What's the Difference Between Fraternal and Identical Twins?
When a single egg is fertilized by a single sperm, then divides into two embryos, the result is identical twins. These separate embryos are genetically identical and are always the same sex. Fraternal (or nonidentical) twins occur when two separate eggs are fertilized by two different sperm. The babies will not look exactly alike. Naturally, twins made up of one boy and one girl are always fraternal.
In the case of triplets, quads, and quints, either -- or both -- of the processes above may be involved. For example, one fertilized egg could split three times (resulting in identical triplets), three different eggs could be fertilized by three different sperm (fraternal triplets), or two eggs could be fertilized by two sperm, then one of those eggs could split (fraternal and identical triplets).