GENETICS - Textbook - A.V. Sivolob - 2008

CHAPTER 7. Human Genetics

HUMAN QUANTITATIVE AND MULTIFACTORIAL TRAITS

Twin and Adoption Studies

The strong dependence of multifactorial traits on environmental factors complicates their genetic analysis and the identification of the genes responsible for them. Even within the same family, it is sometimes difficult to determine whether similarities in certain traits among relatives confirm the heritability of the trait or merely reflect identical living conditions. To separate environmental factors from the Influence of the genotype on trait expression, Human Genetics employs Twin and Adoption studies.

The twin method is based on comparing the concordance of trait expression in twins. In humans, twins are born at a frequency of approximately 1 pair per 84 births (triplets: 1 per 7,000). One-third of these are monozygotic or identical twins. They develop from the blastomeres of a single fertilized egg that split during early Cleavage stages (most commonly at the two-Cell stage), giving rise to separate organisms. Consequently, monozygotic twins share identical genotypes. Dizygotic twins, on the other hand, develop from two eggs that matured and were fertilized simultaneously. Genetically, they are no more alike than full siblings born at different times.

When examining same-sex twin pairs, it is crucial to accurately determine their zygosity. This is done either by assessing the similarity of traits that show minimal dependence on environmental factors (such as Blood Groups for various Antigens, eye color, The ability to taste PTC, etc.) or by using DNA fingerprinting. Traits whose expression matches in a pair of twins are called concordant (if the trait is absent in one of them, discordant).

METABOLISM/18.html">The Influence of heredity on trait expression is estimated by comparing trait concordance (the percentage of twin pairs with identical expression of the trait in question) between monozygotic and dizygotic twins (Table 7.3). Naturally, since monozygotic twins share the same genotypes, any phenotypic differences between them are exclusively due to environmental influences during prenatal development and after birth. As for trait similarities, they can be explained by both identical genotypes and similar living conditions. Of particular interest in this regard are cases where pairs of monozygotic twins were separated in childhood and raised and grew up in different environments for a prolonged period. Concordance in such pairs is explained

by the decisive influence of the genotype on the expression of certain traits. The analysis of dizygotic twin pairs provides information on how identical environmental conditions (including during prenatal development) affect The Development of traits in individuals with different genotypes.

Class="center">Table 7.3. Concordance of certain human traits in monozygotic (MZ) and dizygotic (DZ) twins

Quantitatively, THE CONTRIBUTION OF the genotype to the development of a given trait is expressed as the heritability coefficient H, which in this case can be calculated using the formula

where K_m is the concordance (in percentage) of monozygotic twins, and K_d is that of dizygotic twins. If H = 1, the trait is entirely determined by the genotype, whereas H = 0 indicates determination solely by environmental factors. For instance, the heritability coefficient for hair color is 0.86, which indicates a predominant contribution of genetic factors to The formation of this trait, alongside a minor environmental influence (0.14). For excess body weight, the heritability coefficients are 0.4 (for ages 18-20) and 0.35 (for individuals over 45), pointing to a greater IMPACT OF ENVIRONMENTAL factors, though the genetic component still plays a substantial role.

Another approach to assessing the roles of genotype and environment in the development of multifactorial traits is the analysis of phenotypic expressions in adopted children—the adoption method. Adopted children and their adoptive parents have entirely unrelated genotypes yet share similar living conditions. The adoption method is most frequently used in combination with the twin method, providing more objective results regarding genetic influences on the development of Quantitative Traits.