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 of 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



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.