Dizygotic twins are born from the fertilization of two distinct oocytes during the same cycle. This phenomenon, linked to double ovulation, has a documented hereditary component. Monozygotic twins, on the other hand, result from the spontaneous division of a single embryo, with no identified family predisposition to date. Understanding this distinction is the starting point for unraveling what pertains to genetics and what is a matter of chance.
Hyperovulation: the genetic mechanism behind dizygotic twins
The birth of dizygotic twins depends on a specific physiological trait: hyperovulation, meaning the release of multiple oocytes during the same menstrual cycle. This trait is primarily passed down through the maternal line.
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It is not a single gene. The tendency for hyperovulation is a polygenic trait, influenced by a set of genetic variants that regulate ovulation. Several genes act simultaneously, each with a modest effect, and their combination increases the likelihood of double ovulation.
A woman whose mother or maternal grandmother had dizygotic twins therefore has a higher probability of experiencing a twin pregnancy. To better understand the hereditary genes of twins from the father or mother, one must examine the contribution of each parent separately.
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Monozygotic twins, however, do not follow this logic. Their formation is still considered a spontaneous biological event. No study has identified a clear family predisposition for this type of twinning.
The father’s role in the genetic transmission of twins
The father does not directly cause a twin pregnancy. He cannot trigger hyperovulation in his partner. Therefore, his genetic heritage has no immediate effect on the likelihood of having twins among his own children.
However, the father can pass on variants related to hyperovulation to his daughters. These daughters, carriers of these variants, will themselves have an increased probability of releasing multiple oocytes. The effect only manifests in the next generation.
This role as a “genetic relay” explains a frequent observation in family trees: a man whose family has dizygotic twins will not necessarily see twins born among his own children, but his grandchildren (through his daughters) might be affected. The confusion between direct effect and indirect transmission fuels the myth that “twins skip a generation.”
Twins skipping a generation: a persistent myth
The idea that twins appear every other generation is one of the most widespread beliefs. It is based on a partial observation, but its interpretation is incorrect.
Here is what actually happens in a family:
- A man inherits genetic variants favoring hyperovulation from his mother or father.
- He cannot express this trait himself, as he does not ovulate. Therefore, he has no greater chance of having twins with his partner.
- He passes these variants to his daughters, who can then ovulate twice during the same cycle and conceive dizygotic twins.
The apparent skipping of a generation is not a genetic mechanism in itself. It is simply the consequence of the fact that only a woman can express the trait of hyperovulation. When the carrier is a man, the trait remains silent until a female descendant expresses it.
This nuance changes the reading of family history. The presence of twins in a paternal grandfather does not increase the probability unless that grandfather passed the variants to his daughter, and then to his granddaughters.
Non-genetic factors that modify the frequency of twin pregnancies
Genetics represents only part of the equation. Two external factors have been documented to influence the frequency of multiple births.
Maternal age plays a direct role. The older a woman gets, the higher her FSH (follicle-stimulating hormone) levels rise, which promotes the simultaneous maturation of multiple follicles. This natural hormonal increase explains why dizygotic twin pregnancies are more common after the age of thirty.
Ovarian stimulation treatments and assisted reproductive technologies constitute the second factor. These protocols intentionally induce the maturation of multiple oocytes, increasing the likelihood of multiple fertilizations. Their growing use over the past few decades has contributed to a notable rise in twin births in several countries.
These two factors modify the frequency of multiple births more than heredity alone. A woman without a family history of twins but undergoing fertility treatment has a much higher probability of a twin pregnancy than a woman with a family history but without treatment.
Monozygotic twins: chance rather than genes
Monozygotic twins represent a small portion of multiple births. Their formation results from the division of a single fertilized embryo, usually in the first days after conception.
No genetic variant responsible for this division has been identified. The available data do not show significant family clustering for monozygotic twins. This type of twinning is therefore considered a random event, independent of parental heredity.
Confusing monozygotic and dizygotic twins in discussions about heredity is a frequent source of misunderstandings. When a family claims that “twins run in our genes” without specifying the type of twinning, the statement is only valid if the twins in question are dizygotic.
The genetic component in the birth of twins is therefore real but targeted. It concerns a specific trait (hyperovulation), transmitted in a polygenic manner, expressed only in women, and limited to dizygotic twins. Everything else – monozygotic twins, direct paternal effect, systematic skipping of a generation – is more a matter of simplification than biology.