The genetics of identical twins is a fascinating field of research. If you have identical twins in the family, or if you are one yourself, you must have closely experienced the unique bond that twins share. They have a very close relationship and are believed to have a kind of unusual communication, perhaps telepathy. They resemble so much in physical appearance, that it is difficult to tell one from the other, but they aren't identical in the true sense of the term. Two offspring born from the same pregnancy are known as twins. They may or may not be alike or of the same gender. Statistics say that 31 in every 1000 women give birth to twins.

• When one egg is fertilized to form a zygote, which then splits to form two embryos, identical twins are born. They are also known as monozygotic. Their DNA is very similar but not identical.
• When two eggs are fertilized at the same time, non-identical twins are born. They are also referred to as fraternal or dizygotic. These twins have different DNA.

The study of twins interests genetic researchers around the world. Research has revealed some of the genetic factors in women that are responsible for multiple births while much research is still to be done. To begin, let us take a closer look at the concept of identical twins. When a single egg is fertilized forming one zygote, which later divides into separate embryos, identical twins are born. As they result from the same zygote, they are known as monozygotic twins. In most cases, the zygote divides into two embryos after the first two days of its formation and results in a shared placenta with separate sacs. Identical twins belong to the same gender and share many aspects of their physical appearance. The DNA of identical twins is nearly identical but environmental conditions influence certain genetic factors. Environmental factors and external elements affect the switching on and off of the genes in them. This phenomenon is known as epigenetic modification. A survey conducted on twins of varying age groups revealed that the epigenetic differences increase with growing age. It also brought out the fact that twins who had spent their lives apart showed greater differences. Epigenetic differences result from epigenetic processes that the DNA undergoes. One of the major epigenetic processes is methylation. It is a process by which the gene expression changes with ascending age. Due to this process, identical twins grow more and more dissimilar with growing age. Though identical twins share the same DNA, genes express themselves differently, leading to differences between the twins. In other words, though identical twins have the same genotype (because of identical genomes), their phenotype isn't the same. For example, they do not have identical fingerprints. In most cases, they have the same hair and eye colors. In rare cases, the colors are different. And then, it's not only genes that determine one's personality. Many external factors (one's environment and life experiences) also contribute to shaping one's personality. Identical twins, though genetically similar, are two different individuals, and thus, different in some ways. In certain cases, identical twins have different copy-number-variations (CNVs). By this we mean that one of the identical twins can have a DNA segment missing, have multiple copies of the segment, or may even have a different orientation of the genome. This explains the reason for dissimilarities between identical twins. Human beings receive one chromosome from their mother and one from their father, thus receiving two copies of the genome. If bits of DNA are missing from a chromosome, the offspring is left with only one copy of that DNA bit. In certain cases, multiple copies of a bit of DNA are produced, resulting in mutations. This results in differences in the health and development of identical twins. While nuclear DNA is inherited from both the parents, mitochondrial DNA is inherited from only one parent, the mother. Mitochondrial genome mutations are passed on from the mother to all her offspring. But the number of affected mitochondrial DNA molecules that each offspring inherits, may vary. In case of twins, one may receive bulk, while the other receives only a small portion of the mutant mtDNA molecules. According to a recent genetic research, monozygotic or identical twins are formed as a result of the collapse of a blastocyst, leading to division of the genetic material on two sides of the embryo. Wondering how this happens? Let's start with what a blastocyst is and then understand how the genetic material is divided. A blastocyst consists of an inner cell mass known as embryoblast and an outer cell mass known as trophoblast. The embryoblast eventually forms the embryo, while the placenta is formed from the trophoblast. When the blastocyst collapses, the progenitor cells, which contain the genetic material of the body, split. This results in a division of the genetic material in two on the opposite sides of the embryo. This leads to the formation of two fetuses having similar genetic material, thus resulting in the birth of identical twins. Interestingly, the children of identical twins are half-siblings and not cousins. Similarly, children reproduced from the marriage of twin brothers to twin sisters, are full-siblings. Isn't that amazing? Imagine, your mother has a twin sister, who has given birth to a daughter. Now, you and her daughter are half-siblings since you were born to mothers of the same genetic constitution. Now imagine, identical twin brothers of family A marry identical twin sisters of family B. Genetically speaking, their children will be full-siblings. The formation of identical fetuses during pregnancy, resulting in the birth of twins, is a wonder of nature. Much is still unknown about the genetics of identical twins and scientific research in this field continues.