Environmental distinctions between twins arise and at the subsequent stages of development even at education in one family. Most often leads the prejudiced attitude of parents towards each of twins, thus the physical features which arose at a stage of pre-natal development and childbirth to it are aggravated. Also often there is a division of duties between twins (a case of the complementary relations), division of couples according to the principle "the leader - conducted".
During pre-natal development twins often appear in unequal conditions. So, all nutrients and oxygen come to a fruit through a placenta. In total DZ twins and about one third of MZ of twins has separate horiona and placentae. Other two thirds of MZ of twins have the general and a placenta. In this case in fetal covers of so-called monokhorionny twins various connections (shunts) between vascular systems of twins are formed. In case of formation of the arterio-venous shunt there is a connection of an artery of one twin with a vein of another. Thus to one of twins can not get rich with oxygen and nutrients of arterial blood, possible surplus of that and another at the second twin can also not promote normal development. Fortunately, usually there are some shunts, approximately equal on power, compensating each other. If compensation is insufficient, one of twins develops in the conditions of deficiency of oxygen and nutrients. In this case at the birth the considerable difference between twins, first of all in weight is observed. The similar difference can be observed and at DZ of twins and dikhorionny MZ of twins because of uneven squeezing of placentae at polycarpous pregnancy.
Therefore the experiments made according to the above-stated schemes demand additional verification. It can be two types. First, it is possible to check a hypothesis of similarity of the MZ and DZ circle of twins, that is to prove that the studied characteristic is not influenced by distinctions in the environment of MZ and DZ of twins. But similar check is very difficult and possesses low reliability.
Methods assume the obligatory statistical processing specific to each method. The most informative ways of the mathematical analysis demand simultaneous use at least of the two first methods.
In this method intra pair comparison of the twins separated at early age is carried out. If MZ twins were separated in this way and grew in different conditions, all their similarity has to be defined by their gene identity, and distinctions – influence of environmental factors (see drawing).
The stage of childbirth can also cause strong environmental distinctions for twins. The twin born the first has bigger chance to get a patrimonial trauma. At the same time the second twin most often holds the wrong position in a uterus that results in need of artificial obstetric aid. Besides the second twin is in childbirth and respectively more long more long and more sharply experiences oxygen starvation that has an adverse effect on development of nervous system.
Assumes research of specific twin effects and features of the intra pair relations. It is used as an auxiliary method for check of justice of a hypothesis of equality of environmental conditions for partners of MZ and DZ of couples.
If the provision on equality of environmental conditions of development of MZ and DZ of twins is not observed, estimates a component of fenotipichesky dispersion (heritability, dispersions of effects of the general and differing environment) are distorted. Similar distortion can happen in some cases:
Because of features of development of DZ and MZ of couples of twins it is considered to be a classical twin method and its versions "nonrigid" experiments: in them it is impossible to divide unambiguously influence of genetic and environmental factors as for a variety of reasons conditions of development of twins for a number of reasons are incomparable.
Thus, if environmental conditions have various impact on formation of the studied characteristic at MZ and DZ of twins, the indicator of heritability of this characteristic can be distorted: underestimated if the general environment makes a smaller contribution to similarity of MZ of twins, than to similarity of DZ of twins; the overestimated – otherwise.