Identical twins are also known as monozygotic twins. They result from the fertilization of a single egg that splits in two. Identical twins share all of their genes and are always of the same sex. In contrast, fraternal, or dizygotic, twins result from the fertilization of two separate eggs during the same pregnancy. They share half of their genes, just like any other siblings. Fraternal twins can be of the same or different sexes.
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Identical twins are also called monozygotic twins. They result from the fertilization of a single egg with a single sperm. And as those cells divide and multiply, at some point very early in embryonic growth they split into two individuals. So that you can imagine that the genetic material in these two identical twins is identical to each other, hence the name. The contrast to this would be fraternal twins, which are twins born at the same time. However, they are actually the result of fertilization of two different eggs, and thus can be different genders and look different, and have different characteristics. What's becoming increasingly clear is even though we call identical twins identical, and we think of their genetic material as being identical, we need to be reminded that there are environmental influences that may change, actually, the characteristics of identical twins, reminding us, as we already know, that even though identical twins come from the same genetic material, in characteristics they may actually have differences as well.
Donna Krasnewich, M.D., Ph.D.
Former Deputy Clinical Director, Office of the Clinical Director; Former Staff Clinician, Medical Genetics Branch
Dr. Krasnewich's research has investigated metabolically and biochemically based developmental delay disorders, especially those involving defects in sugar metabolism. Sugars are important for brain development and the proper functioning of other organs in the body. A group of genetic defects in sugar metabolism are termed congenital disorders of glycosylation (CDG), which result from abnormal carbohydrate metabolism, specifically the abnormal synthesis of N-linked oligosaccharides. About 400 cases of CDG are known worldwide, although given the variable clinical presentations of CDG, many individuals likely go undiagnosed.