November 8, 2012

Eva Gusnowski

In the 1980s, Universal would have had us believe that Danny DeVito and Arnold “The Governator” Schwarzenegger were Twins. While the “genetics” of this show is highly suspect (read: false), twins do seem to run in families. But since I’ve never seen a School House Rock episode explaining twinning, here’s an overview of the real genetics.

Let's start with an overview of twins. There are 2 types of twins: fraternal and identical. Fraternal twins are ones that are essentially siblings that have been fertilized and born at the same time, meaning that they have come from two separate eggs and two separate fertilization events. Because they come from two different eggs, you may see these referred to as dizygotic twins (di = two, zygote = what a fertilized egg is initially called). Identical twins are ones that come from only one egg that has split into 2 people...these are the ones that are "real" twins, and share essentially identical copies of their genomes. Because identical twins come from a single egg, they are referred to as monozygotic (mono = one, zygote = fertilized egg). So the question is: why do these duplicitous individuals exist?



Let's first start with identical twins to get it out of the way. There is no known genetic link as of yet to the development of identical twins. There are a number of theories in place, but so far the development of monozygotic twins appears to be entirely due to chance. In the future there may be a hereditary cause found for the development of these duplicates, but as of now anyone who wants them will have to rely on a hope and a prayer.

Fraternal twins, on the other hand, do have a known genetic component in their conception. This has absolutely nothing to do with the male contribution to fertilization, other than the fact that two fertilization events occur. Rather, this is the result of an event in the female reproductive tract: the liberation of multiple eggs at one time from the ovaries (called "hyperovulation"). Hyperovulation has been linked to genes that direct an increase in the hormones responsible for the release of eggs from the ovaries. Since these alleles can from both their mother and/or their father, females can inherit the hyperovulation ability from either side of their family.

So if you're looking to start an Olsen twin empire, you do have a few options. Just don't rely on Uncle Joey and Uncle Jesse to help babysit.




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