The inheritance of twins often sparks curiosity. While identical twins are largely a spontaneous event, the predisposition for fraternal twins can have a genetic component. This article explores what this “twin gene” entails and how its influence travels through families.
Understanding Different Types of Twins
Understanding twin inheritance requires distinguishing between identical (monozygotic) and fraternal (dizygotic) twins. Identical twins develop from a single fertilized egg that splits, resulting in two individuals with nearly identical genetic information and always sharing the same sex. This type of twinning occurs at a consistent rate of approximately 3 to 4 births per 1,000 and is generally considered a random occurrence, not influenced by family history.
Fraternal twins arise when two separate eggs are released and fertilized by two different sperm during the same menstrual cycle. These twins are genetically distinct, sharing about as much DNA as any other siblings, and can be either the same or different sexes. Fraternal twinning accounts for about 70% of spontaneous twin pregnancies and demonstrates a clear genetic predisposition.
The Genetic Basis of Fraternal Twinning
The “twin gene” refers to a complex genetic predisposition primarily linked to hyperovulation, which is the tendency for a woman’s ovaries to release more than one egg during a single menstrual cycle. Several genes and genetic regions are implicated in this increased likelihood of multiple egg release. For instance, variants near the FSHB gene are associated with higher levels of Follicle-Stimulating Hormone (FSH), a key hormone for follicle growth and egg maturation. Elevated FSH levels can lead to multiple follicles developing and releasing eggs.
Another gene, SMAD3, may influence how the ovaries respond to FSH, further contributing to hyperovulation. Genes like Growth Differentiation Factor 9 (GDF9) and Bone Morphogenetic Protein 15 (BMP15) also play roles in ovarian function. Research indicates rare variants in GDF9 are more common in mothers of dizygotic twins. While BMP15 affects ovulation in some animals, its role in human dizygotic twinning is less definitively established.
These genetic variations can make a woman’s ovaries more sensitive to hormonal signals, increasing the chances of releasing multiple eggs. Men can carry these genes, but they do not express hyperovulation themselves as they do not ovulate. The genetic influence on fraternal twinning is polygenic, meaning multiple genes interact to contribute to the overall predisposition.
Inheritance Patterns and Who Carries the Trait
The genetic predisposition for fraternal twinning is primarily inherited through the maternal line. A woman’s chance of having fraternal twins is influenced by her mother’s and other close female relatives’ history of fraternal twins. If a woman has a close relative, such as a mother or sister, who has had fraternal twins, her own likelihood of having them can be approximately doubled compared to the general population.
Men can carry the genes associated with hyperovulation and pass them to their children. While a father will not have twins himself, he can transmit the genetic tendency to his daughters. If a daughter inherits these genes from her father, she will have an increased chance of hyperovulating and conceiving fraternal twins. This explains why the trait might appear to “skip” a generation if it passes through male family members who do not express it directly.
The inheritance pattern is polygenic, involving the combined effect of multiple genes, rather than a simple dominant or recessive trait. This complex genetic interplay means that while a family history of fraternal twins indicates an increased probability, the exact manifestation can vary.