DNA Day experts answer your questions about the genetics of twins

Above: Image © RichardUpshur, iStockphoto.com

Are there any cases of fraternal twins where one has a rare genetic disorder but not the other one?

Absolutely. In fraternal twins, they do not share the exact same DNA, so, as with other non-twin siblings, one might have inherited the disease causing mutation while the other does not.

- Answer provided by Dr. Karen Bedard

Do identical twins have the same DNA?

The answer is yes and no. Does every cell in twin #1 contain an identical sequence to every cell in twin #2 - NO. But, then again, cells within twin #1 are not all genetically identical. Somatic mutations (mutations during mitosis in your body cells, not your gametes) will result in small genetic differences between identical twins.

- Answer provided by Dr. Sean Myles

It is said that the ability to give birth to twins is genetic, does that information rely on DNA? If so are there other reproduction factors rely on DNA also?

Monozygotic twins is most likely due to information on the DNA, but the exact factors are still unknown. I know of many scientists that are looking for factors to help with in vitro fertilization. Right now we have some rudimentary ways of isolating eggs for fertilization, but our success rate for fertilizing these eggs is still low. I would think that there are DNA factors that make eggs more successful for fertilization, but we do not know what these are currently. I suspect that there are also proteins or other cellular factors that increase the viability of the cells.

- Answer provided by Dr. Dennis McCormac

If twin brothers married twin sisters, would their children look like twins?

Well it depends on whether the twins were monozygotic or dizygotic twins. I look like identical to my brother and we are not twins, so this would really depend on the families. But they could definitely look alike. In fact, the probability would be high that they would share many features.

- Answer provided by Dr. Dennis McCormac

In identical twins, can one have a disorder and the other be normal?

This is more complex. In identical twins, you would expect that if one has the disease causing mutation, the other does as well. However, for some genetic disorders there is something called "incomplete penetrance" where someone could have the mutation, but somehow manage to be just fine. Genetically inherited cancers for example sometimes also have an environmental component. Unfortunately, we don't always know what those factors are.

- Answer provided by Dr. Karen Bedard

Which DNA can make twins?

Hi there! From my understanding twins are not a result of a single gene or a set of DNA codes, although there seems to be a propensity for families with a history of twins to have more twins. In the case of identical twins, the fertilized egg separates into two and both develop into babies, while in fraternal twins there are two eggs both of which gets fertilized. If you want more information as a start, you can always check out wikipedia (http://en.wikipedia.org/wiki/Twin#Mechanism) but be sure to read the articles cited to get a better understanding!

- Answer provided by Ethan Yang

Is DNA different in identical twins?

- Answer provided by Dr. Gijs van Rooijen

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