the twinsGemini man has two or more children, developed jointly in one uterus and born almost at the same time. On the mechanism of occurrence of twins are identical and fraternal. Identical twins arise from one fertilized egg. After it was divided into two (rarely four) cells (blastomere), each of them in further developing in an independent germ. In the result of the two (or four) primary blastomeres are two (or four) of identical twins. In case of incomplete separation of embryos or as a result of their secondary mergers have so-called United twin (double or complex deformities), many of which are viable. One pair of twins conjoined twins(Siamese), fused at the chest, lived 63 years.
Identical twins are always the same sex, and, as a rule, are very similar to each other. Mutual transplantation between organs and tissues are always successful. Fraternal twins arise from two or more eggs, fertilized by different sperm. The similarity between fraternal twins are no more than between brothers and sisters by bliznetsami. Fraternal twins can be one - and heterosexual; their organs and tissue transplants are incompatible. How to determine odnoyaytsevy or raznoyaytsevy the pair of twins based on the so-called method of similarity. For a number of hereditary traits (groups and factors of blood, the color of eyes, hair, skin, the shape of the parts of the face and other) you can set the type of any pair of twins at the age of about one year and older.

Gemini - and two more calves, developed jointly in one uterus and born almost at the same time the animal, usually giving birth to one calf. The person on average, twins accounts for 85 singleton births, triplets - 8000; more multiple births occur only rarely. The frequency of births of twins varies in the countries with different climate. In countries with temperate climates, it is higher than in the South.
There are two main types of twins: one is so similar to each other, which is often not visible to parents; others like no more than the usual brothers and sisters. First derived from the same fertilized egg, divided into two (rarely more) independent education, each of which is a separate germ. This is identical, or monozygotic twins (ABOUT). Other twins come from two or more eggs, fertilized by different sperm. It's dizygotic, fraternal, or reseautage, Gemini (RB).
Each pair has ABOUT similar heredity, but to speak about the complete identity of genotype ABOUT because in somatic cells, as in sex, with some frequency, mutations occur (see), and division of chromosomes possible those or other aberrations. So, Wolff and others (E. Wolff, 1962) described a couple ABOUT to be uneven karyotype: one of them suffering from down's syndrome, was 47, others have 46 chromosomes, and he developed normally. Belarus inherited similar and hereditary different as brothers and sisters, born at different times. ABOUT - always the same sex; RB can be of different sexes, and the number of heterosexual Republic of Belarus is approximately equal to the number of same-sex.
Knowing the frequency of opposite-sex couples among bezotvetnoi series of twins, one can calculate the relative frequency and ABOUT RB. The mother who gave birth to a pair of twins, the possibility of re-birth of twins is several times more than the other women of her age. Still unclear role of heredity at birth or ABOUT RB. It is shown that with increasing age of the mother, the probability of birth of the Republic of Belarus increases, reaching a maximum of 35 to 40 years, and then decreases again. At birth, ABOUT the age of the mother is almost irrelevant.
Studies on twins, the so-called twin method is the best way to clarify the respective roles of heredity and environment variability in normal and pathological features of man (see Heredity). Both partners ABOUT and Belarus are moving around in the same conditions of the environment; therefore, mapping and ABOUT Belarus allows to judge about the role of heredity in the variability of the sign, and the comparison ABOUT, educated separately - on the influence of environmental conditions on the same genotype. You can study for and ABOUT the influence of different environmental factors (including unequal treatment and education), exposing one of the partners, their effects and using the second as control.
A comparative study of the incidence in and ABOUT the Republic of Belarus allows to establish the role of heredity and environment in the appearance of different diseases and defects (see table).

The concordance (similarity) twins in various pathological conditions (in %) [on the stern (C., Stern, 1960)]
Pathological conditions The concordance
Diabetes mellitus
Scarlet fever
Manic-depressive psychosis

It is established that heredity plays no role when measles, but has a value in susceptibility to tuberculosis, with the disease schizophrenia, diabetes and other
To determine whether data B. to the group or ABOUT the Republic of Belarus only on the basis of a study of embryonic membranes it is impossible, as Belarus has two CVS, but there are one or two harinama. This is widely used by so-called method of similarity, the essence of which is that B. vnutrivenno compare several any hereditary characteristics, and are considerably changing under normal conditions of environment. From ABOUT these symptoms are the same in most cases, and in the Republic of Belarus only in part. Practically use the comparison of some (about 20) legkoobratimy characteristics: colour of eyes, hair, the size and shape of the eyebrows and eyelashes, the shape of the nose and ear, blood group and other Necessary detailed assessment and vnutrivenno compared taken signs, not the solution of the question of their similarity in overall impression. With a certain skill and a thorough study of the older twins of the year "affinity" provides almost 100% accuracy.
To judge about the role of heredity on the basis of the summary published anecdotal cases it is impossible, as published mainly "interesting" cases where the characteristic features of both b, resulting in a role of heredity exaggerated. We must use bezotosnyi series B. there are two possibilities: 1) you can collect a series of twins (for example, in schools, in the lists of the maternity homes, and so on), to determine the type of their zivotnosti and compare the distribution among them of the studied trait; 2) if the sign of a rare (for example, illness, ugliness), among the ridge B. identify and received bezotvetnoi series compare concordant and discordant couples. Relative role of heredity and environment varies widely depending on the age of the study, as well as the time and place of gathering the material. Critical to use twin method. Sometimes abroad this method has been studies signs, completely socially determined. Twin method is important in the study of the genetic material of incomplete probleemist.