What defines an identical twin?

Identical twins are defined as individuals that originate from a single zygote, which is the fertilized egg formed when one sperm fertilizes an egg. This process involves the zygote undergoing early cell division, resulting in two embryos that share the same genetic material. Since they come from the same zygote, they have identical DNA, making them genetically identical.

In contrast, the other options refer to different concepts. For instance, clones produced by division generally imply that an organism is created from a single cell, resulting in an exact genetic duplicate, but this terminology usually refers more broadly to laboratory cloning techniques rather than biological twin formation. Twins from two separate zygotes refer to fraternal twins, which are genetically distinct and come from two different eggs fertilized by two different sperm. Lastly, a twin that has unique genes would not qualify as identical since genetic differences define this option. Therefore, the defining characteristic of identical twins is their origin from a single zygote.