Also called monozygotic twins, identical twins are generally two lookalike babies born at the same time. Contrary to common belief, the term “identical” refers to the twins’ conception and development, and not their appearance. These twins conceive from a single sperm and egg and therefore have the same DNA. They share the same chorion and amnion (sac), and may share the same placenta or have separate placentas. The biology behind a fertilized egg splitting into two is still not clear. But an identical twin pregnancy is not hereditary and such pregnancies occur randomly.   

Identical twins. Image credit: Flickr
Identical twins. Image credit: Flickr

Approximately a third of all twins born are identical. And 25 percent of identical twins are symmetrical to each other. In other words, a twin’s right side matches the left side of the other twin. Generally, identical twins have the same hair and eye color, and blood group. Teeth marks and fingerprint details can help tell the twins apart.

Behavior

Identical twins don’t behave alike; they are as unique as other siblings with distinct preferences and dislikes. But since most of these twins spend extensive time with each other, they’re assumed to have similar behavior too.

Male-Female Identical Twins

Identical twins are usually the same gender, but there are extremely rare exceptions. This is due to chromosomal birth defects. Therefore, though the opposite-gender twins originated from the same ovum or egg, they cannot be technically referred to as genetically identical, as they have distinct karyotypes.

Mirror Image Twins

Also called mirror twins, mirror image twins is a subpopulation of identical twins, who have similar physical traits but opposite features. In other words, one will be right-handed and the other a lefty. That’s why they’re referred to as mirror images of the other. These mirror twins could even have matching fingerprints and the same DNA structure.

Non-Lookalike Identical Twins

Some identical twins may not look alike. Though biologically programmed to look identical, they may not share the same facial features due to their positioning in the womb and exposure to environmental factors and life experiences post birth such as bruises, bumps, and differing hairstyles. One’s diet, sleep, illnesses, etc. also play a role in the overall appearance. As a result, some identical twins may have different weight and height. This thus proves genetics aren’t solely responsible for an individual’s physical make-up.