Among twins, one in five is left-handed, while among non-twins, left-handed people are 2 times less. Scientists do not yet know the reason for this, but apparently there is some connection between the genetic causes that cause the development of two fetuses in the mother's body and their "appointment" by the leading hand with the left.
The number of twins in the modern population depends on the place of residence. In the USA, for example, the most "prolific" states for twins are New Jersey and Connecticut. A quarter more twins are born there. But in Hawaii and New Mexico, twins are almost a third less. The reasons, as usual, are unknown, but most likely this pattern is associated with dietary habits, taking certain medications, and, of course, heredity.
In addition to the most famous identical and fraternal twins, there are several more types of them. These are mirror twins, which outwardly are a reflection of each other, and in special cases, mirroring even extends to the internal organs. Polar twins are an extremely rare type, half of their genes are the same (from the father), the other half are different (from the father), so they are also called semi-identical. Chimerism - only 25 cases of such people are known, with this type two eggs are fertilized by two different spermatozoa, and then "mixed", exchanging genetic material. Outwardly, it may look as if one half of a person outwardly differs significantly (for example, in skin color) from the other. This phenomenon is much more common in cows. Superfecundation is the development of twins, when two eggs are fertilized by sperm from different fathers, after which two different babies are born (for example, a black and a white). There are even rarer types, but that is beyond the scope of this post.
Gemini doesn't have to be born at the same time. The longest period of time between births of twins is 85 days. Most often, a child with some kind of deviation or developmental difficulties is born first, after which the chances of survival for the child remaining in the uterus and developing are significantly increased.
Identical twins, with maximum external similarity, have fingerprints that differ from each other. This is often the only way to tell them apart.
In identical twins, the patterns of the encephalogram, that is, brain waves, practically coincide. This is one of the confirmations that they not only look, but even think almost identical, knowing what the other will say or feeling his state, including from a distance.
Early in development, twins often come up with their own language. This is especially evident if you observe children from the age of two, when the child begins to speak his "language" and the second twin picks him up, supporting the development of such a "dialect". In the case of a single child, this stage ends quickly, since he has no "like-minded people"
Identical twins can be heterosexual. It sounds strange, but scientists are familiar with such cases, and it is called Klinfelter syndrome (link).
Despite the seemingly absolute identity, the DNA of identical twins differs from each other. Previously, it was believed that environmental conditions lead to differences at the genetic level, but research has shown that despite the process of DNA copying during conception, individual sections of DNA remain unique from birth.
The maximum recorded weight of two twins is 12 kilograms 200 grams, such a happiness went to a mother from South Carolina in 2008.
Scientists believe twin syndrome is far more common than it seems. It's just that in most cases, the second fertilized egg in the uterus dissolves and does not receive further development.
The number of twins obeys the laws of symmetry. Among all twins, one third are identical, one third are simple twins of different sexes, and the remaining third are same sex. Among identical twins, half are twins, girls, and half are twins, boys. Among same-sex twins, half are girls, half are boys.