Identical twins (also called monozygotic twins) result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two. Identical twins share the same genomes and are always of the same sex. In contrast, fraternal (dizygotic) twins result from the fertilization of two separate eggs with two different sperm during the same pregnancy. They share half of their genomes, just like any other siblings. Fraternal twins may not be of the same sex or have similar appearances.
Identical twins. There are many classical studies that looked at twins to try to figure out how genetic a health condition was. That is, these studies will look at identical twins and nonidentical twins. The question was, if one twin has the condition, does the other twin have it, too? By comparing these rates between identical and nonidentical twins, researchers could try to estimate how much genetics was involved in that particular health condition. So twins have always had a very special place in genetics.
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