The arrival of twins often sparks curiosity about their inheritance. Understanding the science behind twin conception reveals the distinct roles each parent’s genetics can play.
Different Kinds of Twins: A Quick Look
Twins are broadly categorized into two main types: monozygotic (identical) and dizygotic (fraternal). Identical twins develop from a single fertilized egg that splits into two separate embryos. This division results in two individuals who share nearly identical genetic information and are always the same sex. Identical twinning is a random event and is not influenced by heredity.
Fraternal twins occur when two separate eggs are released during an ovulation cycle and are each fertilized by different sperm cells. These twins are genetically no more alike than other siblings, sharing about 50% of their genes, and can be of the same or opposite sexes. The conception of fraternal twins is where genetic predisposition primarily comes into play.
The Mother’s Role in Twin Conception
The mother’s genetic makeup plays a direct role in the likelihood of conceiving fraternal twins. This influence centers on hyperovulation, the release of more than one egg during a single menstrual cycle. When a woman hyperovulates, multiple eggs become available for fertilization, increasing the chance of conceiving fraternal twins if two eggs are successfully fertilized.
A genetic predisposition for hyperovulation can be inherited, making a mother more likely to release multiple eggs. Research has identified specific gene variants that contribute to this tendency, such as those affecting follicle-stimulating hormone (FSH) production or ovarian response. For instance, variants of the FSHB and SMAD3 genes have been linked to an increased likelihood of fraternal twinning by influencing FSH levels or ovarian sensitivity. This explains why fraternal twinning often appears to run in families, primarily through the maternal line.
The Father’s Contribution to Twin Births
While the mother’s body directly facilitates hyperovulation, the father’s genetic contribution to twin births is indirect. A father cannot directly cause his partner to release multiple eggs, as hyperovulation is a female biological mechanism.
However, a father can carry genes associated with hyperovulation and pass these traits to his daughters. If a daughter inherits these genes, she will have an increased chance of hyperovulating and conceiving fraternal twins. This explains why fraternal twins may appear to “skip” a generation; a father might carry the gene without experiencing twinning himself, but his female offspring could then manifest the trait. Thus, while a father does not directly influence his partner’s current pregnancy, his genetic legacy can contribute to twinning in future generations through his daughters.