Heredity is the property inherent in all organisms of reproducing the appearance of substances and associated structures, morphology and functions, similar to the parent type, based on the transmission to the offspring of material factors that determine the development of the characteristics of the organism in specific environmental conditions.
The science of heredity - genetics (from the Greek genes - “something that arises and develops”) studies not only the mechanisms of transmission of hereditary characteristics, but also the chain of processes that lead to their manifestation during a person’s life. The founder of genetics is the Czech naturalist G. Mendel.
Heredity is always accompanied by variability of characteristics. When organisms reproduce, along with the preservation of some characteristics, others change.
Basic research methods:
1) hybridological analysis: the use of a system of crossings to establish the nature of inheritance of traits and genetic differences of the organisms being studied.
Hybridological analysis, supplemented after the work of G. Mendel with a number of specific methods and techniques for studying heredity, entered as the most important component of genetic analysis - the main method of genetics;
2) cytological method - the study of cell structures in connection with the reproduction of organisms and the transmission of hereditary information. Based on this method, using the latest methods for studying chromosomal structures, a new science arose - cytogenetics;
3) ontogenetic method - used to study the action of genes and their manifestation in the individual development of organisms - ontogenesis in different environmental conditions;
4) a statistical method with the help of which the statistical patterns of heredity and variability of organisms are studied.
Legend
P - parent form (from Latin parent - “parent”);
F - hybrid generation (lat. “children”);
F 1 - first generation hybrids (offspring obtained from crossing parental forms);
F 2 - second generation hybrids (offspring obtained by crossing F hybrids with each other);
♀ - maternal specimen (mirror of the ancient Roman goddess Venus);
♂ - paternal individual (shield and spear of the ancient Roman god Mars);
X - crossing.
Hybridological method
Hybridological analysis (method) requires the following conditions:
1) parental forms must belong to the same species and reproduce sexually;
2) parental forms must be homozygous (have only a dominant or recessive gene in the zygote) for the genes (traits) being studied;
3) parental forms must differ in the genes (characters) being studied;
4) parental forms are crossed once, then the first generation hybrids (F,) self-pollinate or cross with each other to obtain second generation hybrids (F 2);
5) in the first and second generations of hybrids, a strict quantitative count of individuals having the trait being studied is carried out;
6) to assess the degree of correspondence of the actually obtained number of individuals in certain phenotypic classes to the theoretically expected one, the Pearson correspondence criterion is used.
Hybridological analysis allows:
1) establish the number of genes that control the traits being studied;
2) determine the presence and type of non-allelic interaction of genes;
3) establish gene linkage;
4) determine the distance between linked genes;
5) establish sex-linked or sex-limited inheritance;
6) determine the genotypes of the parental forms being studied.
Hybridological analysis involves crossing
individuals differing in one, two or several pairs of alternative characters. Such crosses are respectively called monohybrid (one pair of alternative characters), dihybrid (two pairs of alternative characters), polyhybrid (more than two pairs of alternative characters).
Mendel's laws
The results of monohybrid crosses were summarized by Mendel in three points:
Mendel's first law (law of uniformity): all first-generation hybrids are uniform in genotype and phenotype.
Mendel's second law (law of segregation): all second-generation hybrids are split into phenotype and genotype. In a monohybrid crossing, F 2 splitting by genotype occurs in a ratio of 1:2:1, by phenotype 3:1 (with complete dominance) or 1:2:1 (with incomplete dominance of the trait). In dihybrid crossing, the splitting of F2 by phenotype and genotype is the result of the product of numerical ratios for each of the allelic pairs:
by genotype:
(1:2: 1) ((1:2: 1)=1: 2: 1: 2:4: 2: 1: 2: 1;
by phenotype:
(3: 1) ((3: 1) = 9:3: 3: 1 (with complete dominance of both characteristics);
(3: 1) ((1:2: 1) = 3:6: 3:3:2: 1 (with complete dominance of one and incomplete dominance of another trait);
(1: 2: 1) ((1: 2: 1) = 1: 2: 1: 2: 4: 2: 1: 2: 1 (with incomplete dominance of both characteristics).
Mendel's third law (law of independent combination): different pairs of traits, the genes of which are located on non-homologous chromosomes, are inherited independently of each other, as a result of which new combinations of traits arise in hybrids that are absent in the parental forms.
Gamete purity hypothesis: each gamete contains only one hereditary factor (allelic gene) from a pair. When hybrids are formed, hereditary factors are not mixed, but remain unchanged. Using the hybridological method, it is possible to study the inheritance of not only two, but also three or many pairs of alternative characters. The crosses carried out in this case will be called trihybrid and polyhybrid, respectively.
These are hybridological, genealogical, cytogenetic, biochemical, dermatoglyphic, twin, population statistical, genetic engineering methods and modeling method.
Hybridological method (crossing method) has been a staple for many years. Developed by G. Mendel. It consists of crossing (hybridization) of organisms that differ from each other in one or more hereditary characteristics.
Using crossing, it is possible to establish: 1) whether the trait under study (and the gene corresponding to it) is dominant or recessive; 2) genotype of the organism; 3) interaction of genes and the nature of this interaction; 4) linkage of genes with sex, etc.
The method has one drawback - it cannot be used in human research, since it is not possible to cross homo sapiens in an experiment.
Genealogical method consists of analyzing pedigrees, and allows you to determine the type of inheritance of a trait (dominant, recessive, autosomal or sex-linked), as well as its monogenic or polygenic nature. Based on the information obtained, the probability of manifestation of the studied trait in the offspring is predicted, which is of great importance for the prevention of hereditary diseases; to study the mutation process, especially in cases where it is necessary to distinguish newly emerged mutations from those that are familial in nature, i.e., arose in previous generations. As a rule, the genealogical method forms the basis for conclusions in medical genetic counseling (if we are not talking about chromosomal diseases).
This is how the inheritance of a person’s individual characteristics is established: facial features, height, blood type, mental and psychological makeup, as well as some diseases. For example, when studying the pedigree of the royal Habsburg dynasty, a protruding lower lip and a hooked nose can be traced over several generations.
Cytogenetic method consists of studying the number, shape and size of chromosomes in animals and plants. It is very valuable for studying both the normal karyotype (morphological features of the chromosome set) and for diagnosing hereditary diseases and mutations.
For example, when during meiosis (division of sex cells) homologous chromosomes do not diverge, then the zygote contains three homologous (responsible for the same characteristics) chromosomes instead of two. If this chromosomal aberration (trisomy) is noted in the 21st pair of chromosomes, Down syndrome occurs: Mongoloid face, irregularly shaped ears, short stature, short arms, mental retardation.
Biochemical method allows you to identify violations of the internal chemistry of the body, which may indicate carriage of an abnormal gene. Diseases based on metabolic disorders constitute a significant part of genetic hereditary pathology. These include diabetes mellitus, phenylketonuria, galactosemia (impaired absorption of milk sugar) and others. This method allows you to identify the disease at an early stage and treat it. Screening for biochemical markers of genetic diseases is now mandatory for newborns.
Dermatoglyphic method.Subject studies - drawings on the palms, soles and fingers. With chromosomal diseases, the patterns change, for example, the monkey fold on the palm in Down syndrome.
Twin method - allows us to determine the influence of the environment on identical twins who are genetically identical. This allows us to reliably assess the role of external conditions in the implementation of gene action.
Population method. Consists of determining the frequency of a gene in a population according to the Hardy-Weinberg law. Based on this method, the distribution of individuals of different genotypes is assessed, and the dynamics of the genetic structure of populations under the influence of various factors is analyzed. For example, the color blindness gene: manifests itself more in men - up to 7-8% (in women - 0.5%, although 13% are carriers of the gene).
Genetic engineering method– with its help, scientists change the genotypes of organisms: they remove and rearrange certain genes, introduce others, combine genes of different species in the genotype of one individual, etc.
Modeling method – studies human diseases in animals. This method is based on Vavilov's law.
1. Genealogical method.
The method is based on tracing a characteristic in a number of generations, indicating family ties (drawing up a pedigree).
The collection of information begins with the proband.
Proband is a person whose pedigree needs to be compiled. The proband's brothers and sisters are called sibs.
The method includes two stages:
1. Collection of information about the family.
2. Genealogical analysis.
Special symbols are used to build a pedigree. The methods allow us to establish the type of inheritance of a trait: autosomal dominant, autosomal recessive, sex-linked.
With autosomal dominant inheritance the gene appears in a heterozygous state in individuals of both sexes; immediately in the first generation; a large number of patients, both vertically and horizontally. Freckles, brachydactyly, cataracts, brittle bones, chondrodystrophic dwarfism, and polydactyly are inherited according to this type.
With autosomal recessive inheritance the mutation gene appears only in the homozygous state in individuals of both sexes. As a rule, sick children are born to healthy parents (the gene is in a heterozygous state). The symptom does not appear in every generation. This is how the following traits are inherited: Left-handedness, red hair, blue eyes, myopathy, diabetes mellitus, phenylketonuria.
With X-linked dominant inheritance Persons of both sexes are affected; it is more common in women. This is how the following symptoms are inherited: pigmentary dermatosis, keratosis (loss of hair), blistering of the feet, brown tooth enamel.
With an X-linked recessive inheritance Mostly males are affected. Half (50%) of the boys in the family are sick; 50% of the girls are heterozygous for the mutant gene. This is how hemophilia A, Duchenne muscular dystrophy, and color blindness are inherited.
With Y-linked inheritance Only men are sick. Such signs are called holandric: syndactyly, hypertrichosis.
2. Cytogenetic method.
The method is based on microscopic examination of chromosomes, analysis of normal and pathological human karyotype. The study of the chromosome set is carried out on metaphase plates of lymphocytes and fibroblasts cultured under artificial conditions. Chromosome analysis is carried out using microscopy. To identify chromosomes, a morphometric analysis of chromosome length and the ratio of their arms (centromere index) is carried out, then karyotyping is carried out according to the Denver classification. This method allows us to establish hereditary human diseases, chromosome structures, translocations, and build genetic maps.
In 1969, T. Kasperson developed a method for differential staining of chromosomes, which made it possible to identify chromosomes by the nature of the distribution of stained segments. The heterogeneity of DNA in different areas along the length of the chromosome causes different staining of segments (hetero- and euchromatic areas). This method makes it possible to detect aneuploidies, chromosomal rearrangements, translocations, polyploidies (trisomy 13, 18, 21 - autosomes; deletions). Deletions on chromosome 5 form the “cry of the cat” syndrome; on the 18th - violation of the formation of the skeleton and mental retardation.
If the disorder concerns sex chromosomes, then the method of studying sex chromatin is used. Sex chromatin (Barr body) is a spiralized X chromosome, which is inactivated in the female body on the 16th day of embryonic development. The Barr body is disc-shaped and is found in the interophase cell nuclei of mammals and humans under the nuclear membrane. Sex chromatin can be detected in any tissue. Most often, epithelial cells of the buccal mucosa are examined (buccal scraping).
In the karyotype of a normal woman there are two X chromosomes, and one of them forms a sex chromatin body. The number of sex chromatin bodies in humans and other mammals is one less than the number of X chromosomes in an individual. In a woman with the XO karyotype, the cell nuclei do not contain sex chromatin. With trisomy (XXX) - 2 bodies are formed, i.e. using sex chromatin, determine the number of sex chromosomes in blood smears; in the nuclei of neutrophilocytes, sex chromatin bodies look like drumsticks extending from the nucleus of leukocytes.
Normally, in women, chromatin - positive nuclei is 20-40%, in men - 1-3%. Y-chromatin can also be detected in the buccal epithelium. It is an intensely luminous large chromocenter located at any point in the nucleus. Normally, in males, 20-90% of nuclei contain Y-chromatin.
3. Population statistical method.
The method allows you to calculate the frequency of heterozygous carriage of a pathological gene in human populations. Distribution of gene and chromosomal abnormalities. The method uses demographic and statistical data, the mathematical processing of which is based on the Hardy-Weinberg law.
Studying the frequency of gene distribution is important for analyzing the distribution of hereditary human diseases. It is known that the overwhelming number of recessive alleles are presented in the heterozygous state. The Hardy-Weinberg law allows us to determine the frequency of carriage of a pathological gene. For example: the frequency of albinism (aq 2) is 1:20000, i.e. q 2 aa = 1/20000, which means q = √ 1/20000 = 1/141
p + q = 1, then p = 1- q = 1 1/141= 140/141; frequency of heterozygotes (carriers of the albinism gene) 2 pq Aa = 2 x140/141 x 1/141 = 1/70.
4. Twin method.
The method is based on the study of signs changing under the influence of living conditions in mono- and dizygotic twins. In genetic studies of twins, it is necessary to study both types comparatively. This is the only way to evaluate the influence of different environmental conditions on the same genotypes (in monozygotes), as well as the manifestation of different genotypes in the same environmental conditions (in dizygotes).
The similarity of characteristics in twins is called concordance, the differences in characteristics are called discordance. Comparing the degree of similarity in two groups of twins allows us to judge the role of heredity and environment in pathological symptoms. The method is based on a comparative study of the characteristics of twins. It allows you to identify a list of diseases with a hereditary predisposition, determine the role of the environment and heredity in the manifestation of the disease. To do this, use the heredity coefficient (H) and the influence of the environment (E), which are calculated using the Holzinger formula:
Н =(%MZ - %DZ/100 - %DZ) x 100
MZ - concordance of monozygotic twins, DZ - dizygotic twins.
If the value H = 1, the trait is formed to a greater extent (100%) under the influence of hereditary factors; H = 0 - the trait is influenced by the environment (100%); H = 0.5 - equal degree of influence of environment and heredity.
For example: the concordance rate of monozygotic twins for the incidence of schizophrenia is 70%, and for dizygotic twins it is 13%. Then H = 70-13 / 100-13 = 57/87 = 0.65 (65%). Therefore, the predominance of heredity is 65%, and environment - 35%.
Using the method, they study:
1. The role of heredity and environment in the formation of the characteristics of an organism;
2. Specific factors that enhance or weaken the influence of the external environment;
3. Correlation of characteristics and functions;
5. Biochemical methods.
These methods are used to diagnose metabolic diseases caused by changes in the activity of certain enzymes (gene mutations). About 500 molecular diseases have been discovered using these methods.
In various types of diseases, it is possible to determine either the abnormal protein-enzyme itself or intermediate metabolic products.
The methods include several stages:
1) Identification using simple, accessible methods (express methods), qualitative reactions of metabolic products in urine and blood.
2) Clarification of the diagnosis. For this purpose, precise chromatographic methods are used to determine enzymes, amino acids, carbohydrates, etc.
3) The use of microbiological tests based on the fact that some strains of bacteria can grow on media containing only certain amino acids and carbohydrates. If there is a substance required for bacteria in the blood or urine, then active growth of bacteria is observed on such a prepared substrate, which does not happen in a healthy person.
Biochemical methods are used to detect hemoglobinopathies, diseases of metabolic disorders of amino acids (phenylkentonuria, alkaptonuria), carbohydrates (diabetes mellitus, galactosemia), lipids (amaurotic idiocy), copper (Konovalov-Wilson disease), iron (hemochromatosis), etc.
6. Dermatoglyphics method.
Dermatoglyphics is a branch of genetics that studies hereditary conditioned skin reliefs on the fingers, palms and soles of the feet. These parts of the body have epidermal projections - ridges that form complex patterns. Skin patterns are strictly individual and genetically determined. The process of formation of capillary relief occurs during 3-6 months of intrauterine development. The mechanism of ridge formation is associated with the morphogenetic relationship between the epidermis and underlying tissues.
Genes that ensure the formation of patterns on the fingertips are involved in the regulation of fluid saturation of the epidermis and dermis.
Gene A - causes the appearance of an arch on the finger pad, gene W - the appearance of a curl, gene L - the appearance of a loop. Thus, there are three main types of patterns on the fingertips (Fig. 5.5). Frequency of occurrence of patterns: arcs - 6%, loops - about 60%, curls - 34%. A quantitative indicator of dermatoglyphics is the ridge count (the number of papillary lines between the delta and the center of the pattern; delta is the points of convergence of papillary lines that form a figure in the form of the Greek letter delta Δ).
On average, there are 15 - 20 ridges on one finger, on 10 fingers in men - 144.98; for women - 127.23 combs.
Palmar relief (palmoscopy) is more complex. It reveals a number of fields of pads and palmar lines. At the bases of the II, III, IY, Y fingers there are finger triradii (a, b, c, e), at the base of the palm - palmar (t). The palmar angle - a t d normally does not exceed 57 0 (Fig. 5.6).
Skin patterns are hereditary. The ridge texture of the skin is inherited polygenically.
The formation of dermatoglyphic patterns can be influenced by some damaging factors in the early stages of embryogenesis (for example, intrauterine exposure to the rubella virus produces deviations in patterns similar to Down's disease).
The dermatoglyphics method is used in clinical genetics as an additional confirmation of the diagnosis of chromosomal syndromes with karyotype changes.
7. Immunological methods.
The methods are based on studying the antigenic composition of cells and body fluids - blood, saliva, gastric juice. The most commonly used antigens are erythrocytes, leukocytes, and blood proteins. Different types of erythrocyte antigens form blood group systems - AB0, Rh - factor. Knowledge of the characteristics of blood immunogenetics is necessary during blood transfusion.
8. Ontogenetic method.
The ontogenetic method allows us to study the patterns of manifestation of traits during development. The purpose of the method is early diagnosis and prevention of hereditary diseases. The method is based on biochemical, cytogenetic and immunological methods. In the early stages of postnatal ontogenesis, diseases such as phenylketonuria, galactosemia, and Vitamin D-resistant rickets appear, the timely diagnosis of which contributes to preventive measures that reduce the pathology of the diseases. Diseases such as diabetes, gout, and alkaptonuria appear at later stages of ontogenesis. The method is of particular importance when studying the activity of genes that are in a heterozygous state, which makes it possible to identify recessive X-linked diseases. Heterozygous carriage is revealed by studying the symptoms of the disease (for anophthalmia - reduction of eyeballs); using stress tests (increased levels of phenylalanine in the blood in patients with phenylketonuria); using microscopic examination of tissue blood cells (accumulation of glycogen during glycogenosis); using direct determination of gene activity.
9. Method of somatic cell genetics.
Based on the study of hereditary material in cell clones from tissues grown outside the body on nutrient media. In this case, it is possible to obtain genes in their pure form and obtain hybrid cells. This allows us to analyze the linkage of genes and their localization, mechanisms of gene interaction, regulation of gene activity, gene mutations.
The use of anthropogenetics methods allows for a timely diagnosis of a hereditary disease.
The main method of genetics is hybridological(crossing certain organisms and analyzing their offspring, this method was used by G. Mendel).
The hybridological method is not suitable for humans for moral and ethical reasons, as well as due to the small number of children and late puberty. Therefore, indirect methods are used to study human genetics.
1) Genealogical- study of genealogies. Allows you to determine patterns of inheritance of traits, for example:
- if a trait appears in every generation, then it is dominant (right-handedness)
- if after a generation - recessive (blue eye color)
- if it occurs more often in one sex, this is a sex-linked trait (hemophilia, color blindness)
2) Twin- comparison of identical twins allows us to study modification variability (determine the impact of genotype and environment on the development of the child).
Identical twins occur when one embryo at the 30-60 cell stage divides into 2 parts and each part grows into a child. Such twins are always of the same sex and are very similar to each other (because they have exactly the same genotype). The differences that occur in such twins throughout life are associated with exposure to environmental conditions.
Fraternal twins (not studied in the twin method) are produced when two eggs are simultaneously fertilized in the mother's reproductive tract. Such twins can be of the same or different sexes, similar to each other like ordinary brothers and sisters.
3) Cytogenetic- study under a microscope of the chromosome set - the number of chromosomes, features of their structure. Allows detection of chromosomal diseases. For example, in Down syndrome there is one extra chromosome 21.
4) Biochemical- study of the chemical composition of the body. Allows you to find out whether patients are heterozygotes for a pathological gene. For example, heterozygotes for the phenylketonuria gene do not get sick, but an increased content of phenylalanine can be found in their blood.
5) Population genetic- study of the proportion of various genes in the population. Based on the Hardy-Weinberg law. Allows you to calculate the frequency of normal and pathological phenotypes.
Choose one, the most correct option. What method is used to identify the influence of genotype and environment on a child’s development?
1) genealogical
2) twin
3) cytogenetic
4) hybridological
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. The twin research method is used
1) cytologists
2) zoologists
3) genetics
4) breeders
5) biochemists
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. Geneticists, using the genealogical research method, make
1) genetic map of chromosomes
2) crossing scheme
3) family tree
4) the scheme of ancestral parents and their family ties in a number of generations
5) variation curve
Answer
1. Choose two correct answers out of five and write down the numbers under which they are indicated. The genealogical research method is used to establish
1) the dominant nature of inheritance of the trait
2) sequence of stages of individual development
3) causes of chromosomal mutations
4) type of higher nervous activity
5) linkage of the trait with sex
Answer
2. Select two correct answers out of five and write down the numbers under which they are indicated in the table. The genealogical method allows us to determine
1) the degree of influence of the environment on the formation of the phenotype
2) the influence of upbringing on human ontogenesis
3) type of inheritance of the trait
4) intensity of the mutation process
5) stages of evolution of the organic world
Answer
3. Select two correct answers out of five and write down the numbers under which they are indicated in the table. The genealogical method is used to determine
3) patterns of inheritance of traits
4) number of mutations
5) hereditary nature of the trait
Answer
4. Choose two correct answers out of five and write down the numbers under which they are indicated. The genealogical method is used to
1) studying the influence of education on human ontogenesis
2) obtaining gene and genomic mutations
3) studying the stages of evolution of the organic world
4) identification of hereditary diseases in the family
5) studies of human heredity and variability
Answer
5. Choose two correct answers out of five and write down the numbers under which they are indicated. The genealogical method is used to determine
1) the degree of influence of environmental factors on the formation of the trait
2) the nature of inheritance of the trait
3) the probability of transmitting a trait over generations
4) chromosome structure and karyotype
5) frequency of occurrence of the pathological gene in the population
Answer
Choose one, the most correct option. The main method for studying patterns of inheritance of traits
1) genealogical
2) cytogenetic
3) hybridological
4) twin
Answer
Choose one, the most correct option. To determine the nature of the influence of the genotype on the formation of the phenotype in humans, the nature of the manifestation of traits is analyzed
1) in the same family
2) in large populations
3) in identical twins
4) in fraternal twins
Answer
Establish a correspondence between the characteristic and the method: 1) cytogenetic, 2) genealogical. Write numbers 1 and 2 in the correct order.
A) family pedigree is examined
B) the linkage of the trait with sex is revealed
C) the number of chromosomes is studied at the metaphase stage of mitosis
D) a dominant trait is established
D) the presence of genomic mutations is determined
Answer
Choose one, the most correct option. A method that allows one to study the influence of environmental conditions on the development of traits
1) hybridological
2) cytogenetic
3) genealogical
4) twin
Answer
Choose one, the most correct option. What genetic method is used to determine the role of environmental factors in the formation of a person’s phenotype?
1) genealogical
2) biochemical
3) paleontological
4) twin
Answer
Choose one, the most correct option. What method is used in genetics when studying genomic mutations?
1) twin
2) genealogical
3) biochemical
4) cytogenetic
Answer
1. Choose two correct answers out of five and write down the numbers under which they are indicated. The cytogenetic method is used to determine
1) the degree of influence of the environment on the formation of the phenotype
2) inheritance of sex-linked characteristics
3) karyotype of the organism
4) chromosomal abnormalities
5) the possibility of manifestation of traits in descendants
Answer
2. Choose two correct answers out of five and write down the numbers under which they are indicated. The cytogenetic method makes it possible to study in humans
1) hereditary diseases associated with genomic mutations
2) development of symptoms in twins
3) the metabolic features of his body
4) its chromosome set
5) the pedigree of his family
Answer
3. Choose two correct answers out of five and write down the numbers under which they are indicated. Cytogenetic method for studying human genetics
1) based on the compilation of human pedigrees
2) used to study the characteristic inheritance of a trait
3) consists of a microscopic examination of the structure of chromosomes and their number
4) used to identify chromosomal and genomic mutations
5) helps to establish the degree of influence of the environment on the development of traits
Answer
All but two of the following research methods are used to study human heredity and variability. Identify these two methods that are “outliers” from the general list, and write down the numbers under which they are indicated.
1) genealogically
2) hybridological
3) cytogenetic
4) experimental
5) biochemical
Answer
Select three sentences from the text that correctly characterize the methods of studying human genetics and heredity. Write down the numbers under which they are indicated. (1) The genealogical method used in human genetics is based on the study of the family tree. (2) Thanks to the genealogical method, the nature of inheritance of specific characteristics was established. (3) The twin method allows us to predict the birth of identical twins. (4) When using the cytogenetic method, the inheritance of blood groups in humans is determined. (5) The inheritance pattern of hemophilia (poor blood clotting) was established through pedigree analysis as an X-linked recessive gene. (6) The hybridological method makes it possible to study the spread of diseases throughout the natural zones of the Earth.
Answer
Below is a list of genetics methods. All of them, except two, relate to methods of human genetics. Find two terms that “fall out” from the general series and write down the numbers under which they are indicated.
1) twin
2) genealogical
3) cytogenetic
4) hybridological
5) individual selection
Answer
1. Choose two correct answer options out of five and write down the numbers under which they are indicated. The biochemical research method is used for:
1) studying the karyotype of the organism
2) establishing the nature of inheritance of a trait
3) diagnosis of diabetes mellitus
4) determination of enzyme defects
5) determining the mass and density of cell organelles
Answer
2. Choose two correct answers out of five and write down the numbers under which they are indicated. The biochemical research method is used for
1) determining the degree of influence of the environment on the development of traits
2) studying metabolism
3) studying the karyotype of the organism
4) studies of chromosomal and genomic mutations
5) clarification of diagnoses of diabetes mellitus or phenylketonuria
Answer
1. Choose three options. The essence of the hybridological method is
1) crossing individuals that differ in several characteristics
2) studying the nature of inheritance of alternative traits
3) use of genetic maps
4) the use of mass selection
5) quantitative accounting of phenotypic characteristics of descendants
6) selection of parents according to the norm of reaction of signs
Answer
2. Choose two correct answers. The features of the hybridological method include
1) selection of parental pairs with alternative characteristics
2) the presence of chromosomal rearrangements
3) quantitative accounting of the inheritance of each trait
4) identification of mutant genes
5) determination of the number of chromosomes in somatic cells
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. What scientific research methods are used to diagnose diabetes mellitus and identify the nature of its inheritance?
1) biochemical
2) cytogenetic
3) twin
4) genealogical
5) historical
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated in the table. Methods used in human genetics
1) cytogenetic
2) genealogical
3) individual selection
4) hybridological
5) polyploidization
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. To study hereditary human diseases, cells of amniotic fluid are examined using methods
1) cytogenetic
2) biochemical
3) hybridological
4) physiological
5) comparative anatomical
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. The population statistical method for studying human genetics is used to
1) calculating the frequency of occurrence of normal and pathological genes
2) studying biochemical reactions and metabolism
3) predicting the likelihood of genetic abnormalities
4) determining the degree of influence of the environment on the development of traits
5) studying the structure of genes, their number and location in the DNA molecule
Answer
Establish a correspondence between examples and methods for detecting mutations: 1) biochemical, 2) cytogenetic. Write numbers 1 and 2 in the order corresponding to the letters.
A) loss of the X chromosome
B) formation of meaningless triplets
B) the appearance of an additional chromosome
D) change in DNA structure within a gene
D) change in chromosome morphology
E) change in the number of chromosomes in the karyotype
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. The twin method for studying human genetics is used to
1) studying the nature of inheritance of a trait
2) determining the degree of influence of the environment on the development of traits
3) predicting the probability of having twins
4) assessment of genetic predisposition to various diseases
5) calculating the frequency of occurrence of normal and pathological genes
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. Used in genetics
1) convergent similarity of individuals
2) hybridological analysis
3) crossing of individuals
4) artificial mutagenesis
5) centrifugation
Answer
Analyze the table “Methods for studying human heredity.” For each cell indicated by a letter, select the corresponding term from the list provided.
1) establishing the nature of inheritance of various traits
2) microscopic examination of the number and structure of chromosomes
3) biochemical method
4) cytogenetic method
5) twin method
6) study of family ties between people
7) study of the chemical composition of blood
8) identification of metabolic disorders
Answer
© D.V. Pozdnyakov, 2009-2019
For genetic research, a person is an inconvenient object, since in humans: experimental crossing is impossible; a large number of chromosomes; puberty occurs late; small number of descendants in each family; it is impossible to equalize living conditions for offspring.
A number of research methods are used in human genetics.
Genealogical method
The use of this method is possible 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. After compiling the pedigree, it is analyzed in order to establish the nature of inheritance of the trait being studied.
Conventions adopted 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 - dizygotic twins; 11 - monozygotic twins.
Thanks to the genealogical method, the types of inheritance of many traits in humans have been determined. Thus, the autosomal dominant type inherits polydactyly (increased number of fingers), the ability to curl the tongue into a tube, brachydactyly (short fingers due to the absence of two phalanges on the fingers), freckles, early baldness, fused fingers, cleft lip, cleft palate, eye cataracts, bone fragility and many others. Albinism, red hair, susceptibility to polio, diabetes mellitus, congenital deafness and other traits are inherited as autosomal recessive.
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).
3 - pedigree for polydactyly (autosomal dominant inheritance).
A number of traits are inherited in a sex-linked manner: X-linked inheritance - hemophilia, color blindness; Y-linked - hypertrichosis of the edge of the auricle, webbed toes. There are a number of genes localized in homologous regions of the X and Y chromosomes, for example, general color blindness.
The use of the genealogical method has shown 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. An example of this is the inheritance of hemophilia in the royal houses of Europe.
- hemophiliac; - female carrier.
Twin method
1 - monozygotic twins; 2 - dizygotic twins.
Twins are children born at the same time. They are monozygotic(identical) and dizygotic(fraternal).
Monozygotic twins develop from one zygote (1), 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 reasons.
Dizygotic twins develop from two or more eggs that were simultaneously ovulated and fertilized by different sperm (2). Therefore, they have different genotypes and can be of the same or different sexes. Unlike monozygotic twins, dizygotic twins are characterized by discordance - dissimilarity in many ways. Data on twin concordance for some characteristics are shown 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 |
| "Harelip" | 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 for all of the above characteristics is significantly higher than that of dizygotic twins, but it is not absolute. As a rule, discordance in monozygotic 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, epilepsy, diabetes mellitus and others.
Observations of monozygotic twins provide material for elucidating the role of heredity and environment in the development of traits. Moreover, the external environment refers not only to physical environmental factors, but also to 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. 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.
The material for karyotypic analysis is most often blood lymphocytes. Blood is taken from a vein in adults, and from a finger, earlobe or heel in newborns. Lymphocytes are cultured in a special nutrient medium, which, in particular, contains added substances that “force” lymphocytes to intensively divide through mitosis. After some time, colchicine is added to the cell culture. Colchicine stops mitosis at the metaphase level. It is during metaphase that the chromosomes are most condensed. Next, the cells are transferred to glass slides, dried and stained with various dyes. Staining can be a) routine (chromosomes are stained evenly), b) differential (chromosomes acquire cross-striations, with each chromosome having an individual pattern). Routine staining makes it possible to identify genomic mutations, determine the group affiliation of a chromosome, and find out in which group the number of chromosomes has changed. Differential staining allows you to identify chromosomal mutations, determine the chromosome by number, and find out the type of chromosomal mutation.
In cases where it is necessary to conduct a karyotypic analysis of the fetus, cells from the amniotic (amniotic fluid) fluid - a mixture of fibroblast-like and epithelial cells - are taken for cultivation.
Chromosome diseases include: Klinefelter syndrome, Turner-Shereshevsky syndrome, Down syndrome, Patau syndrome, Edwards syndrome and others.
Patients with Klinefelter syndrome (47, XXY) are always men. They are characterized by underdevelopment of the gonads, degeneration of the seminiferous tubules, often mental retardation, and high growth (due to disproportionately long legs).
Turner-Shereshevsky 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 Turner-Shereshevsky 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 is 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 signs: shortened limbs, a small skull, a flat, wide nose bridge, narrow palpebral fissures with an oblique incision, the presence of a fold in 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. Yes, deletion R-arm of autosome No. 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.
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 result, 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 is a disease 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 population understand a collection of individuals of the same species, living for a long time 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 collections of individuals of a given species. A population is not only a form of existence of a species, but also a unit of evolution, since the microevolutionary processes that culminate in the formation of a species are based on genetic transformations in populations.
A special branch of genetics deals with the study of the genetic structure of populations - population genetics. In humans, three types of populations are distinguished: 1) panmictic, 2) demes, 3) isolates, which differ from each other in numbers, frequency of intragroup marriages, proportion of immigrants, and population growth. The population of a large city corresponds to a panmictic population. The genetic characteristics of any population include the following indicators: 1) gene pool(the totality of genotypes of all individuals in a population), 2) gene frequencies, 3) genotype frequencies, 4) phenotype frequencies, marriage system, 5) factors changing gene frequencies.
To determine the frequency of occurrence of certain genes and genotypes, it is used Hardy-Weinberg law.
Hardy-Weinberg Law
In an ideal population, from generation to generation, a strictly defined ratio of the frequencies of dominant and recessive genes is maintained (1), as well as the ratio of the frequencies of genotypic classes of individuals (2).
p + q = 1, (1)R 2 + 2pq + q 2 = 1, (2)
Where p— frequency of occurrence of the dominant gene A; q— frequency of occurrence of the recessive gene a; R 2 - frequency of occurrence of homozygotes for the dominant AA; 2 pq— frequency of occurrence of heterozygotes Aa; q 2 - frequency of occurrence of homozygotes for the recessive aa.
The ideal population is a sufficiently large, panmictic (panmixia - free crossing) population in which there is no mutation process, natural selection and other factors that disturb the balance of genes. It is clear that ideal populations do not exist in nature; in real populations, the Hardy-Weinberg law is used with amendments.
The Hardy-Weinberg law, in particular, is used to approximate the number of carriers of recessive genes for hereditary diseases. For example, phenylketonuria is known to occur at a frequency of 1:10,000 in this population. Phenylketonuria is inherited in an autosomal recessive manner, therefore, patients with phenylketonuria have the aa genotype, that is q 2 = 0.0001. From here: q = 0,01; p= 1 - 0.01 = 0.99. Carriers of a recessive gene have the genotype Aa, that is, they are heterozygotes. 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, you can calculate the frequency of occurrence of homozygotes by dominant (AA): p 2 = 0.992, just under 98%.
A change in the balance of genotypes and alleles in a panmictic population occurs under the influence of constantly acting factors, which include: mutation process, population waves, isolation, natural selection, genetic drift, emigration, immigration, inbreeding. 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.
Human genetics is one of the most rapidly developing branches of science. It is the theoretical basis of medicine and reveals the biological basis of hereditary diseases. Knowledge of the genetic nature of diseases allows you to make an accurate diagnosis in time and carry out the necessary treatment.
Go to lectures No. 21"Variability"