Apologia Biology: Twins

Today, we were doing a pedigree activity on the blue-skinned Kentucky Fugates.  I distributed Pedigree Symbols and needed to explain monozygotic (identical) twins and dizygotic (fraternal) twins.  I had heard of maternal twins and mentioned it—but blanked on the details.  Here is an excellent link.  Once in a while an egg and polar body are released and both fertilized, the twins are ‘half-identical’, or more closely related than fraternal twins.  Lastly, here is an article about twins who developed after the egg was fertilized by two sperm and subsequently split.  Yes, all of us are facinated by twins.