Expert: Dana Krempels, Ph.D. - 7/15/2008

Question
Does the decision of twins come from the Female side or the Male side or does it not matter? is there more a likelihood of idential twins rather than non-identical if on the mans side both his parents are identical but on the womans side the twins in the family are non-identical. Thanks

Answer
Dear Valmal,

Identical twins are produced when a single fertilized ovum (zygote) or embryo divides completely, rather than staying connected, during cell division.  A single cell separated from an embryo is capable of growing into a complete fetus if this occurs early in development.  

No one really knows for certain, but it is more likely that the tendency for a zygote to split completely may be more a function of the *egg* than the sperm, which would make it more of a maternal thing.  But in many cases, no cause other than random chance can be named for causing this split.

Non-identical, or fraternal, twins are produced when a woman ovulates more than one ovum, and both are fertilized by the male's sperm.  These are essentially just regular siblings who happen to be conceived and born at the same time, and each is genetically unique.

Again, the tendency for ovulation of multiple ova in a single month is most likely determined by the female parent's own genes.  And also there are many incidences of multiple ovulations that are not necessarily due to a "family tendency," but rather to random chance or environmental factors (such as fertility drugs!).

Hope that helps.

Dana