternal twins varies widely across populations, suggesting an underlying genetic component.
The Genetic Mechanism of Twinning Inheritance
The inherited trait responsible for fraternal twinning is not a gene for “having twins” directly, but rather a genetic tendency toward hyperovulation. Hyperovulation occurs when a woman’s ovaries release two or more eggs during a single ovulatory cycle. If these multiple eggs are fertilized by separate sperm, the result is dizygotic twins.
This tendency is rooted in variations of genes that manage a woman’s reproductive hormones, specifically Follicle-Stimulating Hormone (FSH). FSH stimulates the development of ovarian follicles. Researchers have identified specific gene variants that increase the chances of fraternal twinning by affecting FSH production or how sensitively the ovaries respond to its signals.
One notable gene is \(FSHB\), which is associated with FSH secretion and linked to higher hormone levels. Another gene, \(SMAD3\), regulates the ovarian response to FSH, potentially making the ovaries more sensitive to signaling. These variations mean a woman carrying these traits is more likely to release two eggs, increasing the probability of conceiving fraternal twins by up to 29%.
Who Inherits the Trait and Who Expresses It
The genetic predisposition for hyperovulation can be passed down through both the mother and the father, but only the mother can express the trait. Since hyperovulation is a maternal biological process, the gene’s effect—releasing multiple eggs—can only manifest in a woman. Therefore, the odds of conceiving fraternal twins are solely determined by the genetics of the woman who is ovulating.
A father can carry the hyperovulation gene and pass it to his daughters. If a man inherits the gene, his own chance of having twins with a partner is not increased, as his genes do not influence his partner’s ovulation. However, if he passes this gene to his daughter, she gains the genetic tendency to hyperovulate. This pattern often makes the trait appear to “skip” a generation when a son inherits the gene but cannot express it, only for it to resurface in his daughters.
Factors That Increase Twinning Risk That Are Not Genetic
While genetics influence the natural tendency toward fraternal twinning, several non-inherited factors also affect the rate of multiple births.
Maternal age is a significant influence, as women over 35 naturally experience hormonal changes, including higher levels of Follicle-Stimulating Hormone, which can lead to multiple ovulation. Women with a higher body mass index and those who have had multiple previous pregnancies also show a higher probability of conceiving twins.
Ethnicity plays a notable role; for example, some African populations, particularly the Yoruba people in Nigeria, exhibit the highest rates of fraternal twinning globally, while Asian populations generally have lower rates. Environmental factors, such as diet, have been theorized to contribute to these ethnic differences.
Assisted Reproductive Technologies (ART)
The use of Assisted Reproductive Technologies (ART), such as in vitro fertilization (IVF) and fertility medications, dramatically increases the chance of multiple births. This is achieved by intentionally stimulating the ovaries to produce multiple eggs or by transferring multiple embryos.