Twins are two offspring produced by the same pregnancy. Twins can either be monozygotic (identical), meaning that they can develop from just one zygote that will then split and form two embryos, or dizygotic (fraternal), meaning that they can develop from two different eggs, each are fertilized by separate sperm cells. Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. Identical twins do not have the same fingerprints, due to the fact that even in a small space inside the womb, people have contact with different parts of this environment, which gives small variations in the same digital, making them unique. No photographer credited, 1868.