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Twins method





Twins method suggested by Englishman F. Galton is used to determine the role of heredity and environment in the development of mental properties of human.

Study of signs manifestations of identical twins with the same genotype, especially if they grew up in different conditions, allows evaluating environment role in genes action implementation with high confidence.

Monozygotic twins (MT) arise from a single egg, fertilized by one sperm cell, due to zygote division into two (sometimes three or more) embryo. Dizygotic twins (DT) resulting from simultaneous fertilization of two or more eggs, they are sibs who were born at the same time. MT always are of the same sex, and DT can be both same sex and opposite-sex. If twins are heterosexual, then they are naturally dizygotic, if the same sex, then differentiation of monozygotic and dizygotic twins is difficult. DT are often given birth by women aged 40-45 years and women treated with gonadotropin. Tendency to the birth of twins is maternally transmitted.

 

4. Population-statistical method is used to study the genetic structure of different organisms’ populations, including large groups of people in one or more generations, quantify the distribution of individuals with different genotypes in the population, and analyze the dynamics of the populations’ genetic structure due to various factors. The essential point is the statistical processing of the data. This method can be used to calculate the frequency of different alleles and different genotypes of these alleles occurrence in the population, determine the spread in it of various inherited traits, including diseases. It allows us to study the mutation process, the role of heredity and environment in the formation of human phenotypic polymorphism.

The basis for determination of the population genetic structure is Hardy Weinberg law of genetic equilibrium.

English mathematician Hardy and the German physician Weinberg approximately simultaneously in the 1908 proved the fundamental theorem of population genetics, which explains why from generation to generation the frequency of dominant genes occurrence does not increase. Hardy's discovery was published in the U.S. journal “Science”. He believed that his work would be considered mathematical colleagues too trivial. Weinberg was a practical physician, who made a great contribution to the development of classical genetics. He developed a variety of methods of twins study.

Hardy Weinberg law reflects regularity, according to which, under certain conditions, the ratio of alleles and genotypes in a population's gene pool is kept unchanged in a number of generations of this population. Under this law, with data on the frequency of occurrence in the population of a recessive phenotype that has a homozygous genotype (aa), you can calculate the frequency of occurrence of these alleles in the gene pool of a given generation. Extending this information for the next generation, we can predict the frequency of people with a recessive trait occurrence, and heterozygous carriers of recessive alleles.

Mathematical expression of the Hardy – Weinberg law is an expression

(pA + qa)2 = p2AA + 2pqAa + q2aa, p + q = 1,

where p and q – frequency of alleles A and a of corresponding gene. Formula makes it possible to calculate the frequency of people with different genotypes occurrence.

For example, albinism is caused by the lack of an enzyme involved in the formation of melanin pigment, it is an inherited recessive trait. Frequency of albino (aa) occurrence in the population is 1:20 000. Consequently, q2 = 1/20 000, then q = 1/141, p = 140/141. According to the expression the frequency of heterozygotes is 2pq, i.e. corresponds to 2 × (1/141) × (140/141) = 280/20000 = 1/70. This means that in population heterozygous carriers of albinism alleles occur with a frequency one for 70 people.

If the frequency of signs in the population corresponds to the Hardy – Weinberg law, hence the features are determined by different alleles of one gene.

If a gene in the population gene pool is represented by several alleles, such as blood group gene of ABO system, the ratio of different genotypes is given by expression

(p IA + q IB + r I0)2= p2 IA + q2 IB + r2 I0 + 2 pq IA IB + 2pr IA I0 + 2qr IB I0

 

Date: 2015-09-02; view: 670; Нарушение авторских прав; Помощь в написании работы --> СЮДА...



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