How Fraternal Twins Form

• Fraternal -- or non-identical twins -- develop when two separate egg cells get fertilized by two separate sperm cells. Fraternal twins have different physical characteristics and features because they do not share similar chromosomes. "Fraternal twinning," the most typical type, accounts for about 40 percent of all twin pregnancies. Fraternal twins can be different or the same gender. Scientists also believe that fraternal twinning is a genetic trait.
  
  

How Identical Twins Form

• Identical twins, like a regular pregnancy, start off as a single egg cell that gets fertilized by a single sperm; however, as the zygote (the fertilized egg) develops, it divides itself in half and forms two embryos that develop into babies. Unlike fraternal twinning, scientists still do not know the exact reason why the zygote splits to form identical twins. Identical twins generally have the same sex, eye and hair color, as well as blood type. Their physical features look typically the same, with most twins having an exact mirror face of the other.
  
  

"Paternal" Twins

• The term paternal relates to characteristics or traits received from the father. In terms of twinning, this means that the offspring inherited characteristics mainly related to the father. It doesn't mean that the twinning occurred because of the father's sperm cell. In fraternal twins, the presence of two egg cells in the mother's womb is the main reason why twinning occurs. In identical twins, even though the main reason has not been identified, it has no specific relation to the characteristics of the sperm cell.
  
  

Twins with Different Fathers

• Heteropaternal superfecundation is the term given when twins have the same mother but different fathers.Jupiterimages/Goodshoot/Getty Images
  
  There are some cases where the fathers of twins are different individuals. With this, you may call the twins "non-paternal" twins because they are fathered by two different people. Although the idea may seem unusual, there are several instances that this occurrence was proven through DNA testing. One such instance occurred in May 2009 in Texas. Msnbc.com reported that a woman in Texas gave birth to two twin boys having different fathers.