If two or more children are born as a result of one pregnancy, they are called twins. The development of embryos in the womb can occur in different ways, there are identical and fraternal twins, which differ in how many eggs gave them life.
In the wild, many animals often have many babies at the same time, but humans have one twin for every 250 births of one child. Scientists distinguish between monozygotic and dizygotic twins, and in colloquial speech they are called identical and fraternal or twins and twins.
Identical twins
Identical twins develop from a single fertilized egg that has split into two or more parts that have begun to develop separately. Thus, all embryos receive the same set of genes and develop in about the same way. Such children always have the same sex and are very similar to each other, sometimes even their parents cannot distinguish between them. Although sometimes they have mirror features: for example, one has a mole on the right side, another has a mole on the left, one is right-handed, and the other is left-handed. They have similar character traits, almost the same temperaments, predisposition to disease and often even fate.
There are cases when identical twins separated in childhood, who did not know about each other, lived almost identical lives.
Cases of the birth of monozygotic twins are several times more rare than that of fraternal twins, since the division of an egg into several parts is uncharacteristic for a normal pregnancy.
Fraternal twins
Fraternal twins have a different set of genes, since different sperm and eggs were involved in their conception. In some cases, in women, two or more eggs ripen in one menstrual cycle at once, if all of them are fertilized, then a multiple pregnancy will begin, and two or more children will be born. These are quite common cases, in some nationalities they are very common - it is believed that multiple pregnancy is of a genetic nature.
The genotypes of fraternal twins, which are also called twins, are different, so children can be of different sexes and generally differ from each other, like ordinary brothers and sisters. It is known that with age, the likelihood of multiple pregnancies in a woman increases.
In countries where it is customary to give birth after the age of thirty, for example, in the United States or European countries, there are more cases of birth of fraternal twins.
There is another type of twins, which is called transitional between the two described - polar or semi-identical. This happens when two different spermatozoa fertilize the egg and the polar body that forms with it (it usually dies off). As a result, embryos have a half different set of genes, the same from the mother.
