The twin method

Twins

 * Dizygotic – DZ = "two ovoid", fraternal;
 * they arise from the fertilization of two eggs, each with one sperm ;
 * from a genetic point of view, individuals created in this way are in the same relationship as normal siblings ( relationship coefficient r = 0.5).
 * Monozygotic - MZ = "single egg", identical
 * they arise from the division of one zygote in the early stages of ontogeny ;
 * from a genetic point of view, such individuals have the same genetic make-up ( relative coefficient r = 1);
 * their epigenetic makeup (e.g. DNA methylation ) is not completely identical and this difference continues to increase during life.

The Twin Method

 * Also name geminological method ;
 * is a special genetic method that is largely specific to human genetics;
 * in the case of alternative signs, it is based on the determination of concordance (similarity) and discordance (difference) between members of a twin pair and on the ratio of the number of concordant and discordant pairs;
 * the first step is determining the so-called zygosity of the twin pair;
 * divide the set of twins into a group of MZ pairs and DZ pairs;
 * if obstetrical data are available, it is possible to base them on them; in other cases, it is possible to assume that the members of the MZ couple must be concordant in all genetically determined traits, such as blood groups ;
 * with a sufficient number of characters of a similar type, we can fairly reliably divide a set of twins into a group of MZ and DZ;


 * the so-called Weinberg method :
 * determines the so-called a priori probability of zygosity ;
 * the phenotypic character used here is sex ;
 * all MZ couples must be of the same sex ;
 * if we assume that the probability of a boy being born is the same as that of a girl, then gender-discordant pairs represent half the number of DZ pairs

(boy, boy: girl, girl; girl, boy: boy, girl);


 * then twice the number of discordant pairs is the total number of DZ twins in the set; the remaining (concordant) pairs are MZ twins;
 * after dividing the set of twins into DZ set and MZ set, it is possible to compare between these sets the representation of concordant or discordant pairs for a given character.

A number of statistical procedures have been proposed for the quantitative expression of heritability in twin studies, among the classics is the Holzinger heritability index:

$$ H = \dfrac{Kmz-Kdz}{1-Kdz}$$.

where Kmz is the relative representation of concordant pairs in the MZ twin group and Kdz is the relative representation of concordant pairs in the DZ twin group.


 * The twin method is also widely used in the heredity of quantitative human traits - appropriate statistical procedures allow heritability estimates for multifactorial traits

Related articles

 * Genealogy
 * Heritability
 * Multifactorial inheritance

source
ŠTEFÁNEK, Jiří. Medicine, diseases, studies at the 1st Faculty of Medicine, UK  [online]. [feeling. 2/11/2010]. < https://www.stefajir.cz/ >.

Reference

 * 1) FRAGA, MF. Epigenetic differences arise during the lifetime of monozygotic twins.  PNAS. 2005, year 102, pp. 10604-9, ISSN 1091-6490.