are identical twins genetically identical
Zygosity of fraternal and identical twins. Zygosity refers to the similarity between the genomes of each twin. Fraternal twins are dizygotic, meaning that they developed from two different eggs fertilized by two different sperms. Their genetic code is as similar (or dissimilar) as any other siblings. Identical twins are monozygotic, meaning that they developed from the same fertilized egg, and so have nearly identical genetic code. They almost always have the same gender and many of the same physical characteristics (phenotypes).