|MadSci Network: Genetics|
Well, unlike identical twins which arise from the spliting of a single fertilized egg, fraternal twins arise from two different fertilized eggs (i.e. each fertilized by a different sperm). These separate fertalized eggs have the same genetic relatedness as ordinary siblings (brothers and sisters). Thus only half of the genes in the twins are identical. Basically fraternal twins are no different than two siblings born several years apart. It was just that 2 eggs were released into the fallopian tube of the female instead of one. Hope this helps! -Mike Reference: Hartl DL, Jones EW (1998) Genetics : principles and analysis. Jones and Bartlett Publishers, Sudbury, Mass. pp 682.
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