A method to study the relative effects of heredity and environment on individual phenotypes by comparing the differences and similarities between twins
Twin method is a method to study the relative effect of heredity and environment on individual phenotype by comparing the differences and similarities between twins.It is a classic method in human genetics research, which was first created by British scholar F. Galton in 1875.Twins are two fetuses formed simultaneously in the mother's womb, commonly known as twins, and are the most common type of human multiple pregnancy.
Twins are usually divided into monozygotic twins (monozygotic twins or identical twins) and dizygotic twins (dizygotic twins or fraternal twins).
Monozygotic twins are generally two fetuses developed after fertilization of one sperm and one egggenotypeThey are identical, so they are also called full twins (see figure).In general, twins are formed by fertilizing two eggs discharged from one or both ovaries at the same time with spermgeneThe same degree of type A is the same as that of each single pregnancy fetus of the same mother, so it is also called heterozygotic twins.
The incidence of monozygotic twins is roughly the same among different ethnic groups, but there are some differences in the incidence of dizygotic twins. The incidence of dizygotic twins is higher among white and black people, but lower among yellow people.The incidence of twins is also related to the maternal reproductive age, birth order and other factors. The larger the reproductive age, the higher the birth order, the higher the incidence, especially for the twin twins.In addition, the occurrence of twins also has a certain tendency of family aggregation, which indicates that the occurrence of twins is related to genetic factors.
Identification of twins
Announce
edit
To apply the twin method, we must first identify the type of twins accurately.There are generally three methods for egg sex identification:
transplantation
According to the identification method of compatibility between twins for mutually transplanted tissues.The commonly used transplantation tissue is skin. If the genotype of donor and recipient is different, the transplanted skin will be rejected due to tissue incompatibility, which indicates that the twins are twins.On the contrary, monozygotic twins have the same genotype, so the graft is easily accepted without rejection.The result of this method is relatively reliable.
Fetal membrane method
Identification method according to the fetal membrane structure of twins.Dizygotic twins generally have two placentas, and a few have conjoined placentas. The diaphragm of conjoined placentas consists of two layersChorionAnd two floorsamnionform;Monozygous twins only have a single placenta, and the fetal membrane diaphragm consists of only two layers of amnion, and a few may have no amnion diaphragm.This method is not reliable and can only be identified at the time of delivery, so it has certain limitations in use.
Similarity method
It is a discrimination method to compare the similarities between twins by using lifelong and independent anthropological indicators.This law originated from the Sherman Multiple Similarities Law in 1924.The indicators used in the early years included skin texture, hair color, iris color, tooth shape and other general body surface characteristics. Later, a variety of immunological indicators were widely used, such as ABO, P, MN, Rh, Xg and other antigens of the red blood cell blood group system, ABH secretion type, HLA antigen and so on, which is also called blood group similarity method.This method is the most reliable, and it avoids the traumatic operation in the transplantation method. It is an ideal method.
The reliability of using similarity method to judge monozygous twins can be estimated according to statistical methods.The so-called reliability refers to the probability that the fetus is a monozygous twin when several indicators adopted are consistent, expressed as P (MZ), and then expressed as P (DZ) when several indicators are consistent due to accidental opportunities, then P (MZ)+P (DZ)=1.
Since the probability of consistency of n indicators (PD) of a dizygotic twin is the continuous product of the occurrence rate of each indicator, that is, PD=P0D × P1D × P2D ×... × PnD (P0, P1, P2,..., Pn respectively represent the probability of consistency of each indicator due to accidental opportunities, including the gender probability and egg sex ratio).Therefore, the total probability of a dizygotic twin is P (DZ)=PD/(1+PD);The total probability of monozygotic twins is P (MZ)=1 - [PD/(1+PD)].
Twins
If these indicators of twins' parents are known, the probability of meeting the same indicator in a pair of twins can be calculated according to the genetic law.In reality, parents' indicators are often not measured, which can only be based on the long-term survey of the indicators in the regiongene frequencyTo estimate.According to the calculation, if more than ten blood type and serotype identification items are consistent, the reliability of identifying monozygotic twins can be as high as 98~99%.
application
Announce
edit
Classical genetics shows that the phenotype of any organism is the product of the interaction between genes and environment, but the degree is different.The monozygotic twins with one sperm and one egg are naturally identical in genotype, and any difference between them should be the result of different environmental factors in utero or after birth. Therefore, the relative effects of heredity and environment can be estimated through the comparative study of twins on the basis of accurately judging the type of twins.
The similarity between twins in a certain character is called consistency, and the dissimilarity is called inconsistency.Under the same environmental conditions, the genetic effect can be estimated by comparing the consistency ratio between monozygous twins and dizygous twins.The research results of twin method indicate that for human diseases such asSchizophrenia、Manic melancholia、Congenital stupidityIn other words, monozygotic twins have higher concordance rate of the same disease than dizygotic twins, which indicates that genetic factors play an important role in the occurrence of these diseases (Table 1).
There are different calculation methods for the consistency ratio (CW) of twins, one of which is the paired method, and the calculation formula is CW=C/(C+D), where C is the total logarithm of twins with identical traits, and D is the total logarithm of twins with inconsistent traits.
The degree of genetic effect can be expressed by heritability (h2), h2=r/R. Where r is the correlation coefficient and R is the genetic coefficient.Therefore, for monozygotic twins, h2=r; for dizygotic twins, h2=2r. For polygenic genetic diseases, the correlation coefficient (r) of the threshold character can be estimated according to (xg xra)/ag (x represents the normal deviation between the threshold value and the population average value; b represents the average deviation between the patient and the population average value;Subscript g refers to general population;Subscript ra refers to relatives of patients), where x and α values can be obtained from the population frequency of this trait and the consistency rate of twins.For example, the correlation coefficient (r) of quantitative traits such as height, weight and blood pressure can be estimated according to (twin pair variance - twin pair internal variance)/(twin pair variance+twin pair internal variance).If the genetic effect is only roughly estimated, it can be calculated as follows: H=(CMZ-CDZ)/(1-CDZ) or H=(rMZ-rDZ)/(1-rDZ).Where CMZ and rMZ are the consistency rate and correlation coefficient in monozygous twins, and CDZ and rDZ are the consistency rate and correlation coefficient in dizygous twins.
By comparing the consistency of several pairs of monozygotic twins raised under the same and different environmental conditions, we can calculate the size of environmental effects: E=(rMZT rMZA)/(1-rMZA) (E: environmental effects; r: correlation coefficient;MZT: cohabiting monozygotic twins;MZA: separated monozygotic twins).The calculation results of environmental effects show that height has little relationship with environmental factors, while IQ has a great dependence on environmental factors (Table 2).
summary
Announce
edit
The genetic effect and environmental effect derived from the twin method are only a general concept, which is not conducive to understanding the complex interaction mechanism between them. Twins can have certain differences between the two members after delivery due to different conditions in the womb.The fact that twins are lighter in birth weight and lower in intelligence than the general population also affects the application of the twin method.These are the limitations of the twin method.However, because of its inherent convenience, the twin method was once one of the main methods of human genetics research, and is still widely used in medical genetics, eugenics, behavioral genetics, human intelligence and other research fields.Since the 1970s, the family investigation of twins, especially the research on genetic markers such as biochemistry, cytogenetics and immunology (such as histocompatibility antigen), will certainly open up a broader research field for twins.[1]
