Monozygotic or identical twins are the only people in the world who have the same genotype. Twins' embryos are formed from a single zygote that is divided into two or more parts. Since ancient times, people who did not know the genetics and anatomy felt that the birth of identical people was something special. The twins were credited with either miraculous or diabolical properties.
For example, American Indians believed that one twin in the pair is always good, and the other is evil, so the evil twin was subjected to expulsion from the tribe. The Slavs believed that twins shared one soul, therefore the presence of one twin at a wedding or funeral of another was not allowed. In many cultures - Slavic, Indian - there was a belief that twins are able to protect against the weather and elements. Apparently, the very appearance of twins was seen as a whim of nature, therefore they were considered capable of making a deal with it. Indian shamans during rituals painted twins' bottoms black and white.
Modern scholars of genetics obviously do not do anything like this. Yet, they are also very interested in monozygotic twins and love to perform various studies with their participation. This is indeed a gift of nature that provides a unique opportunity to learn where the boundaries of environmental influence and genetic predetermination of certain features run.
Conducting studies for twins was first suggested in 1875 by the English scientist Francis Galton. His cousin, Charles Darwin, in a letter supported his brother: "Nothing strikes me as more interesting than the similarities and differences between twins," he wrote. This method is used in the study of probability of occurrence of diseases, the formation of personality traits, physical characteristics and so on. After all, with the same genotype, the similarities are most likely due to the genotype. Differences, respectively, are explained by the environment. However, this is not that simple.
First of all, not that long ago, scientists learned that even the genomes of monozygotic twins have differences. In 2008, the international group of scientists from America, Sweden and the Netherlands concluded that identical twins did differ genetically. The researchers analyzed the genomes of 19 pairs of identical twins and found differences in some copies of segments of DNA in somatic cells. In general, the genomes of monozygotic twins are very close, but almost all of them had small differences, the researchers noted.
Another difficulty is that most brothers and sisters grow up in a very similar environment. Especially the twins who were born at the same time and thus receive the same toys as gifts and are likely to go to the same school, and so on. Twins, perhaps, would love different writers, but their parents read the same books to them, they might have different preferences in food, but mom and dad offered them the same thing at lunch. Of course, it would be inhumane to forcibly put twins in the different living conditions. Yet, sometimes life separates people against their will.
In some cases twins discover that they possess different interests and professional inclinations. They choose different schools, acquire their own circle of contacts, learn different profession and each form their own environment. It happens so that brothers and sisters become separated due to historical events or family dramas after the death of their parents. In the 20th century this was especially true during World War Two. Family members were lost in the evacuation and children of the parents killed in the war were adopted by different families. To this day we see intriguing headlines to the stories of elderly twins who were born in those years and who got united at a much older age.
One of the larger studies of twins was conducted in the early 1980s by American psychologist Thomas Buchard. In all, he has studied 30 pairs of twins separated at an early age. One the described pairs of monozygotic twins were brothers Oscar and Jack. The boys were born from the marriage of a Jewish father and a German mother. Shortly after the birth the couple parted. The mother took Oscar to her homeland, Germany, and for a long time was hiding the fact that the boy had Jewish blood. Later, Oscar joined the Hitler Youth organization and became a devoted Catholic. Jack spent his childhood and youth with his father, at first in Palestine, then at the Caribbean islands, and was raised in the traditions of Judaism.
The brothers met when they were both over forty. Incredibly, despite the radically different education, place of residence and a different language, there was surprising number of similarities in their habits and characters. Both brothers had the same temperament, dressed in a similar fashion, loved the same food, and even wore similar glasses and trimmed their mustache the same way. Oscar and Jack were equally distracted, and their styles of communication in the family were incredibly similar.
Another glaring example is sisters Lois and Louise. The girls were born in 1920 and had been separated shortly after birth. They were adopted by different families and hardly saw each other in childhood. Their education level and residence were different: Louise lived in a small town and went to a local, very simple school, while Lois lived in a big city and attended a quite prestigious school. It is worth noting, however, that economic and cultural status of the families where they lived was very similar. Perhaps this was the reason why at the age of 18 the girls managed to enter the same college. The researchers note that in terms of a number of physical and mental traits this pair of twins proved to be very similar. The girls were even more similar than many brothers and sisters who have grown up together.
There is an amusing example when in the United States two twins separated in childhood have chosen the same, very narrow, "specialization". Independently of each other young people have become safecrackers. However, scientists draw conclusions about the role of heredity with great caution. For example, it was found that 50-70 percent of monozygotic twin pairs have similar criminal history, while in dizygotic twins (who in terms of genetics are regular brothers and sisters) this similarity occurs much less frequently, in 20 percent of cases. However, the results of some other studies contradict these data.
Researchers revealed some features that are most likely caused by genes. Nancy Peterson, a Swedish scientist, found that the level of extroversion or introversion of a personality is largely dependent on heredity. She also found that the level of neuroticism (emotional instability) is dictated by genes. In general, scientists agree that certain properties of an organism depend not only on the genotype or the living environment but their interaction. Yet, gemellology - science dedicated to studying twins - continues to evolve and occupy the minds of researchers.