Tasks:
- Describe the main methods of studying human genetics.
- Study the genetic basis of the structure and evolution of populations.
Methods for studying human genetics
Each major stage in the development of genetics was associated with the use of certain objects for genetic research. The theory of the gene and the basic patterns of inheritance of traits were established in experiments with peas, the Drosophila fly was used to substantiate the chromosomal theory of heredity, and viruses and bacteria were used to develop molecular genetics. Currently, humans are becoming the main object of genetic research.
Rice. 1. Legend taken when compiling pedigrees:
1 - man; 2 - woman; 3 - gender is unknown; 4 - owner of the trait being studied; 5 - heterozygous carrier of the recessive gene being studied; 6 - marriage; 7 - marriage of a man with two women; 8 - consanguineous marriage; 9 - parents, children and their order of birth; 10 - fraternal twins; 11 - identical twins.
For genetic research, a person is a very inconvenient object, since a person: a large number of chromosomes, experimental crossing is impossible, puberty comes late, a small number of descendants in each family, equalization of living conditions for offspring is impossible.
However, despite these difficulties, human genetics is quite well studied. This was made possible through the use of a variety of research methods.
Genealogical method. The use of this method is possible only when direct relatives are known - the ancestors of the owner of the hereditary trait (proband) on the maternal and paternal lines in a number of generations or the descendants of the proband also in several generations. When compiling pedigrees in genetics, a certain notation system is used (Fig. 1). After compiling the pedigree, it is analyzed in order to establish the nature of inheritance of the trait being studied.
Thanks to the genealogical method, it was established that in humans all types of inheritance of traits known for other organisms are observed, and the types of inheritance of some specific traits were determined. Thus, the autosomal dominant type inherits polydactyly (increased number of fingers) (Fig. 2), the ability to roll the tongue into a tube (Fig. 3), brachydactyly (short fingers, caused by the absence of two phalanges on the fingers), freckles, early baldness, fused fingers , cleft lip, cleft palate, eye cataracts, brittle bones and many others. Albinism, red hair, susceptibility to polio, diabetes mellitus, congenital deafness and other traits are inherited as autosomal recessive.
Rice. 2. Pedigree for polydactyly (autosomal dominant inheritance).
Rice. 3. The dominant trait is the ability to roll the tongue into a tube (1) and its recessive allele is the absence of this ability (2).
A number of traits are inherited in a sex-linked manner: X-linked inheritance - hemophilia, color blindness; Y-linked - hypertrichosis (increased hair growth in the auricle), membranes between the fingers. There are a number of genes localized in homologous regions of the X and Y chromosomes, for example, general color blindness.
The value of the method is not limited to establishing the type of inheritance of characteristics. Usage genealogical method showed that with a related marriage, compared to an unrelated marriage, the likelihood of deformities, stillbirths, and early mortality in the offspring increases significantly. In consanguineous marriages, recessive genes often become homozygous, resulting in the development of certain anomalies. A striking example of this is the inheritance of hemophilia in the royal houses of Europe.
Twin method. Twins are children born at the same time. They are monozygotic (identical) and dizygotic (fraternal) (Fig. 4).
Rice. 4. Formation of monozygotic (1) and dizygotic (2) twins.
In gametes and zygotes, only sex chromosomes are conventionally designated, as well as chromosomes carrying the gene for dark hair (black) and the gene for blond hair (white).
Monozygotic twins develop from one zygote, which at the cleavage stage is divided into two (or more) parts. Therefore, such twins are genetically identical and always of the same sex. Monozygotic twins are characterized by a high degree of similarity (concordance) for many characteristics.
Dizygotic twins develop from eggs that were simultaneously ovulated and fertilized by different sperm.
Therefore, they are hereditarily different and can be either the same or different sexes. Unlike monozygotic twins, dizygotic twins are often characterized by discordance - dissimilarity in many ways. Data on twin concordance for some characteristics are shown in the table.
Concordance of some human characteristics
As can be seen from the table, the degree of corcondancy of monozygotic twins for all of the above characteristics is significantly higher than that of dizygotic twins, but it is not absolute. As a rule, discordance between identical twins occurs as a result of disturbances in the intrauterine development of one of them or under the influence of the external environment, if it was different.
Thanks to the twin method, a person’s hereditary predisposition to a number of diseases was determined: schizophrenia, mental retardation, epilepsy, diabetes mellitus and others. Observations of identical twins provide material for elucidating the role of heredity and environment in the development of traits. And under external environment understand not only physical environmental factors, but also social conditions.
Cytogenetic method is based on the study of human chromosomes in normal and pathological conditions. Normally, a human karyotype includes 46 chromosomes - 22 pairs of autosomes and two sex chromosomes. The use of this method made it possible to identify a group of diseases associated with either changes in the number of chromosomes or changes in their structure. Such diseases are called chromosomal. These include: Klinefelter syndrome, Shereshevsky-Turner syndrome, trisomy X, Down syndrome, Patau syndrome, Edwards syndrome and others.
Patients with Klinefelter syndrome(47,ХХУ) always men. They are characterized by underdevelopment of the gonads, degeneration of the seminiferous tubules, often mental retardation, tall(due to disproportionately long legs).
Shereshevsky-Turner syndrome(45,X0) is observed in women. It manifests itself in delayed puberty, underdevelopment of the gonads, amenorrhea (absence of menstruation), and infertility. Women with Shereshevsky-Turner syndrome are short, their body is disproportionate - the upper part of the body is more developed, the shoulders are wide, the pelvis is narrow - the lower limbs are shortened, the neck is short with folds, the "Mongoloid" shape of the eyes and a number of other signs.
Down syndrome- one of the most common chromosomal diseases. It develops as a result of trisomy on chromosome 21 (47, 21,21,21). The disease is easily diagnosed, as it has a number of characteristic features: shortened limbs, small skull, flat, wide nose bridge, narrow palpebral fissures with an oblique incision, the presence of a fold of the upper eyelid, mental retardation. Disturbances in the structure of internal organs are also often observed.
Chromosomal diseases also arise as a result of changes in the chromosomes themselves. Thus, deletion of chromosome 5 leads to the development of “cry of the cat” syndrome. In children with this syndrome, the structure of the larynx is disrupted, and in early childhood they have a peculiar “meowing” voice timbre. In addition, there is retardation of psychomotor development and dementia. Deletion of chromosome 21 leads to the occurrence of one of the forms of leukemia.
Most often, chromosomal diseases are the result of mutations that have occurred in the germ cells of one of the parents.
Biochemical method allows you to detect metabolic disorders caused by changes in genes and, as a consequence, changes in the activity of various enzymes. Hereditary metabolic diseases are divided into diseases of carbohydrate metabolism (diabetes mellitus), metabolism of amino acids, lipids, minerals, etc.
Phenylketonuria refers to diseases of amino acid metabolism. The conversion of the essential amino acid phenylalanine to tyrosine is blocked, while phenylalanine is converted to phenylpyruvic acid, which is excreted in the urine. The disease leads to the rapid development of dementia in children. Early diagnosis and diet can stop the development of the disease.
Human genetics- one of the most intensively developing branches of science. She happens to be theoretical basis medicine, reveals biological basis hereditary diseases. Knowledge of the genetic nature of diseases allows you to make an accurate diagnosis in time and carry out the necessary treatment.
Population genetics
A population is a collection of individuals of the same species long time living in a certain territory, freely interbreeding with each other, having a common origin, a certain genetic structure and, to one degree or another, isolated from other such populations of individuals of a given species. A population is not only a unit of a species, a form of its existence, but also a unit of evolution. Microevolutionary processes that culminate in speciation are based on genetic transformations in populations.
A special branch of genetics deals with the study of the genetic structure and dynamics of populations - population genetics.
From a genetic point of view, a population is an open system, while a species is a closed one. In general form, the process of speciation comes down to genetic transformation open system into a genetically closed one.
Each population has a specific gene pool and genetic structure. The gene pool of a population is the totality of the genotypes of all individuals in the population. The genetic structure of a population is understood as the ratio of different genotypes and alleles in it.
One of the basic concepts of population genetics is genotype frequency and allele frequency. The frequency of a genotype (or allele) is understood as its share divided by the total number of genotypes (or alleles) in the population. The frequency of a genotype, or allele, is expressed either as a percentage or as a fraction of a unit (if the total number of genotypes or alleles in a population is taken to be 100% or 1). So, if a gene has two allelic forms and the share of the recessive allele a is 3/4 (or 75%), then the share of the dominant allele A will be equal to 1/4 (or 25%) total number alleles of a given gene in a population.
The method of reproduction has a great influence on the genetic structure of populations. For example, populations of self-pollinating and cross-pollinating plants differ significantly from each other.
The first study of the genetic structure of a population was undertaken by V. Johannsen in 1903. Populations of self-pollinating plants were selected as objects of study. Having studied the seed mass of beans for several generations, he discovered that in self-pollinators the population consists of genotypically heterogeneous groups, the so-called pure lines, represented by homozygous individuals. Moreover, from generation to generation in such a population it remains equal ratio homozygous dominant and homozygous recessive genotypes. Their frequency increases in each generation, while the frequency of heterozygous genotypes will decrease. Thus, in populations of self-pollinating plants, a process of homozygotization, or decomposition into lines with different genotypes, is observed.
Most plants and animals in populations reproduce sexually through free mating, which ensures an equal occurrence of gametes. The equal occurrence of gametes during free crossing is called panmixia, and such a population is called panmictic.
Hardy-Weinberg Law
In 1908, the English mathematician G. Hardy and the German physician N. Weinberg independently formulated a law governing the distribution of homozygotes and heterozygotes in a panmictic population, and expressed it in the form of an algebraic formula.
The frequency of occurrence of gametes with the dominant allele A is denoted by p, and the frequency of occurrence of gametes with the recessive allele a is denoted by q. The frequencies of these alleles in a population are expressed by the formula p + q = 1 (or 100%). Since gametes are equally likely to occur in a panmictic population, genotype frequencies can also be determined.
Hardy and Weinberg, summing up data on the frequency of genotypes formed as a result of equally probable occurrence of gametes, derived a formula for the frequency of genotypes in a panmictic population:
p 2 + 2pq + q 2 = 1.
AA + 2Aa + aa = 1
Using these formulas, it is possible to calculate the frequencies of alleles and genotypes in a specific panmictic population. However, this law is subject to the following conditions: an unlimitedly large population size, all individuals can freely interbreed with each other, all genotypes are equally viable, fertile and are not subject to selection, direct and reverse mutations occur with the same frequency or are so rare that they can be neglected, there is no outflow or influx of new genotypes into the population .
In real existing populations, these conditions cannot be met, so the law is valid only for an ideal population. Despite this, the Hardy-Weinberg law is the basis for the analysis of some genetic phenomena occurring in natural populations. For example, if it is known that phenylketonuria occurs with a frequency of 1:10,000 and is inherited in an autosomal recessive manner, you can calculate the frequency of heterozygotes and homozygotes for a dominant trait. Patients with phenylketonuria have a genotype q2(aa) = 0.0001. Hence q = 0.01. p = 1 - 0.01 = 0.99. The frequency of occurrence of heterozygotes is 2pq, equal to 2 x 0.99 x 0.01 0.02 or about 2%. Frequency of occurrence of homozygotes for dominant and recessive traits: AA = p2 = 0.992 98%, aa = 0.01%.
Changes in the balance of genotypes and alleles in a panmictic population occur under the influence of constantly acting factors, which include: the mutation process, population waves, isolation, natural selection, genetic drift and others.
It is thanks to these phenomena that an elementary evolutionary phenomenon arises - a change in the genetic composition of the population, which is the initial stage of the process of speciation.
Literature.
1. Green N., Stout W., Taylor D. Biology. - M.: world, 1990. - T.1-3.
2. Goncharov O.V. Pimenov A.V. Biology. Part 1, Cytology, genetics, selection: A guide for applicants to universities. - Saratov: Lyceum boarding school at SSAU named after. N.I. Vavilova, 2001.
3. Yarygin V.N. Biology for university applicants. - M.: graduate School, 2006.
Population genetics, a branch of genetics that studies the gene pool of populations and its changes in space and time. Let's take a closer look at this definition. Individuals do not live alone, but form more or less stable groups, jointly mastering their habitat. Such groups, if they self-reproduce over generations and are not supported only by newcomers, are called populations. For example, a school of salmon spawning in one river forms a population because the descendants of each fish tend to return to the same river, to the same spawning grounds, from year to year. In farm animals, a population is usually considered to be a breed: all individuals in it are of the same origin, i.e. have common ancestors, are kept in similar conditions and are supported by uniform selection and breeding work. Among aboriginal peoples, the population consists of members of related camps.
In the presence of migrations, the boundaries of populations are blurred and therefore indefinable. For example, the entire population of Europe are descendants of the Cro-Magnons who settled our continent tens of thousands of years ago. Isolation between ancient tribes, which increased with the development of each of them own language and culture, led to differences between them. But their isolation is relative. Constant wars and seizures of territory, and in Lately– the gigantic migration led and is leading to a certain genetic rapprochement of peoples.
The examples given show that the word “population” should be understood as a grouping of individuals related by territorial, historical and reproductive community.
The individuals of each population are different from each other, and each of them is unique in some way. Many of these differences are hereditary, or genetic—they are determined by genes and passed from parents to children.
The totality of genes in individuals of a given population is called its gene pool. In order to solve problems of ecology, demography, evolution and selection, it is important to know the characteristics of the gene pool, namely: how large genetic diversity in each population, what are genetic differences between geographically separated populations of the same species and between various types how the gene pool changes under the influence environment how it is transformed during evolution, how it spreads hereditary diseases how effectively the gene pool of cultivated plants and domestic animals is used. Population genetics studies these issues.
Basic concepts of population genetics
Frequencies of genotypes and alleles. The most important concept population genetics is the frequency of a genotype - the proportion of individuals in a population having a given genotype. Consider an autosomal gene with k alleles, A1, A2, ..., Ak. Let the population consist of N individuals, some of which have alleles Ai Aj. Let us denote the number of these individuals as Nij. Then the frequency of this genotype (Pij) is determined as Pij = Nij/N. Let, for example, a gene have three alleles: A1, A2 and A3 - and let the population consist of 10,000 individuals, among which there are 500, 1000 and 2000 homozygotes A1A1, A2A2 and A3A3, and heterozygotes A1A2, A1A3 and A2A3 - 1000, 2500 and 3000 respectively. Then the frequency of A1A1 homozygotes is P11 = 500/10000 = 0.05, or 5%. Thus we obtain the following observed frequencies of homo- and heterozygotes:
P11 = 0.05, P22 = 0.10, P33 = 0.20,
P12 = 0.10, P13 = 0.25, P23 = 0.30.
Another important concept in population genetics is allele frequency—its proportion among those that have alleles. Let us denote the frequency of the Ai allele as pi. Since a heterozygous individual has different alleles, the frequency of the allele is equal to the sum of the frequency of homozygous and half the frequencies of individuals heterozygous for this allele. This is expressed the following formula: pi = Pii + 0.5jPij. In the example given, the frequency of the first allele is p1 = P11 + 0.5(P12 + P13) = 0.225. Accordingly, p2 = 0.300, p3 = 0.475.
Hardy–Weinberg relations. When studying the genetic dynamics of populations, a population with random crossing, having an infinite number and isolated from the influx of migrants, is taken as a theoretical, “zero” reference point; It is also believed that the rate of gene mutation is negligible and there is no selection. It is mathematically proven that in such a population the allele frequencies of the autosomal gene are the same for females and males and do not change from generation to generation, and the frequencies of homo- and heterozygotes are expressed in terms of allele frequencies as follows:
Pii = pi2, Pij = 2pi pj.
This is called the Hardy-Weinberg relationship, or law, after the English mathematician G. Hardy and the German physician and statistician W. Weinberg, who simultaneously and independently discovered them: the first theoretically, the second from data on the inheritance of traits in humans.
Real populations can differ significantly from the ideal one described by the Hardy–Weinberg equations. Therefore, the observed genotype frequencies deviate from the theoretical values calculated using the Hardy–Weinberg relationships. Thus, in the example discussed above, the theoretical frequencies of genotypes differ from the observed ones and are
P11 = 0.0506, P22 = 0.0900, P33 = 0.2256,
P12 = 0.1350, P13 = 0.2138, P23 = 0.2850.
Such deviations can be partially explained by the so-called. sampling error; After all, in reality, the experiment does not study the entire population, but only individual individuals, i.e. sample. But main reason deviations in genotype frequencies are undoubtedly processes that occur in populations and affect their genetic structure. Let us describe them sequentially.
Population genetic processes
Genetic drift. Genetic drift refers to random changes in gene frequencies caused by a finite population size. To understand how genetic drift occurs, let us first consider a population of the smallest possible size N = 2: one male and one female. Let the female in the initial generation have the genotype A1A2, and the male have the genotype A3A4. Thus, in the initial (zero) generation, the frequencies of alleles A1, A2, A3 and A4 are each 0.25. Individuals of the next generation are equally likely to have one of the following genotypes: A1A3, A1A4, A2A3 and A2A4. Let's assume that the female will have the A1A3 genotype, and the male will have the A2A3 genotype. Then in the first generation, the A4 allele is lost, the A1 and A2 alleles retain the same frequencies as in the original generation - 0.25 and 0.25, and the A3 allele increases the frequency to 0.5. In the second generation, the female and male can also have any combination of parental alleles, for example A1A2 and A1A2. In this case, it turns out that the A3 allele, despite its high frequency, disappeared from the population, and the A1 and A2 alleles increased their frequency (p1 = 0.5, p2 = 0.5). Fluctuations in their frequencies will eventually result in either the A1 or A2 allele remaining in the population; in other words, both male and female will be homozygous for the same allele: A1 or A2. The situation could have developed in such a way that the A3 or A4 allele would have remained in the population, but in the case considered this did not happen.
The process of genetic drift described by us takes place in any population of finite size, with the only difference that events develop at a much lower speed than with a population of two individuals. Genetic drift has two important consequences. First, each population loses genetic variation at a rate inversely proportional to its size. Over time, some alleles become rare and then disappear altogether. In the end, only one allele remains in the population, which one is a matter of chance. Secondly, if a population splits into two or larger number new independent populations, then genetic drift leads to an increase in differences between them: in some populations some alleles remain, and in others - others. Processes that counteract the loss of variability and genetic divergence of populations are mutations and migrations.
Mutations. When gametes are formed, random events– mutations, when the parent allele, say A1, turns into another allele (A2, A3 or any other), which was or was not previously present in the population. For example, if in the nucleotide sequence “...TCT TGG...”, encoding a section of the polypeptide chain “...serine-tryptophan...”, the third nucleotide, T, as a result of mutation was passed on to the child as C, then in the corresponding section of the amino acid chain of the protein synthesized in the body child, alanine would be located instead of serine, since it is encoded by the TCC triplet. Regularly occurring mutations have formed, in a long series of generations of all species living on Earth, the gigantic genetic diversity that we now observe.
The probability with which a mutation occurs is called the frequency, or rate, of mutation. The rate of mutation of different genes varies from 10–4 to 10–7 per generation. At first glance, these values seem insignificant. However, it should be taken into account that, firstly, the genome contains many genes, and, secondly, that the population can have a significant size. Therefore, some gametes always carry mutant alleles, and in almost every generation one or more individuals with mutations appear. Their fate depends on how strongly these mutations affect fitness and fertility. The mutation process leads to an increase in the genetic variability of populations, counteracting the effect of genetic drift.
Migrations. Populations of the same species are not isolated from each other: there is always an exchange of individuals—migration. Migrating individuals, leaving offspring, pass on to the next generations alleles that might not exist at all in this population or they might be rare; This is how gene flow is formed from one population to another. Migrations, like mutations, lead to an increase in genetic diversity. In addition, gene flow connecting populations leads to their genetic similarity.
Crossing systems. In population genetics, crossing is called random if the genotypes of individuals do not affect the formation of mating pairs. For example, based on blood groups, crossing may be considered random. However, coloring, size, and behavior can greatly influence the choice of a sexual partner. If preference is given to individuals of a similar phenotype (i.e., with similar individual characteristics), then such positive assortative crossing leads to an increase in the proportion of individuals with the parental genotype in the population. If, when selecting a mating pair, preference is given to individuals of the opposite phenotype (negative assortative crossing), then new combinations of alleles will be presented in the genotype of the offspring; Accordingly, individuals of either an intermediate phenotype or a phenotype that is sharply different from the phenotype of the parents will appear in the population.
To conduct experiments on laboratory animals, it is necessary to know the genotypes of not only certain individuals, but also the genetic structure of the entire line and species. For this purpose, for renewal and development biological science, its analysis, a special field of genetics was created - population genetics or population genetics. The methods of this science make it possible to reveal patterns that are realized in the aggregate of individuals, that is, in populations.
From a genetic point of view, a population is considered as a collection of individuals of the same species inhabiting a certain territory and differing in their phenotypic and genotypic properties. For analysis, a freely interbreeding, so-called panmictic population is usually considered as the initial population structure and its changes. All individuals included in it can mate with each other in any combination, regardless of genetic structure. Freely interbreeding populations are possible only in species that reproduce sexually. Research genetic processes, occurring in natural conditions of reproduction of animals, birds, reptiles, insects have great importance for knowledge biological features, specificity of differences and homogeneity by genotype in various environmental conditions.
In a panmictic population, there is an equal probability of any member of the population combining with each other, as well as an equal probability of producing offspring, but this does not mean the purely physical mating of any females with any males, but only the fundamental possibility of its implementation. Hence the need to build another model, namely: we can consider the entire set of germ cells formed by individuals of a freely crossing population as a single whole, as if they were all placed in a vessel and mixed with each other. IN in this case the union of female and male germ cells occurs purely by chance, and its results will depend only on the frequency (or probability measured by the frequency) of certain germ cells. And also, before fertilization, each germ cell contains only one gene from a pair or series of alleles, then the totality of genes located in the germ cells of all individuals of the population, as a single gene pool. The proportion of certain genes of the same series of alleles is usually called gene frequency.
Depending on the frequencies of individual genes found in a population, the ratio of genotypes and phenotypes can be determined. Knowing this ratio, it is possible to determine gene frequencies as the most important parameters for characterizing a population.
To analyze the method for determining gene frequencies, we can give specific example. On the experimental rabbit farm there were 729 gray rabbits (AA), 111 black ones that were heterozygous (Aa) and 4 white rabbits (aa). If all categories of individuals do not differ from each other in terms of the number of germ cells formed, then, taking for simple calculation only two germ cells, we get the following number of genes A and a in the general gene pool of the rabbit farm.
Gene A (2A) (729 x 2) +111=1569 germ cells.
Gene aa and aa 111+(4+2)=119 germ cells.
TOTAL: 1688 germ cells.
Composing ratio: 1688 - 1.0
Ratio: 1688 - 1.0
Total sum of genes: p(A)=0.93
In this simple example Gene frequencies are calculated based on the known numbers or proportions of groups of individuals that are genotypically different from each other. Knowing the frequencies of genes, one can predict the specific ratios that will be obtained in the next generation of a freely interbreeding population. It's best to do this in general view for any values of p and q in the gene pool. Both females and males will form gametes of two types A and a in the ratio p(A):q(a). The results of combining male and female gametes can be shown using four-field table 1.
Table 1 - Results of combining male and female gametes
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Men's Women's |
Gametes and their frequencies, ♀ |
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Gametes and their frequencies ♂ |
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Three genotypes were formed in the offspring in the ratio expressed by the coefficient: Р², 2рq and q² (the sum of the upper and lower fields of the table) or Р²АА+22рqАа+ q²аа.
This ratio of genotypes was called the formula or the Hardy-Weinberg law, or the law of stabilizing equilibrium, since it expresses a certain pattern that characterizes a population in the presence of free crossing in it. Such a population is in equilibrium according to the ratio of genotypes, which is confirmed by the above formula:
Р²АА+22рqАа+ q²аа =1.
According to this law Hardy-Weinberg, the absence of factors that determine and change the frequency of genes, the population, at any ratio of alleles from generation to generation, keeps these frequencies constant. Despite some limitations, using the Hardy-Weinberg formula it is possible to calculate the population structure and determine the frequencies of heterozygotes, for example, for lethal or sublethal genes, knowing the frequencies of homozygotes for recessive traits and the frequencies of individuals with dominant traits, analyze shifts in gene frequencies for specific traits as a result of selection, mutations and other factors.
In all populations of laboratory animals and in nature, during free crossing, splitting occurs according to a given number of genes that determine various morphological and physiological characteristics. In some cases, it is relatively easy to isolate alleles of individual genes, and then a grandiose picture of the genetic complexity of the population emerges.
This is the case with the analysis of the genetic structure of populations in animals, but we need to know the factors that can change this structure. There are many of them, but the most important place belongs to selection.
Under selection in classical sense words usually mean elimination certain group individuals from reproduction, i.e., the formation of the next generation. In the absence of selection, each individual in a population has the same chance of producing offspring. Although they are random, they are characterized by a normal distribution curve.
If a group of individuals is eliminated from reproduction, then the structure of the future generation will be influenced only by the remaining part of the population, which will inevitably affect the frequency of genes in the next generation. However, K. Pearson showed that as soon as the state of panmixia (free crossing) arises, the ratio of genotypes returns to the type that corresponds to the Hardy-Weinberg formula, but in a different ratio. Thus, in the absence of culling of heterozygous carriers of recessive anomalies, the frequency of appearance of abnormal animals in the population remains unchanged.
Dividing gene alleles into wild and mutant, as we did when getting acquainted with the basics of genetics, is not entirely correct, and such a division can lead to an incorrect understanding of evolution. Research natural populations show that not all members of the population have a common genotype, which we conventionally call wild. In fact, there is significant genetic diversity in many populations. Dobzhansky and his colleagues conducted studies of wild fruit flies in the southwestern United States and found that among them there are carriers of several inversion variants of each chromosome. (An inversion is a rotation of one of the chromosome sections.) In the salivary glands of fruit flies there are giant chromosomes with a clear pattern of black and white stripes that are visible under a microscope. This makes it easy to compare the chromosomes of different individuals and determine how close they are to each other. The basic concept of population genetics is allele frequency, that is, the share certain type gene or chromosome in a population. Suppose, for example (using Dobzhansky's notation) that 37% of flies in a certain population have a second chromosome with the "standard" gene sequence, 16% have the "Arrowhead" inversion, and 47% have the "Chiricahua" inversion. In this case, the frequencies of these forms will be 0.37, 0.16 and 0.47, respectively. Dobzhansky and his colleagues mapped the frequencies of various inversions throughout the region and showed that the frequency of each inversion varies in a specific way from California east and north to Mexico. It is assumed that some gene sequences give their owners some advantages in one way or another. geographical region. Other studies have found approximately the same results. Many genes and chromosomes exist in different allelic forms and are maintained in a population at significant frequencies, which are likely to change regularly (for example, depending on the season). Such variability is a rich source of evolution.
The diversity of gene forms is maintained by mutations that occur constantly in the population with low frequency. Some genotype changes are beneficial, so individuals with genetic changes receive more chances leave offspring. Over time, the percentage of individuals with a beneficial mutation increases. Natural selection and suggests such a reproductive advantage for some individuals. Each genotype has its own degree fitness, measured according to reproduction frequency. To say that a certain genotype has high fitness means that individuals with that genotype have more possibilities pass on copies of your genes to your offspring.
To form a new species or larger taxonomic unit such as a genus, changes must affect many genes. Let us assume that adaptive changes occur in some form, corresponding to changes in genes: the genome AA BB mm QQ stst becomes aa bb MM qq StSt. This requires mutations A - a, B - b, t - M, Q - q And st- St. They are likely to occur independently of each other, in different time and in different individuals, and the final genotype is formed through recombination. One can imagine how mutations lengthen and shorten the limbs of vertebrates, make their bones thinner or thicker, and gradually create the appearance of the animal to which we are accustomed. Some researchers have modeled selection for a particular genotype in laboratory conditions.
Population genetics describes these processes statistical methods. Let's start with a single gene model. Let us assume that the population has alleles A and a the same gene, and that the frequency A is equal to 0.6p, and the frequency A - 0.4q. (Note that in such a simple model p + q= 1 because all alleles in the population belong to either type A, or to type A.) Allele frequencies can be determined by counting the number of their carriers, both homozygotes and heterozygotes. Each homozygote carries two copies of the same allele, while a heterozygote carries one copy of each.
What will be the frequencies of the different genotypes in this population? The processes of mutation and selection act slowly, over several generations, and to begin with, let's assume that they do not act at all. Suppose also that the population is large enough for the principles of probability to apply, and that individuals mate randomly. This means that neither males nor females specifically choose their partners (for example, a partner AA does not prefer to mate with partners of the same genotype). Let us now remember that gametes contain one allele either A, or A, therefore gametes A And A will occur at the same frequencies as the alleles, that is R And q. For clarity, you can imagine the alleles A in the form of red balls, and alleles A- in the form of blue ones, and the entire gene pool of the population - in the form of a bag with these balls. To obtain a new individual, without looking with both hands, we take two balls out of this bag. The probability that they are both red is equal R X R= p 2, that they are both blue - q x q = q 2 . Sometimes it happens that with our left hand we take out a red ball, and with our right hand we take out a blue one (frequency p x q = pq), and sometimes vice versa: with the left - blue, and with the right - red (frequency q X p = qp). From here we obtain the following genotype frequencies: p 2 for AA, 2pq For Aa; q 2 For ah.
This is an approximate formula called Xapdu-Weinberg formula, underlies population genetics. Its more complex variants take into account the frequency of mutations and the selective fitness of various alleles. It can also be used to estimate the prevalence in the human population of a hereditary disease caused by a single allele. Let's take for example an autosomal recessive disease such as phenylketonuria, which occurs with frequency in the population q2. If in a certain population one person in 10 thousand suffers from phenylketonuria, then q 2 ="/ 10000 - It follows that q should be equal square root from "/10000, that is, "/100. Because p + q = I, then p = 99/100. Then, according to the Hardy-Weinberg formula, the frequency of heterozygous carriers 2pq= 2 x 99/100 x 1/100= 1/50 (approximately). These estimates show that heterozygous carriers are much more common (approximately one in 50 people) than homozygous patients. Knowing the frequency of heterozygotes is very helpful in genetic counseling. Knowing the data on the distribution of heterozygotes, you can also try to eliminate the recessive allele from the population by selection, as will be described below.
POPULATION GENETICS
a branch of genetics that studies the gene pool of populations and its changes in space and time. Let's take a closer look at this definition. Individuals do not live alone, but form more or less stable groups, jointly mastering their habitat. Such groups, if they self-reproduce over generations and are not supported only by newcomers, are called populations. For example, a school of salmon spawning in one river forms a population because the descendants of each fish tend to return to the same river, to the same spawning grounds, from year to year. In farm animals, a population is usually considered to be a breed: all individuals in it are of the same origin, i.e. have common ancestors, are kept in similar conditions and are supported by uniform selection and breeding work. Among aboriginal peoples, the population consists of members of related camps. In the presence of migrations, the boundaries of populations are blurred and therefore indefinable. For example, the entire population of Europe are descendants of the Cro-Magnons who settled our continent tens of thousands of years ago. The isolation between the ancient tribes, which increased as each of them developed their own language and culture, led to differences between them. But their isolation is relative. Constant wars and seizures of territory, and more recently, gigantic migration have led and are leading to a certain genetic rapprochement of peoples. The examples given show that the word “population” should be understood as a group of individuals related by territorial, historical and reproductive community. The individuals of each population are different from each other, and each of them is unique in some way. Many of these differences are hereditary, or genetic—they are determined by genes and passed on from parents to children. The totality of genes in individuals of a given population is called its gene pool. In order to solve problems of ecology, demography, evolution and selection, it is important to know the features of the gene pool, namely, how much genetic diversity is in each population, what are the genetic differences between geographically separated populations of the same species and between different species, how the gene pool changes under the influence of the environment how it is transformed during evolution, how hereditary diseases spread, how effectively the gene pool of cultivated plants and domestic animals is used. Population genetics studies these issues.
BASIC CONCEPTS OF POPULATION GENETICS
Frequencies of genotypes and alleles. The most important concept of population genetics is genotype frequency - the proportion of individuals in a population having a given genotype. Consider an autosomal gene with k alleles, A1, A2, ..., Ak. Let the population consist of N individuals, some of which have alleles Ai Aj. Let us denote the number of these individuals as Nij. Then the frequency of this genotype (Pij) is determined as Pij = Nij/N. Let, for example, a gene have three alleles: A1, A2 and A3 - and let the population consist of 10,000 individuals, among which there are 500, 1000 and 2000 homozygotes A1A1, A2A2 and A3A3, and heterozygotes A1A2, A1A3 and A2A3 - 1000, 2500 and 3000 respectively. Then the frequency of A1A1 homozygotes is P11 = 500/10000 = 0.05, or 5%. Thus we obtain the following observed frequencies of homo- and heterozygotes:
P11 = 0.05, P22 = 0.10, P33 = 0.20, P12 = 0.10, P13 = 0.25, P23 = 0.30.
CHANGES IN ALLELE FREQUENCIES DURING DRIFT. The results of modeling the process of genetic drift in two populations of N = 25 and two populations of N = 250, with an allele frequency of 0.5 in the initial generation, are presented. Under the influence of drift, the frequency of a given allele changes chaotically from generation to generation, with frequency “jumps” being more pronounced in smaller populations. Over 50 generations, drift led to the fixation of the allele in one population of N = 25, and to its complete elimination in another. In populations larger numbers this allele is still at intermediate frequencies, but the populations are already noticeably different from each other starting from the 60th generation.
LITERATURE
Timofeev-Resovsky N.V., Yablokov A.V., Glotov N.V. Essay on the doctrine of population. M., 1973 Ayala F., Kaiger J. Modern genetics, vol. 1-3, M., 1988 Vogel F., Motulski A. Human Genetics, vol. 1-3. M., 1990
Collier's Encyclopedia. - Open Society. 2000 .