Fraternal twins usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilized by two different sperm cells, fraternal twins result.
Dizygotic twins, like any other siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be a different sex or the same sex. Mixed-race twins, or twins born to parents of mixed racial origin, can vary considerably in their skin colouration and other features.