The twin research method is used to identify the conditionality of signs of a hereditary nature. This method demonstrates the relationship of the genotype and the environment. The twin genetic method made it possible to evaluate the significance of a genetic predisposition to many pathologies, expressivity, penetrance, and also the conditions for the appearance of certain types of diseases. Research data are very useful in quantifying the hereditary determinism (determinability) of some traits. In this regard, the twin method is considered one of the most important research methods in quantitative heredity.
Gemini can be identical. They are formed at the early stages of zygote division, in which organisms (full-fledged) develop from two (rarely more) blastomeres. The identical twins are distinguished by genetic identity.
The authorship of this research method is attributed to F. Galton.
Despite the opportunities that the twin method possesses, today it does not have the wide practical application as before. This is due to the discovery of more accurate modern research methods that allow us to unequivocally identify a hereditary predisposition to a specific pathology.
The twin method involves the comparison of dizygotic and monozygotic twins (offspring, which consists of individuals born at the same time in mammals with a singleton).
Monozygous offspring develops in one fertilized egg. These twins possess 100% of the total genes. Thus, the differences revealed between them are not related to the hereditary factor. The development of dizygotic twins occurs in different eggs, which are fertilized by different sperm. This offspring has 50% of the total genes, as in ordinary siblings (descendants of one pair of parents - brothers, sisters). However, their simultaneous birth, co-education forms the factors of the general environment. Thus, the degree of difference of dizygotic offspring is determined by the degree of difference of genotypes.
As a result of comparing the data of two groups of twins, indicators of inconsistency (discordance) and conformity (concordance) are calculated. In addition, the frequency is calculated at which a sign or disease develops in each of the aforementioned hereditary groups.
The twin method is divided into several stages:
- Stage sampling.
- The stage of determining the type of zygosity.
- The stage of evaluating the results when matching pairs.
The above research method was of great importance in the study of heredity in behavior, many infectious and "multifactorial" (resulting from several reasons) pathologies.
In human genetics , not only the classical, but also the modified method of twin study is used. So, to study the possibilities for improving some intellectual characteristics using psychological training, we use the control method for the partner, and in the study of diabetes mellitus - the method of twin families.
It is convenient to observe the heredity of the psychological characteristics of intelligence and personality in descendants separated in early or infancy, brought up separately and not affected by the general environment and special interaction with each other.
Moreover, reliable data can not always be obtained. This is due to systematic differences between non- twins and twins on certain grounds, as well as social and psychological characteristics in the development process in the postnatal period. These factors and limit the scope of the above method.