Research in genetics. Methods of studying human heredity. Differences between human genetics and general genetics
Genealogical method based on the compilation of the genealogy of a person and the study of the nature of inheritance of the trait. This method was first proposed by F. Galton in 1865. This is the oldest method. Its essence is to establish pedigree relationships and determine the dominant and recessive traits and the nature of their inheritance. This method is especially effective in the study of gene mutations.
The method includes two stages: collecting family information for as many generations as possible and genealogical analysis. The pedigree is compiled, as a rule, on one or more grounds. For this, information is collected on the inheritance of the trait among close and distant relatives. When compiling a pedigree are usedspecial symbols.
Representatives of one generation are placed in the same row in the order of their birth.
Next, the second stage begins - the analysis of the pedigree in order to establish the nature of the inheritance of the trait. First of all, it is established how the sign manifests itself in representatives of different sexes, i.e. linkage of a sign with sex. Next, it is determined whether the trait is dominant or recessive, whether it is linked to other traits, etc. With the recessive nature of inheritance, the trait appears in a small number of individuals not in all generations. It may be absent from parents. With dominant inheritance, the trait is often found in almost all generations.
A characteristic feature of the inheritance of traits linked to the floor is their frequent manifestation in persons of the same sex. If this feature is dominant, then it is more common in women. If the symptom is recessive, then in this case it is more often manifested in men.
The analysis of numerous genealogies and the nature of the distribution of the trait in the vast human population helped geneticists to establish the inheritance pattern of many normal human traits, such as curly hair color, eye color, freckles, earlobe structure, etc., as well as anomalies such as color blindness, sickle cell anemia, etc.
It was the genealogical method that was able to determine the nature of hemophilia inheritance. A study of the British Royal House's pedigree revealed that the attribute is recessive and sex-linked. The carrier of the recessive gene was the British Queen Victoria.
Thus, using the genealogy method, the dependence of the trait on genetic material is established, the type of inheritance (dominant, recessive, autosomal, linked to sex chromosomes), the presence of gene linkage, the zygosity (homozygosity or heterozygosity) of family members, the probability of gene inheritance in generations,trait inheritance type. In autosomal dominant inheritance (the appearance of a trait is associated with the dominant gene), the trait usually manifests itself in each generation (horizontal inheritance). In autosomal recessive inheritance, the trait rarely appears, not in every generation (vertical inheritance), however, in related marriages, sick children are born more often. In case of sex-linked inheritance, the frequency of manifestation of the trait in individuals of different sexes is not the same.
Genealogical studies have shown that some human abilities - musicality, mathematical mindset - are also determined by hereditary factors. The genealogical method proved the inheritance of diabetes mellitus, deafness, schizophrenia, blindness in humans. This method is used for the diagnosis of hereditary diseases and genetic counseling. The nature of inheritance determines the probability of having a baby with genetic anomalies.
Twin method based on the study of the phenotype and genotype of twins to determine the degree of influence of the environment on the development of various characters. This method was proposed in 1876 by the English researcher F. Galton to distinguish between the influence of heredity and environment on the development of various traits in humans.
Among the twins stand outidentical and double.
Identical twins (identical) are formed from one zygote, which was divided into two parts at an early stage of crushing. In this case, one fertilized egg gives rise to not two, but two embryos at once. They have the same genetic material, always of the same sex, and are most interesting to study. The similarities in these twins are almost absolute. Minor differences can be explained by the influence of developmental conditions.
The twin twins (non-identical) are formed from various zygotes, as a result of the fertilization of two eggs by two sperm. They are similar to each other no more than siblings born at different times. Such twins can be same-sex and same-sex.
The twin method allows you to determine the degree of manifestation of a sign in a couple, the influence of heredity and environment on the development of signs. All differences that occur in identical twins with the same genotype are associated with the influence of external conditions. Of great interest are cases when such a couple was separated for some reason in childhood and the twins grew and were brought up in different conditions.
The study of heterogeneous twins allows us to analyze the development of different genotypes in the same environmental conditions. The twin method made it possible to establish that for many diseases, the environmental conditions under which the phenotype is formed play a significant role.
For example, signs such as blood type, eye and hair color are determined only by the genotype and are independent of the environment. Some diseases, although caused by viruses and bacteria, to some extent depend on a genetic predisposition. Diseases such as hypertension and rheumatism are largely determined by external factors and, to a lesser extent, by heredity.
Thus, the twin method allows you to identify the role of genotype and environmental factors in the formation of the trait, for which the degrees of similarity (concordance) and differences (discordance) of monozygotic and dizygotic twins are studied and compared.
Cytogenetic method consists in a microscopic study of the structure of chromosomes and their numbers in healthy and sick people. Of the three types of mutations, only chromosomal and genomic mutations can be detected under the microscope. The simplest method is express diagnostics - the study of the number of sex chromosomes by X-chromatin. Normally, in women, one X chromosome in the cells is in the form of a chromatin body, while in men this body is absent. With trisomy in a sexual pair, two bodies are observed in women, and one in men. To identify trisomy for other pairs, a sary cell karyotype is examined and an idiogram is compiled, which is compared with the standard one.
Chromosomal mutations associated with a change in the number or structure of chromosomes. Of these, under a microscope, with special staining, translocations, deletions, and inversions are well detected. With translocation or deletion, the chromosomes respectively increase or decrease in size. And with the inversion, the pattern of the chromosome (alternating bands) changes.
Chromosomal mutations can be markers in the cytogenetic methodology for the study of a disease. In addition, this method is used to determine radiation doses absorbed by people in other scientific studies.
Biochemical method based on the study of the nature of biochemical reactions in the body, metabolism to establish the carriage of an abnormal gene or to clarify the diagnosis. Diseases based on metabolic disorders make up a significant part of genetic inherited pathology. These include diabetes mellitus, phenylketonuria (impaired metabolism of phenylalanine), galactosemia (impaired absorption of milk sugar) and others. This method allows you to establish the disease at an early stage and treat it.
Population-statistical method makes it possible to calculate the frequency of occurrence of normal and pathological genes in a population, to determine the ratio of heterozygotes - carriers of abnormal genes. Using this method, the genetic structure of the population is determined (frequency of genes and genotypes in human populations); phenotype frequencies; environmental factors that change the genetic structure of a population are studied. The method is based onhardy – Weinberg Law, according to which the frequencies of genes and genotypes in numerous populations living in constant conditions, and in the presence of panmixia (free crosses) remain constant for a number of generations. Calculations are made according to the formulas: p + q \u003d 1, p2 + 2pq + q 2 \u003d 1. In this case, p is the frequency of the dominant gene (allele) in the population, q is the frequency of the recessive gene (allele) in the population, p2 - the frequency of dominant homozygotes, q2 - recessive homozygotes, 2pq - frequency of heterozygous organisms. Using this method, one can also determine the frequency of carriers of pathological genes.
Molecular genetic methods. In recent years, the level of development of modern genetics makes it possible to widely use molecular methods to study the molecular foundations of heredity and variability of organisms, the chemical and physico-chemical structure of genetic material, its functions.
For genetic research, a person is an uncomfortable object, since a person has: experimental crossing is impossible; a large number of chromosomes; late puberty; a small number of descendants in each family; it is impossible to equalize living conditions for posterity.
In human genetics, a number of research methods are used.
Genealogical method
The use of this method is possible when direct relatives are known - the ancestors of the owner of the hereditary trait ( probanda) on the maternal and paternal lines in a number of generations, or the descendants of a proband also in several generations. When compiling pedigrees in genetics, a certain notation system is used. After the genealogy is compiled, its analysis is carried out in order to establish the nature of inheritance of the trait under study.
Symbols adopted in the compilation of pedigrees:
1 - man; 2 - woman; 3 - gender is not clarified; 4 - the owner of the studied trait; 5 - heterozygous carrier of the studied recessive gene; 6 - marriage; 7 - marriage of a man with two women; 8 - family marriage; 9 - parents, children and the order of their birth; 10 - dizygotic twins; 11 - monozygotic twins.
Thanks to the genealogical method, the types of inheritance of many traits in humans have been identified. So, in an autosomal dominant type, polydactyly (increased number of fingers), the ability to curl the tongue into a tube, brachydactyly (short-toedness due to the absence of two phalanges on the fingers), freckles, early baldness, fused fingers, cleft lip, cleft palate, cataract, are inherited. fragility of bones and many others. Albinism, red hair, susceptibility to polio, diabetes mellitus, congenital deafness and other signs are inherited as autosomal recessive.
The dominant feature is the ability to curl the tongue into a tube (1) and its recessive allele is the absence of this ability (2).
3 - pedigree on polydactyly (autosomal dominant inheritance).
A number of signs are inherited in conjunction with sex: X-linked inheritance - hemophilia, color blindness; Y-linked - hypertrichosis of the margin of the auricle, webbing of the toes. There are a number of genes localized in homologous regions of X and Y chromosomes, for example, total color blindness.
The use of the genealogical method showed that in a kinship, compared with an unrelated marriage, the likelihood of deformities, stillbirths, and early mortality in offspring increases significantly. In related marriages, recessive genes often turn into a homozygous state, as a result of which these or other anomalies develop. An example of this is the inheritance of hemophilia in the royal houses of Europe.
- hemophilic; - carrier woman.
Twin method
1 - monozygotic twins; 2 - dizygotic twins.
Twins are called simultaneously born children. They are monozygous (identical) and dizygotic (opposite).
Monozygotic twins develop from one zygote (1), which at the crushing stage was 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 reasons.
Dizygotic twins develop from two or more ovulated and fertilized ovules from different sperm cells (2). Therefore, they have different genotypes and can be of the same or different sexes. Unlike monozygous, dizygotic twins are characterized by discordance - dissimilarity in many ways. Data on the concordance of twins for some signs are given in the table.
| Signs | Concordance,% | |
|---|---|---|
| Monozygotic twins | Dizygotic twins | |
| Normal | ||
| Blood type (AB0) | 100 | 46 |
| Eye color | 99,5 | 28 |
| Hair color | 97 | 23 |
| Pathological | ||
| Clubfoot | 32 | 3 |
| "Cleft lip" | 33 | 5 |
| Bronchial asthma | 19 | 4,8 |
| Measles | 98 | 94 |
| Tuberculosis | 37 | 15 |
| Epilepsy | 67 | 3 |
| Schizophrenia | 70 | 13 |
As can be seen from the table, the degree of concordance of monozygotic twins is significantly higher in all of the above signs than in dizygotic twins, but it is not absolute. As a rule, the discordance of monozygotic twins arises as a result of violations of 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 hereditary predisposition of a person to a number of diseases was clarified: schizophrenia, epilepsy, diabetes mellitus and others.
Observations of monozygotic twins provide material for elucidating the role of heredity and the environment in the development of characters. Moreover, the external environment is understood not only as physical environmental factors, but also social conditions.
Cytogenetic method
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. Using this method allowed us to identify a group of diseases associated with either a change in the number of chromosomes or with changes in their structure. Such diseases are called chromosome.
The most commonly used material for karyotypic analysis is blood lymphocytes. Blood is taken in adults from a vein, in newborns from a finger, earlobe or heel. Lymphocytes are cultivated in a special nutrient medium, in which, in particular, substances are added that “make” lymphocytes intensively divide mitosis. After some time, colchicine is added to the cell culture. Colchicine stops mitosis at the metaphase level. It is during metaphase that chromosomes are the most condensed. Next, the cells are transferred to glass slides, dried and stained with various dyes. Coloring can be a) routine (chromosomes are stained evenly), b) differential (chromosomes become transversely striated, with each chromosome having an individual pattern). Routine staining allows you to identify genomic mutations, determine the group affiliation of the chromosome, find out in which group the number of chromosomes has changed. Differential staining allows you to identify chromosomal mutations, determine the chromosome to the number, to find out the type of chromosomal mutation.
In cases where it is necessary to carry out a karyotypic analysis of the fetus, cells of amniotic (amniotic fluid) are taken for cultivation - a mixture of fibroblast-like and epithelial cells.
Among the chromosomal diseases include: Klinefelter syndrome, Turner-Shereshevsky syndrome, Down syndrome, Patau syndrome, Edwards syndrome and others.
Patients with Klinefelter's syndrome (47, XXY) are always men. They are characterized by underdevelopment of the sex glands, degeneration of the seminiferous tubules, often mental retardation, high growth (due to disproportionately long legs).
Turner-Shereshevsky syndrome (45, X0) is observed in women. It manifests itself in a slowdown in puberty, underdevelopment of the sex glands, amenorrhea (absence of menstruation), and infertility. Women with Turner-Shereshevsky syndrome have short stature, the body is disproportionate - the upper body is more developed, the shoulders are wide, the pelvis is narrow - the lower extremities are shortened, the neck is short with folds, the “Mongoloid” eye section and a number of other signs.
Down Syndrome is one of the most common chromosomal diseases. It develops as a result of trisomy on the 21st chromosome (47; 21, 21, 21). The disease is easily diagnosed, as it has a number of characteristic signs: shortened limbs, a small skull, a flat, wide nose, narrow eye slits with an oblique incision, the presence of an upper eyelid fold, and mental retardation. Often observed and violations of the structure of internal organs.
Chromosomal diseases also arise as a result of changes in the chromosomes themselves. So, deletion r-shoulder of autosome No. 5 leads to the development of the “cry of a cat” syndrome. In children with this syndrome, the structure of the larynx is disturbed, and in early childhood they have a kind of “mewing” timbre of the voice. In addition, there is a retardation of psychomotor development and dementia.
Most often, chromosomal diseases are the result of mutations that occur in the germ cells of one of the parents.
Biochemical method
It allows to detect metabolic disorders caused by a change in genes and, as a consequence, a change in the activity of various enzymes. Hereditary metabolic diseases are divided into diseases of carbohydrate metabolism (diabetes mellitus), the exchange 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.
Population-statistical method
This is a method for studying the distribution of hereditary traits (hereditary diseases) in populations. An essential point when using this method is the statistical processing of the data obtained. Under the population they understand the totality of individuals of one species, living for a long time in a certain territory, freely crossing with each other, having a common origin, a certain genetic structure and to some extent isolated from other such populations of individuals of this species. A population is not only a form of existence of a species, but also a unit of evolution, since the basis of microevolutionary processes culminating in the formation of a species are genetic transformations in populations.
The study of the genetic structure of populations is engaged in a special section of genetics - population genetics. Three types of populations are distinguished in humans: 1) panmictic, 2) demas, 3) isolates, which differ from each other in number, frequency of intragroup marriages, the share of immigrants, and population growth. The population of a large city corresponds to the panmictic population. The genetic characteristics of any population include the following indicators: 1) gene pool (the set of genotypes of all individuals in a population), 2) the frequency of genes, 3) the frequency of genotypes, 4) the frequency of phenotypes, the marriage system, 5) factors that change the frequencies of genes.
To determine the frequency of occurrence of certain genes and genotypes used hardy-Weinberg Act.
Hardy-Weinberg Law
In an ideal population from generation to generation, a strictly defined ratio of the frequencies of dominant and recessive genes is preserved (1), as well as the ratio of frequencies of genotypic classes of individuals (2).
p + q = 1, (1)r 2 + 2pq + q 2 = 1, (2)
where p - frequency of occurrence of dominant gene A; q - the frequency of occurrence of the recessive gene a; r 2 - frequency of occurrence of homozygotes for dominant AA; 2 pq - the frequency of occurrence of heterozygotes Aa; q 2 - frequency of occurrence of homozygotes for recessive aa.
An ideal population is a rather large panmictic (panmixia - free crossing) population, in which there is no mutation process, natural selection, and other factors that disrupt the balance of genes. It is clear that ideal populations do not exist in nature; in real populations, the Hardy-Weinberg law is used as amended.
Hardy-Weinberg's law, in particular, is used for the approximate calculation of carriers of recessive genes of hereditary diseases. For example, it is known that in this population phenylketonuria occurs with a frequency of 1: 10000. Phenylketonuria is inherited in an autosomal recessive manner, therefore, patients with phenylketonuria have the aa genotype, i.e. q 2 \u003d 0.0001. From here: q = 0,01; p \u003d 1 - 0.01 \u003d 0.99. Carriers of the recessive gene have the Aa genotype, that is, they are heterozygotes. The frequency of occurrence of heterozygotes (2 pq) is 2 · 0.99 · 0.01 ≈ 0.02. Conclusion: in this population, about 2% of the population are carriers of the phenylketonuria gene. At the same time, it is possible to calculate the frequency of occurrence of homozygotes by dominant (AA): p 2 \u003d 0.992, slightly less than 98%.
The change in the balance of genotypes and alleles in the panmictic population is influenced by constantly acting factors, which include: the mutation process, population waves, isolation, natural selection, gene drift, emigration, immigration, inbreeding. Thanks to these phenomena, an elementary evolutionary phenomenon arises - a change in the genetic composition of a population, which is the initial stage of the speciation process.
Human genetics is one of the most intensively developing branches of science. It is the theoretical basis of medicine, reveals the biological foundations of hereditary diseases. Knowing the genetic nature of the disease allows you to make an accurate diagnosis on time and carry out the necessary treatment.
Go to lecture number 21 "Variability"
Genealogical method
The basis of this method is the compilation and analysis of pedigrees. This method is widely used from ancient times to the present day in horse breeding, breeding of valuable lines of cattle and pigs, when obtaining purebred dogs, as well as when breeding new breeds of fur animals. Pedigrees of man have been compiled over many centuries in relation to the reigning families in Europe and Asia.
As a method for studying human genetics, the genealogical method of steel
apply only from the beginning of the 20th century, when it turned out that the analysis
pedigrees in which transmission of a certain sign (disease) from generation to generation can be traced can replace the hybridological method that is practically inapplicable to humans. When compiling pedigrees, the source is a person - proband,
whose pedigree is being studied. Usually it is either a patient or a carrier
a particular trait whose inheritance needs to be studied. At
genealogy tables using conventions proposed
G. Justus in 1931 (Fig. 6.24). Generations are denoted by Roman numerals; individuals in this generation are Arabic. Using the genealogical method, hereditary conditionality of the studied trait can be established, as well as the type of inheritance (autosomal dominant, autosomal recessive, X-linked dominant or recessive, Y-linked). When analyzing pedigrees for several reasons
the linked nature of their inheritance can be revealed, which is used in compiling chromosome maps. This method allows you to study the intensity of the mutation process, to assess the expressivity and penetrance of the allele. It is widely used in genetic counseling to predict offspring. However, it should be noted that genealogical analysis is significantly complicated with small families.
Cytogenetic method
The cytogenetic method is based on the microscopic study of chromosomes in human cells. It began to be widely used in studies of human genetics since 1956, when the Swedish scientists J. Thillot and A. Levan, proposing a new method for studying chromosomes, found that there are 46, not 48 chromosomes in the human karyotype, as
considered earlier. The current stage in the application of the cytogenetic method is associated with
developed in 1969 by T. Casperson method of differential staining of chromosomes,which expanded the possibilities of cytogenetic analysis, allowing accurate identification of chromosomes by the nature of the distribution of stained segments in them. The use of the cytogenetic method allows not only to study the normal morphology of chromosomes and the karyotype as a whole, to determine the genetic sex of the body, but, most importantly, to diagnose various chromosomal diseases associated with changes chromosome numbers or with a violation of their structure. In addition, this method allows us to study the processes of mutagenesis at the level of chromosomes and
karyotype. Its use in medical and genetic counseling for the purpose of prenatal diagnosis of chromosomal diseases makes it possible by timely termination of pregnancy to prevent the appearance of offspring with gross developmental disorders.
The material for cytogenetic studies is human cells obtained from different tissues, peripheral blood lymphocytes, bone marrow cells, fibroblasts, tumor cells and embryonic tissues, etc. An indispensable requirement for the study of chromosomes is the presence of dividing cells. Direct receipt of such cells from the body is difficult, therefore, readily available material, such as peripheral blood lymphocytes, is used.
Normally, these cells do not divide, but the special treatment of their culture with phytohemagglutinin returns them to the mitotic cycle. The accumulation of dividing cells in the metaphase stage, when the chromosomes are maximally spiralized and are clearly visible under the microscope, is achieved by treating the culture with colchicine or
colcemide, which destroys the spindle of division and prevents the separation of chromatids.
Microscopy of smears prepared from the culture of such cells allows visual observation of chromosomes. Photographing metaphase plates and subsequent processing of photographs with the preparation of karyograms in which chromosomes are arranged in pairs and distributed into groups allow
establish the total number of chromosomes and detect changes in their number and structure in individual pairs. As an express method detecting a change in the number of sex chromosomes, use sex chromatin determination methodin non-dividing cells of the mucous membrane of the cheek. Sex chromatin, or Barr's body, is formed in the cells of the female body by one of two X chromosomes. It looks like an intensely colored clump located near the nuclear envelope. With an increase in the number of X chromosomes in the karyotype of an organism, Barra bodies are formed in its cells in an amount one less than the number of X chromosomes. At
a decrease in the number of X chromosomes (monosomy X) Barr body is absent.
In the male karyotype, the Y chromosome can be detected at more
intense compared with other chromosomes of luminescence during processing
their acrychiniphritis and study in ultraviolet light.
For short-term observation, the cells are simply placed in a liquid medium on a glass slide; if you need long-term monitoring of cells, then special cameras are used. This is either flat bottles with openings covered with thin glass, or collapsible flat cameras.
Biochemical method
Unlike the cytogenetic method, which allows you to study the structure of chromosomes and the karyotype in normal and diagnose hereditary diseases associated with a change in their number and violation of organization, hereditary diseases caused by gene mutations, as well as polymorphism in
normal primary gene products are studied using biochemical methods. For the first time, these methods began to be used to diagnose gene diseases at the beginning of the 20th century. Over the past 30 years, they have been widely used in the search for new forms of mutant alleles. With their help, more than 1000 congenital metabolic diseases have been described. For many of them, a defect in the primary gene product has been identified. The most common among these diseases are diseases associated with defective enzymes, structural, transport or other
proteins. Defects in structural and circulating proteins are identified by studying their structure. So, in the 60s. XX century the analysis was completed (3-globin chain of hemoglobin, consisting of 146 amino acid residues. A large variety of hemoglobins in humans was established associated with a change in the structure of his peptide chains, which is often the cause of the development of diseases. Defects of enzymes are established by determining the content in the blood and urine of products metabolism resulting from the functioning of a given
squirrel. Deficiency of the final product, accompanied by the accumulation of intermediate and by-products of impaired metabolism, indicates an enzyme defect or deficiency in the body. Biochemical diagnosis of hereditary metabolic disorders is carried out in two stages. At the first stage, presumptive cases of diseases are selected, at the second, the diagnosis of the disease is clarified by more accurate and complex methods. The use of biochemical studies to diagnose diseases in the prenatal period or immediately after birth allows timely detection of pathology and the initiation of specific medical measures, as, for example, in the case of phenylketonuria. To determine the content in the blood, urine or amniotic fluid of intermediate, by-products and final metabolic products other than high-quality
reactions with specific reagents for certain substances using chromatographic methods for the study of amino acids and other compounds.
DNA research methods in genetic research
As shown above, violations of the primary products of genes are detected using biochemical methods. Localization of the corresponding lesions in the hereditary material itself can be detected by molecular genetics. Method Development reverse transcriptionDNA on mRNA molecules of certain proteins, followed by the multiplication of these DNAs, led to DNA probesfor various mutations in human nucleotide sequences. The use of such DNA probes for hybridization with the DNA of the patient’s cells makes it possible to identify corresponding changes in the hereditary material, i.e. diagnose certain types of gene mutations (genodiagnosis). Important advances in molecular genetics in recent decades have been the work on sequencing -determination of the nucleotide sequence of DNA. This was made possible thanks to the opening in the 60s. XX century enzymes - restriction enzymeisolated from bacterial cells that cut a DNA molecule into fragments in strictly defined places. In vivo
restrictase gas protects the cell from penetration into its genetic apparatus and the propagation of foreign DNA in it. The use of these enzymes in an experiment makes it possible to obtain short DNA fragments in which the nucleotide sequence can be determined relatively easily. Molecular genetics and genetic engineering methods allow not only to diagnose a number of gene mutations and establish nucleotide
the sequence of individual human genes, but also to multiply (clone) them and get in large quantities proteins - products of the corresponding genes. Cloning of individual DNA fragments is carried out by incorporating them into bacterial plasmids, which, autonomously propagating in the cell, provide a large number of copies of the corresponding fragments of human DNA. Subsequent expression of recombinant DNA in bacteria allows you to get the protein product of the corresponding cloned human gene. Thus, using genetic engineering methods, it has become possible to obtain some primary gene products (insulin) based on human genes.
Twin method
This method consists in studying the patterns of inheritance of characters in pairs of single and double twins. It was proposed in 1875 by Galton initially to assess the role of heredity and the environment in the development of human mental properties. Currently, this method is widely used in the study of
heredity and variability in humans to determine the correlative role of heredity and the environment in the formation of various signs, both normal and pathological. It allows you to identify the hereditary nature of the character, to determine the penetrance of the allele, to evaluate the effectiveness of the action on
the body of some external factors (drugs, training, education).
The essence of the method is to compare the manifestation of the trait in different groups of twins, taking into account the similarities or differences in their genotypes. Monozygotic twinsdeveloping from a single fertilized egg, are genetically identical, as they have 100% of the common genes. Therefore, among monozygotic twins is observed
high percent concordant couplesin which the symptom develops in both twins. Comparison of monozygotic twins brought up under different conditions of the postembryonic period, reveals signs in
the formation of which a significant role belongs to environmental factors. According to these signs, between the twins discordancethose. differences. On the contrary, the preservation of similarities between twins, despite the differences in the conditions of their existence, testifies to the hereditary conditionality of the trait.
Comparison of pair concordance for this trait in genetically identical monozygous and dizygotic twins, which have on average about 50% of the common genes, makes it possible to more objectively judge the role of the genotype in the formation of the trait. High concordance in pairs of monozygotic twins and significantly lower concordance in pairs of dizygotic twins indicate the significance of hereditary differences in these pairs to determine the trait. The similarity of the concordance index in mono- and
dizygotic twins indicates an insignificant role of genetic differences and the decisive role of the environment in the formation of a sign or development of a disease. Significantly different, but rather low indicators of concordance in both groups of twins make it possible to judge a hereditary predisposition to the formation of a trait that develops under the influence of environmental factors.
A number of methods are used to identify the monozygosity of the twins. 1. A polysymptomatic method for comparing twins according to many morphological features (pigmentation of the eyes, hair, skin, hair shape and hairline on the head and body, shape of ears, nose, lips, nails, body, finger patterns). 2. Methods based on the immunological identity of twins in erythrocyte antigens (ABO, MN, Rhesus systems), and in whey proteins (γ-globulin). 3. The most reliable monozygosity criterion provides
transplant test using cross skin transplantation of twins. (NOT USED)
Population-statistical method
Using the population-statistical method, hereditary traits are studied in large groups of the population, in one or several generations. An essential point when using this method is the statistical processing of the data obtained. This method can calculate the frequency
occurrences in the population of different gene alleles and different genotypes for these alleles, to determine the distribution of various hereditary traits in it, including diseases. It allows you to study the mutation process, the role of heredity and the environment in the formation of phenotypic polymorphism
a person according to normal signs, as well as in the occurrence of diseases, especially with a hereditary predisposition. This method is also used to determine the significance of genetic factors in anthropogenesis, in particular in racogenesis. the basis for determining the genetic structure of a population is lawhardy - Weinberg genetic balance . It reflects a pattern, in accordance with
which, under certain conditions, the ratio of alleles of genes and genotypes in the population’s gene pool remains unchanged among the generations of this population. Based on this law, having frequency data
occurrence in a population of a recessive phenotype with a homozygous genotype (aa), it is possible to calculate the frequency of occurrence of the indicated allele (a) in the gene pool of a given generation. The mathematical expression of the Hardy - Weinberg law is the formula ( rA . + qa) ^ 2, where rand q -the frequency of occurrence of alleles A and a of the corresponding gene. The disclosure of this formula makes it possible to calculate the frequency of occurrence
people with different genotypes and especially heterozygotes - carriers of latent
recessive allele: p^ 2AA + 2pqAa + q ^ 2aa.
Simulation method.
A method for studying genetic patterns in biologically and mathematical models of an organism or population.
Biological modeling - the basis of the law homologous to a number of heredities of Vavilov. It is based on the fact that genera and species genetically close have similar series of hereditary variability, with such accuracy that knowing changes in one genus or species can be predicted from the appearance of other genera and species.
The method is based on the creation of models of hereditary human anomalies (mutant animal lines) with the aim of studying the etiology and pathogenesis of hereditary diseases. As well as the development of treatment methods - examples of biological models - hemophilia in dogs, cleft lip in rodents, diabetes in hamsters, alcoholism in rats. Deaf and Dumb in Cats
Mathematical modeling - the creation of mathematical models of populations for the purpose of calculating: the frequencies of genes and genotypes with different interactions and environmental changes, the effects of linked inheritance in the analysis of many linked genes, the role of heredity and the environment in the development of a trait, the risk of having a sick child
Genealogical methodproposed in 1883 by F. Galton. This is a method of analyzing pedigrees (tracing the inheritance of a normal or pathological trait in a family, indicating the type of kinship between members of the pedigree). In medical genetics they call him clinical and genealogical , since pathological signs are traced and clinical research methods are applied.
The essence of the method : identification of family ties and tracing of the studied trait among close, distant, direct and indirect relatives.
Method steps :
1. Collection of information about relatives from a proband (a person who contacted a geneticist).
2. Drawing up a family tree.
3. Analysis of the pedigree.
The method is used to establish the hereditary nature of the trait, type of inheritance, genotypes of members of the pedigree, gene penetrance.
To construct the pedigrees, the system of characters proposed in 1931 by the English scientist Just (Fig. 17) is used.
When building pedigrees, the following rules must be observed:
· It is necessary to find out from the collected history the number of generations;
· The pedigree begins with a proband;
· Each generation is numbered in Roman numerals on the left;
· Symbols designating individuals of one generation are located on a horizontal line and are numbered in Arabic numerals.
An analysis of the pedigree reveals the following inheritance typessigns: autosomal dominant; autosomal recessive; X-linked (with sex) dominant; X-linked (with sex) recessive; hollandric (linked to the Y-chromosome).
Autosomal dominant inheritance type:
· A sick child is born to sick parents with a 100% probability if they are homozygous; 75% if they are heterozygous.
Figure 17. Symbols used in the compilation of pedigrees
Autosomal recessive inheritance type:
· Both men and women are equally affected.
· The probability of having a sick child in healthy parents is 25% if they are heterozygous; 0% if both of them or one of them is homozygous for the dominant gene.
· Often manifested in closely related marriages.
X-linked (with sex) dominant type of inheritance:
· Patients occur in every generation.
· Women are more affected.
· If the father is sick, then all his daughters are sick.
· A sick child is born in sick parents with a 100% probability if the mother is homozygous; 75% if the mother is heterozygous.
· The probability of having a sick child in healthy parents is 0%.
X-linked (with sex) recessive inheritance type:
· Patients are not found in every generation.
· Mostly men are ill.
· The probability of giving birth to a sick boy in healthy parents is 25%, sick girls - 0%.
Hollandic (linked to the Y-chromosome) type of inheritance:
· Patients occur in every generation.
· Only men get sick.
· If the father is sick, then all his sons are sick.
· The probability of giving birth to a sick boy with a sick father is 100%
Twin method(proposed in 1876 by F. Galton for the study of genetic patterns in twins.
The essence of the method : comparison of characters in different groups of twins based on their similarity (concordance) or difference (discordance).
Method Steps:
1. Sampling twins from the entire population.
2. Diagnosis of zygosity of twins.
3. The establishment of the correlative role of heredity and environment in the formation of the trait.
To assess the role of heredity and environment in the formation and development of the trait using holzinger formula:
H \u003d (KMB% -KDB%) / 100% -KDB%
where N is the share of hereditary factors,
KMB% and - the concordance of monozygotic twins in percent
KDB% - concordance of dizygotic twins in percent
If H is greater than 0.5, then the genotype plays a large role in the formation of the trait, if H is less than 0.5, then the environment plays a large role.
Cytogenetic method- This is a study of the karyotype using microscopy.
Method Steps:
1. Obtaining and culturing cells (lymphocytes, fibroblasts) on artificial nutrient media.
2. Adding phytohemagglutinin to the culture medium to stimulate cell division.
3. Stop cell division at the metaphase stage by the addition of colchicine.
4. Treatment of cells with hypotonic solution NaClas a result of which the cell membrane is destroyed and a "scattering" of chromosomes is obtained.
5. Staining of chromosomes with specific dyes.
6. Microscopy and photographing of chromosomes.
7. Drawing up an idiogram and its analysis.
The method allows you to:
· Diagnose genomic and chromosomal mutations;
· Determine the genetic sex of the body.
Biochemical methods.The cause of most hereditary monogenic diseases is metabolic defects associated with fermentopathies (impaired structure of enzymes involved in exchange reactions). At the same time, intermediate metabolic products accumulate in the body, therefore, by determining them or the activity of enzymes using biochemical methods, hereditary metabolic diseases (gene mutations) can be diagnosed. Quantitative biochemical methods (stress tests) make it possible to identify heterozygous carriage of a pathological recessive gene.
Dermatoglyphic analysis- This is a study of human scallop skin (skin of the fingertips, palmar side of the hands and the plantar side of the feet), where the papillary dermis is very pronounced.
The method is applied:
a) to establish the zygosity of the twins;
b) as an express method for the diagnosis of the congenital component of some hereditary diseases.
Usually, in case of genomic pathology, a combination of some indicators is noted: radial loops on 4 and 5 fingers, four-finger groove, main palmar angle from 60 ° to 80 °, etc.
Chemical methodsbased on quality color chemical reactions. Used for preliminary diagnosis of hereditary metabolic diseases. Like a screening test diagnosis of phenylketonuriathe method is used to wet the urine of a child with strips of paper soaked in a 10% solution of FeCl 3 or 2,4 dinitrophenylhydrazine. If phenylpyruvic acid is present in the urine, a greenish stain of filter paper appears.
Determination of X- and Y-sex chromatin.For the study, buccal epithelial cells or leukocytes are used. A "-chromatin is determined by staining the drug acetorsein,and U-chromatin - when stained acrychiniprithom.These methods make it possible to identify the number of sex chromosomes in the karyotype (the number of A-chromosomes is always one more than the number of clumps of A 1 -chromatin, the number of Y chromosomes is equal to the number of clumps of U-chrominome); establish the genetic sex of an individual and diagnose chromosomal sex diseases (in combination with other methods).
Methods of prenatal (prenatal) diagnosishereditary diseases allow to establish hereditary defects of the fetus in the early stages of pregnancy. With their help, it is possible long before the birth of a child to determine the disease, and if it is necessary to terminate the pregnancy.
The main indicators for prenatal diagnosis are:
Accurately established hereditary disease in the family.
· The age of the mother is above 35 years, father - from 40 years.
· The presence in the family of a sex-linked disease.
· The presence of structural rearrangements of chromosomes in one of the parents (especially translocations and inversions).
· Heterozygosity of both parents for one pair of alleles in an autosomal recessive disease.
· The presence in the history of a pregnant woman of long-term work in unhealthy industries or living in places with an increased radiation background, etc.
· Repeated spontaneous abortions or the birth of a child with congenital malformations, diabetes mellitus, epilepsy, infections in a pregnant woman, drug therapy.
Methods of prenatal diagnosis can be divided into:
1) Sifters:allow you to identify women who have an increased risk of having a baby with a congenital pathology or hereditary disease. Methods are available for widespread use and are relatively inexpensive. Screening methods include:
Determination of the concentration of α-fetoprotein (AFP);
Determination of the level of human chorionic gonadotropin (hCG);
Determination of the level of unbound estriol;
Identification of pregnancy-associated plasma protein A;
Isolation of fetal cells or DNA from the mother.
2) Non-invasive: methods of examination of the fetus without surgical intervention. Currently, these include ultrasound examination of the fetus (ultrasound). Ultrasound can be used with both screening and clarifying methods. The accumulated data show that ultrasound does not harm the fetus. In some countries, ultrasound is performed for all pregnant women. This prevents the birth of 2-3 children with serious congenital malformations per 1,000 newborns, which makes up about 30% of all children with this pathology.
3) Invasive:methods based on the analysis of the genetic material of cells or tissues of the fetus. They are carried out according to strict indications. Invasive methods include:
Chorion and placenta biopsy (for cytogenetic, biochemical studies and DNA analysis);
Amniocentesis (intake of amniotic fluid of the fetus for the diagnosis of gene, chromosomal and genomic mutations);
Cordocentesis (taking blood from the umbilical cord for the purpose of early diagnosis of hereditary blood diseases);
Fetoscopy (insertion of a fibrooptic endoscope into the amnion cavity with the aim of examining the fetus, placenta, umbilical cord, etc.);
Lesson Objectives:
- To expand and deepen students' knowledge on the topic - the features of the study of human genetics,
- To form knowledge about the basic methods of studying human heredity; teach to determine the main types of inheritance in humans.
- Show the importance of genetic knowledge for modern man; the role of mutagens and the appearance of mutations in humans.
Equipment: tables on general biology, presentation “Human genetics. Methods of studying human heredity ”, on the tables of the study table“ Examples of pedigrees, “Inheritance of various traits in humans”
DURING THE CLASSES
1. Organizational moment
2. Introduction by the teacher. Knowledge Update
Human genetics studies the phenomena of heredity and variability in human populations, especially the inheritance of characters in the norm and their changes under the influence of environmental conditions. The goal of medical genetics is the development of diagnostic methods, treatment and prevention of hereditary human pathology.
3. Learning new material
The tasks of human genetics are:
- determination of the complete nucleotide sequence of the DNA of the human genome, localization of genes and the creation of their bank;
- early diagnosis of hereditary pathology by improving methods of prenatal and rapid diagnosis;
- widespread introduction of genetic counseling;
- development of genetic therapy methods for hereditary diseases based on genetic engineering;
- identification of genetically dangerous environmental factors and the development of methods for their neutralization.
Man as a specific object of genetics
The study of human genetics is associated with great difficulties:
- complex karyotype - many chromosomes and adhesion groups;
- late puberty and a rare change of generations;
- a small number of descendants;
- inability to experiment
- the impossibility of creating the same living conditions.
Despite these difficulties, human genetics is better studied today than the genetics of many other organisms (for example, mammals) due to the needs of medicine and a variety of modern research methods.
During the lesson, students are invited to fill out a table:
Table 1.Methods of studying human heredity
| method | its essence and meaning |
Research methods for human genetics
The author of many methods for the study of human genetics is F. Galton
Sir Francis Galton (English Francis Galton; February 16, 1822 - January 17, 1911) - English researcher, geographer, anthropologist and psychologist; founder of differential psychology ipsychometrics. Born in Birmingham, England.
Galton was a cousin of Charles Darwin to go to his grandfather - Erasmus Darwin.
A. Genealogy method
The oldest method of human genetics is genealogical analysis, or a pedigree analysis method. The essence of the method is the compilation of the pedigree and its subsequent analysis. It was introduced in 1865. F. Galton.
Pedigree analysis is used to identify dominant, semi-dominant, and recessive traits, map chromosomes (i.e., to determine whether a gene encoding a given trait belongs to a specific group of linkage, linkage to X- or Y-chromosomes), and to study the mutation process (especially in cases when it is necessary to distinguish newly arising mutations from those that are of a family nature, i.e., originated in previous generations).
According to the autosomal dominant type, polydactyly (an increased number of fingers), freckles, early baldness, fused fingers, cataracts of the eyes, brittle bones and many others are inherited.
Albinism, red hair, susceptibility to poliomyelitis, diabetes mellitus, congenital deafness are inherited in an autosomal recessive manner.
A number of signs are inherited, sex-linked: X-linked inheritance - hemophilia, color blindness; Y-linked - hypertrichosis (increased auricle), membranes between the fingers.
The genealogical method is widely used to solve both scientific and applied problems. It allows you to identify the hereditary nature of the trait and determine the type of inheritance .. The genealogical method is the basis of medical and genetic counseling.
Students are encouraged to solve the problem: determine the type of inheritance by pedigree
Task 1In the family genealogy there is a sign - “gray hair lock”, which is inherited as dominant (Fig. 1). Define the genotypes of the original parents. What descendants are expected from the marriage of cousins \u200b\u200band brothers a) 1 and 5; b) 2 and 6 ?.
B. The twin method
This method of studying human genetics was also introduced into medical practice by F. Galton in 1876. It makes it possible to determine the contribution of genetic (hereditary) and environmental factors (climate, nutrition, training, education, etc.) in the development of specific signs or diseases in humans.
It is known that in humans twins come in two categories. In some cases, not an ode to the egg is fertilized, but two. In this case, children of the same or different sexes are born, similar to each other as brothers and sisters who are not twins. But sometimes one egg gives rise to two (three, four) embryos. Then we get identical twins, who always belong to the same sex and find striking similarities with each other. This is understandable, since they have the same genotype, and the differences between them are solely due to the influence of the environment in the development of the physical and mental properties of a person.
B. Biochemical method
For the first time, biochemical methods began to be used to diagnose gene diseases as early as the beginning of the 20th century. Over the past 30 years, they have been widely used in the search for new forms of mutant alleles. With their help, more than 1000 congenital metabolic diseases have been described. For many of them
defect of the primary gene product was detected. The most common among these diseases are diseases associated with defective enzymes, structural, transport or other proteins.
One of the most common diseases of carbohydrate metabolism is diabetes. This disease is associated with a deficiency of the hormone insulin, which leads to a disruption in the process of glycogen formation and an increase in blood glucose.
Phenylketonuria refers to diseases of amino acid metabolism. In this case, the conversion of the essential amino acid phenylalanine to tyrosine is blocked, and phenylalanine turns into 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.
G. Cytogenetic method
The basis of the method is the microscopic study of human chromosomes. Cytogenetic studies have been widely used since the early 20s of the twentieth century to study the morphology and counting of human chromosomes, the cultivation of white blood cells to obtain metaphase plates
The development of modern human cytogenetics is associated with the names of the cytologists D. Thio and A. Levan. In 1956 They were the first to establish that a person has 46, and not 48, as they thought before, chromosomes. This event marked the beginning of a broad study of the mitotic and meiotic chromosomes of humans.
In 1960 In Denver (USA), the first International classification of human chromosomes was developed. It was based on the size of the chromosomes and the position of the primary constriction-centromere.
Each pair of chromosomes is designated by a serial number from 1 to 23, sex chromosomes are X and Y separately. Women have two X - chromosomes, men - X- and Y-chromosomes
Using this method allowed us to identify a group of diseases associated with a change in the number of chromosomes, or with a change in their structure. Such diseases are called chromosomal, they include: violations in somatic chromosomes.
"Human Chromosomal Diseases"
Learning Messages:
To date, more than 2 thousand inherited human diseases have been registered.
According to the World Health Organization, thanks to the use of new diagnostic methods, an average of three new hereditary diseases are annually recorded, which are found in the practice of a doctor of any specialty: therapist, surgeon, neurologist, obstetrician-gynecologist, pediatrician, endocrinologist, etc. Diseases that do not have absolutely no relation to heredity, practically does not exist. The course of various diseases (viral, bacterial, mycoses and even injuries) and recovery after them to one extent or another depend on the hereditary immunological, physiological, behavioral and mental characteristics of the individual.
Chromosomal diseases.
This type of hereditary disease is associated with a change in the number or structure of chromosomes. The frequency of chromosomal abnormalities in newborns is from 0.6 to 1%, and at the stage of 8-12 weeks, about 3% of embryos have them. Among spontaneous miscarriages, the frequency of chromosomal abnormalities is approximately 30%, and in the early stages (up to two months) - 50% and higher.
In humans, all types of chromosomal and genomic mutations are described, including aneuploidy, which can be of two types - monosomy and polysomy. Monosomy is especially severe.
The whole body monosomy is described for the X chromosome. This is Shereshevsky-Turner syndrome (44 + X), which manifests itself in women who are characterized by pathological changes in physique (short stature, short neck, with folds, a “Mongoloid” eye incision, a narrow pelvis, lower extremities), disorders in the development of the reproductive system (the absence of most female secondary reproductive signs), mental disability. The frequency of occurrence of this anomaly is 1: 4000-5000.
Women trisomics (44 + XXX), as a rule, are distinguished by impaired sexual, physical and mental development, although in some patients these signs may not appear. Cases of fertility of such women are known. The frequency of the syndrome is 1: 1000.
Men with Klinefelter Syndrome (44+ XXY) are characterized by impaired development and activity of the gonads, eunuchoid body type (narrower than the pelvis, shoulders, body hair and fat deposition on the female type, elongated in comparison with the trunk of the arm and legs). Hence the higher growth. These signs, combined with some mental retardation, appear in a relatively normal boy from the time of puberty.
Klinefelter's syndrome is observed during polysomy not only on the X chromosome (XXX XXXY, XXXXY), but also on the Y chromosome (XYY. XXYY. XXYYY). The frequency of the syndrome is 1: 1000.
Of the autosomal diseases, trisomy on the 21st chromosome, or down syndrome. According to various authors, the frequency of birth of children with Down syndrome is 1: 500-700 newborns, and over the past decade, the frequency of trisomy-21 has increased.
Typical signs of patients with Down syndrome: a small nose with a wide, flat nose bridge, slanting eyes with an epicanthus - an overhanging fold over the upper eyelid, deformed small auricles, half-open mouth, short stature, mental retardation. About half of patients have heart disease and large vessels.
There is a direct relationship between the risk of having children with Down syndrome and the age of the mother. It was found that 22-40% of children with this disease are born to mothers older than 40 years (2-3% of women of childbearing age).
Patau Syndrome- the syndrome is based on nondisjunction on the 13th pair of chromosomes. In the patient's karyotype, there are 47 chromosomes with an extra chromosome - 13.
Feline scream syndrome
Cytologically, in all patients, a shortening (deletion) of approximately one third of the short arm of one of the homologues of chromosome 5 is detected.
Here we consider only a few examples of human genetic and chromosomal diseases, which, however, give a certain idea of \u200b\u200bthe complexity and fragility of its genetic organization.
Violation of the number of sex chromosomes.
Diseases caused by a change in the number of sex chromosomes are much milder than autosomal abnormalities. They are usually accompanied by a decrease in mental abilities and sterility. Various syndromes are known associated with impaired heterosome numbers.
Genetics and medicine Genetic counseling
Currently, most attention is paid to measures to prevent the birth of children with hereditary pathology.
The main way to prevent hereditary diseases is to prevent them. First of all, his services should be used by persons who marry, who have genetically disadvantaged relatives. Genetic consultation is obligatory upon the marriage of relatives, persons older than 30-40 years, as well as those working in production with harmful working conditions. Doctors and geneticists will be able to determine the degree of risk of having genetically inferior offspring and provide control over the child during his intrauterine development. It should be noted that smoking, alcohol and drug use by the mother or father of the unborn child dramatically increase the likelihood of having a baby with severe hereditary ailments.
The importance of human genetics, a detailed acquaintance with the genealogy of people seeking advice, allows a geneticist to assess the degree of risk in each case.
Thus, current trends in human genetics are an active counteraction to adverse factors causing hereditary anomalies based on knowledge of genetic patterns.
4. Consolidation of acquired knowledge
1. Check the completed table No. 1
2. Checking task number 1
3. To consolidate the knowledge gained, students are offered a test
1) What is the name of the method, the essence of which is the crossing of parental forms that differ in a number of signs, the analysis of their manifestation in a number of generations?
A) hybridological
B) cytogenetic
C) twin
D) biochemical
2) Using what method is the effect of genotype and environment on the development of a child detected?
A) genealogical
B) twin
C) cytogenetic
D) hybridological
3) Chromosome sets of healthy and sick people are studied using the method
A) genealogical
B) cytogenetic
C) twin
D) hybridological
4) Chromosome sets of healthy and sick people are studied using the method
A) genealogical
B) cytogenetic
C) twin
D) hybridological
5) What variability causes the difference in phenotypes of identical twins
A) gene
B) genomic
C) modification
D) mutational
A) can be of different sex
B) always the same gender
C) have the same weight
D) have the same size
7) Using the genealogical method, you can find out
A) the nature of the change in genes
B) the influence of education on the development of mental characteristics of a person
C) patterns of inheritance of traits in humans
D) the nature of chromosome changes
8) The method of studying human heredity, which is based on the study of the number of chromosomes, the features of their structure, is called
A) genealogical
B) twin
C) hybridological
D) cytogenetic
5. Summarizing the lesson. Homework
