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Answers to the questions on the previous page.

As you probably know, twins come in two types – fraternal and identical.  Fraternal twins are produced from two different eggs that are each fertilized by different sperm, which results in two embryos with different genetic make-up.  Identical twins are produced when a fertilized egg divides in two, and both "halves" grow into identical twins that have the same genetic make-up.

To answer your question, the propensity to bear fraternal twins does run in families, but one generation doesn't necessarily have a higher probability of having fraternal twins than another.  Fraternal twins occur in about 12 of every 1000 births.  There is no evidence that the likelihood of having identical twins is impacted by genetics.  Identical twins occur in approximately 4 of every 1000 births, and are no more common in any one group of people.

Fraternal twinning is caused by inheriting a gene that makes it more likely for those with the gene to have fraternal twins than those without that gene.  This gene on the X chromosome may cause hyper-ovulation – when a woman's ovaries release more than one egg per ovulation cycle.  A woman can inherit the hyper-ovulation gene from either of her parents.  However, if she inherits it from her father, the gene will appear to "skip a generation" because men cannot affect whether his partner's ovaries will release multiple eggs. Thus, a man with a family history of twins is not more likely to father twins himself. But, if a man has fraternal twins in his family, he can pass the twin gene on to a daughter. 

The probability of fraternal twins is, therefore, impacted by whether a particular generation has more males or females. If your grandparents had all boys, none of them would contribute to multiple births in the next generation. But if your grandparents had daughters along with your twin uncles, those aunts would also have an increased probability of having twins.

There are other factors that appear to influence the probability of a woman releasing more than one egg per cycle. Women of African descent are two times more likely than Caucasian women to have twins, and four times more likely than Asian women. Women who are well-nourished, who are between the ages of 20 and 35, or who have had children, all have a higher probability of having fraternal twins.

So, if you are female and your uncles were fraternal twins, it is likely that you have a higher probability of having twins than many other people. But no need to run out and buy the double baby carriage yet, because the chances are still pretty small of two babies appearing at once!

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