Fraternal, or
dizygotic twins are formed from two separate fertilized eggs (two ovum and two sperm). They can be same
sex or different sex. Unlike
identical twins,
fraternal twins share fifty percent of their genes as opposed to one hundred, and are akin to normal siblings (from a biological standpoint, anyway).
The likelihood that twins will be fraternal is about two-thirds. About a third of all twins are same-sex fraternal twins, and a third are different-sex fraternal twins. The other third is identical. Fraternal twins can look almost identical, so sometimes it's difficult to tell if twins are fraternal if they are the same gender. The only way to tell conclusively is a DNA test.
Although some studies have shown that generally identical twins are more altruistic towards each other, there are many exceptions to that rule.