Twins

TWINS

Identical Twins

Identical twin births occur when a fertilized egg splits.
Each twin's fetus has the same genetic traits.

Fraternal Twins

Fraternal Twins are much more common than Identical Twins.
These births occur when two eggs are released at the same time,
and both become fertilized. Their genetic composition
is the same as any siblings.

Half-Identical Twins

Half-identical twin births occur when an unfertilized egg splits,
and the two eggs are fertilized by two separate sperm.
The fetuses share approximately half of their genetic composition,
which they receive from their mother.

Twins by Different Fathers

The rarest form of twin births is called
Heteropaternal Superfecundation.
This occurs when an egg is released,
even though another egg has been fertilized.
If the second egg is fertilized by another man's sperm,
these fetuses would be no closer genetically than half siblings.

Here are some links concerning
Heteropaternal Superfecundation.
Twins by two different fathers

http://wsrv.clas.virginia.edu/~rjh9u/twindad2.html

http://wsrv.clas.virginia.edu/~rjh9u/twindads.html

http://www.thetherapist.com/Bows_Rele_040797.html

Four Parents: One Pregnancy: Two Non - Twin Babies
http://wsrv.clas.virginia.edu/~rjh9u/nontwins.html