Identical Twins


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Introduction

Identical twins are the result of a single fertilized egg splitting into two separate embryos after conception. They share a single placenta. Since identical twins come from the same fertilized egg, they have the exact same DNA, gender and blood type.
Even though identical twins have the same DNA, it can be expressed in different ways. This is affected by environment that the twins are exposed to (in the womb or out of the womb). As a result, identical twins usually have different fingerprints. 
How are they formed? 

Naturally


Fertilisation: One sperm fertilises the ovum and the nuclei of the sperm and ovum fuse.
Day 1: The fertillized egg, now called a zygote, divides once, resulting in two cells.
Day 4 to 5: Each zygote develops, and has about 100 cells by the end of Day 5.
Day 6:  The zygotes implant themselves in the endometrium (uterine lining). They will continue to develop for approximately nine months (like normal zygotes).

    Here's a self-made video to illustrate the process!
If the zygote divides after Day 13 of conception, then the cells forming the two embryos do not entirely separate and the twins are born conjoined (read more about conjoined twins in this post).


Artificially

  • Artificial embryo twinning uses the same approach as how identical twins develop naturally, but it occurs in a Petri dish instead of in the mother's body.
  • An early embryo is split into two individual cells.
  • Each cell is allowed to divide and develop on its own.
  •  A surrogate mother will then carry and deliver the embryos. Since all the cells come from the same embryo, the twins will be genetically identical.