What is an allele? Allelic genes. Definition, types of interaction and influence on hereditary traits

Allelic genes– genes located in identical regions of homologous chromosomes and controlling the development of variations of one trait.

Nonallelic genes are located in different parts of homologous chromosomes and control the development of various traits.

The concept of gene action.

A gene is a section of a DNA or RNA molecule that encodes a sequence of nucleotides in tRNA and rRNA or a sequence of amino acids in a polypeptide.

Characteristics of gene action:

The gene is discrete

The gene is specific - each gene is responsible for the synthesis of a strictly specific substance

The gene acts gradually

Pleiotropic effect - 1 gene acts on the change or manifestation of several signs (1910 Plate) phenylketonuria, Marfan syndrome

Polymer action - several genes are needed for the expressiveness of a trait (1908 Nilsson-Ehle)

Genes interact with each other through protein products determined by them

Gene expression is influenced by environmental factors

List the types of interactions between allelic and non-allelic genes.

Between alleles:

Complete Domination

Incomplete dominance

Codominance

Overdominance

Between non-allelic: (a trait or properties are determined by two or more non-allelic genes that interact with each other. Although here the interaction is conditional, because it is not the genes that interact, but the products controlled by them. In this case, there is a deviation from the Mendeleevian laws of segregation).

Complementarity

Polymerism

The essence of complete dominance. Examples.

Complete dominance is a type of interaction of allelic genes in which the dominant gene (A) completely suppresses the action of the recessive gene (a) (freckles)

Incomplete dominance. Examples.

Incomplete dominance is a type of interaction of allelic genes in which the dominant allele does not completely suppress the action of the recessive allele, forming a trait with an intermediate degree of degeneracy (eye color, hair shape)

Overdominance as the basis of heterosis. Examples.

Overdominance is a type of interaction of allelic genes in which a gene in a heterozygous state has a greater phenotypic manifestation of a trait than a homozygous one.

Sickle cell anemia. A – hemoglobinA, and – hemoglobinS. AA – 100% normal red blood cells, more susceptible to malaria; aa – 100% mutated (die), Aa – 50% mutated, practically not susceptible to malaria because already amazed

Codominance and its essence. Examples.

Codominance is a type of interaction of allelic genes in which several alleles of a gene are involved in the determination of a trait and a new trait is formed. One allelic gene complements the action of another allelic gene, the new trait differs from the parental ones (ABO blood group).

The phenomenon of independent manifestation of both alleles in the phenotype of a heterozygote, in other words, the absence of dominant-recessive relationships between alleles. Most famous example- interaction of alleles that determine the fourth human blood group (AB). A multiple series is known, consisting of three alleles of gene I, which determines the sign of a person’s blood group. Gene I is responsible for the synthesis of enzymes that attach certain polysaccharides to proteins located on the surface of red blood cells. (These polysaccharides on the surface of red blood cells determine the specificity of blood groups.) Alleles 1 A and 1 B encode two different enzymes; the 1° allele does not code for anything. In this case, the 1° allele is recessive in relation to both 1 A and I B, and there is no dominant-recessive relationship between the last two. People with the fourth blood group carry two alleles in their genotype: 1 A and 1 B. Since there is no dominant-recessive relationship between these two alleles, both enzymes are synthesized in the body of such people and the corresponding phenotype is formed - the fourth blood group.

The theory of multiple alleles. Inheritance of blood groups of the AB0 system.

Sometimes allelic genes may include not two, but a larger number of genes. They are called multiple alleles. Multiple alleles arise as a result of repeated mutations of the same locus in the chromosome. Thus, in addition to the main dominant and recessive allelic genes, intermediate genes arise between them, which behave as recessive genes in relation to the dominant one, and as dominant genes in relation to the recessive one.

Genetic and physiological characteristics of the AB0 system

From the point of view of genetics, the most studied is the AB0 system, which determines I (0), II (A), III (B) and IV (AB) blood groups. On the surface of erythrocytes there may be agglutinogens (antigens) A and B, and in the blood plasma there may be agglutinins (antibodies)  and . Normally, agglutinogens and agglutinins of the same name are not detected together. It should be noted that A- and B-antigens form a numerous series of antigens (A 1, A 2 ... A; B 1, B 2 ... B).

Inheritance of blood groups of the AB0 system. In the AB0 system, the synthesis of agglutinogens and agglutinins is determined by the alleles of the gene I : I 0 , I A , I B. Gene I controls both the formation of antigens and the formation of antibodies. In this case, complete dominance of the alleles is observed I A And I B over the allele I 0 , but joint dominance (codominance) of alleles I A And I B. The correspondence of genotypes, agglutinogens, agglutinins and blood groups (phenotypes) can be expressed in the form of a table:

Genotypes	Antigens (agglutinogens)	Antibodies (agglutinins)	Blood groups (phenotypes)
I 0 I 0		, 
I A I A , I A I 0
I B I B , I B I 0			III (B)
I A I B			IV (AB)

Normally, normal antibodies (agglutinins) are formed, which are synthesized in very small quantities; they belong to class M; When immunized with foreign antigens, class G immune antibodies are produced (the differences between normal and immune antibodies will be discussed in more detail below). If for some reason agglutinogen A meets agglutinin  or agglutinogen B meets agglutinin , then an agglutination reaction occurs - the gluing of red blood cells. Subsequently, agglutinated red blood cells undergo hemolysis (destruction), the products of which are poisonous.

Due to codominance, inheritance of ABO blood groups occurs in a complex manner. For example, if the mother is heterozygous for II blood group (genotype I A I 0 ), and the father is heterozygous for III blood group (genotype I B I 0 ), then their offspring can equally likely produce a child with any blood type. If the mother I blood type (genotype I 0 I 0 ), and my father's IV blood type (genotype I A I B), then their offspring is equally likely to have a child or a child with II(genotype I A I 0 ), or with III(genotype I B I 0 ) blood type (but not with I, and not with IV).

The concept of complementary gene interaction. Examples.

Complementarity is a type of interaction of non-allelic genes, in which 2 non-allelic genes, located simultaneously in the genotype, complement each other’s action, which leads to the formation of a new trait that is absent in the parental forms.

Moreover, the corresponding trait develops only in the presence of both non-allelic genes. For example, gray coat color in mice is controlled by two genes (A and B). Gene A determines the synthesis of pigment, however, both homozygotes (AA) and heterozygotes (Aa) are albinos. Another gene, B, provides pigment accumulations mainly at the base and ends of the hair. Crossing of diheterozygotes (AaBb x AaBb) leads to the splitting of hybrids in a ratio of 9:3:4. Numerical ratios during complementary interaction can be as high as 9:7; 9:6:1 (modification of the Mendelian split). An example of complementary gene interaction in humans can be the synthesis of a protective protein - interferon. Its formation in the body is associated with the complementary interaction of two non-allelic genes located on different chromosomes.

Epistatic interaction of genes. Examples.

Epistasis is a type of interaction of non-allelic genes in which a gene from one allelic pair suppresses the action of a non-allelic gene from another pair.

Suppressing gene – epistatic

Repressed gene – hypostatic

Oppression can be caused by both dominant and recessive genes (A>B, a>B, B>A, B>A), and depending on this they distinguish epistasis is dominant and recessive. The suppressive gene was named inhibitor or suppressor. Inhibitor genes generally do not determine the development of a particular trait, but only suppress the action of another gene. The gene whose effect is suppressed is called hypostatic. With epistatic gene interaction, the phenotypic segregation in F2 is 13:3; 12:3:1 or 9:3:4, etc. The color of pumpkin fruits and the color of horses are determined by this type of interaction. If the suppressor gene is recessive, then cryptomeria(Greek hristad - secret, hidden).

For a person, such an example could be the “Bombay Phenomenon”. In this case, the rare recessive allele “h” in the homozygous state (hh) suppresses the activity of the jB gene (which determines the B (III) blood group of the ABO system). Therefore, a woman with the genotype jв_hh phenotypically has blood group I - 0 (I).

During epistasis, one of the genes (B) is expressed phenotypically only in the absence of a certain allele of another gene (A) in the genotype. In its presence, the effect of gene B does not manifest itself. In the strict sense of the word, this type of interaction of non-allelic genes can be considered as a variant of the complementary action of certain alleles of these genes, when one of them is capable of ensuring the development of a trait, but only in the presence of a certain allele of another gene. In this situation, the phenotype of an organism depends on the specific combination of alleles of non-allelic genes in their genotypes, and the phenotypic segregation in the offspring of two diheterozygotes for these genes may be different.

At dominant epistasis, when the dominant allele of one gene (A) prevents the expression of alleles of another gene (B or b), segregation in the offspring depends on their phenotypic significance and can be expressed in ratios of 12:3:1 or 13:3 (Fig. 6.19). With recessive epistasis a gene that determines a trait (B) does not appear in homozygotes for a recessive allele of another gene (aa). The splitting in the offspring of two diheterozygotes for such genes will correspond to the ratio 9:3:4 (Fig. 6.20). The inability to form a trait during recessive epistasis is also regarded as a manifestation of a failed complementary interaction that occurs between the dominant allele of the epistatic gene and the alleles of the gene that determines this trait.

From this point of view, we can consider the “Bombay phenomenon” in humans, in which in organisms that carry the dominant allele of the gene that determines the blood group according to the AB0 system (I A or I B), these alleles do not manifest themselves phenotypically and blood group I is formed (see Fig. 3.82). The absence of phenotypic manifestation of dominant alleles of the I gene is associated with the homozygosity of some organisms for the recessive allele of the H gene (hh), which prevents the formation of antigens on the surface of erythrocytes. In a marriage of diheterozygotes for the H and I genes (HhI A I B), 1/4 of the offspring will phenotypically have blood type I due to their homozygosity for the recessive allele of the H gene - hh.

Polymerism and its role in the determination of quantitative traits. Additive effect.

Polymerism is the interaction of non-allelic genes, in which several non-allelic genes influence the formation of one trait (skin color). 1908 Nilsson-Ehle.

An important feature of polymers is the summation of the effect of non-allelic genes on the development of quantitative traits. If with monogenic inheritance of a trait there are three possible variants of gene “doses” in the genotype: AA, Aa, aa, then with polygenic inheritance their number increases to four or more. The summation of the “doses” of polymer genes ensures the existence of continuous series of quantitative changes.

The biological significance of polymers also lies in the fact that the traits encoded by these genes are more stable than those encoded by a single gene. An organism without polymer genes would be very unstable: any mutation or recombination would lead to sharp variability, and this in most cases is unfavorable. Animals and plants have many polygenic traits, among them those that are valuable for the economy: growth rate, early maturity, egg production, amount of milk, content of sugary substances and vitamins, etc. Skin pigmentation in humans is determined by five or six polymer genes. In indigenous Africans (the Negroid race), dominant alleles predominate, while in representatives of the Caucasian race, recessive alleles predominate. Therefore, mulattoes have intermediate pigmentation, but when mulattoes marry, they may have both more and less intensely pigmented children. Many morphological, physiological and pathological features of a person are determined by polymer genes: growth, body mass, blood pressure, etc. The development of such traits in humans is subject to the general laws of polygenic inheritance and depends on environmental conditions. In these cases, there is, for example, a tendency to hypertension, obesity, etc. These signs may not appear or appear slightly under favorable environmental conditions. These polygenic traits differ from monogenic ones. By changing environmental conditions, it is possible to prevent a number of polygenic diseases.

Inheritance of traits during polymeric interaction of genes. In the case when a complex trait is determined by several pairs of genes in the genotype and their interaction is reduced to the accumulation of the effect of certain alleles of these genes, different degrees of expression of the trait are observed in the offspring of heterozygotes, depending on the total dose of the corresponding alleles. For example, the degree of skin pigmentation in humans, determined by four pairs of genes, ranges from the maximum expressed in homozygotes for dominant alleles in all four pairs (P 1 P 1 P 2 P 2 P 3 P 3 P 4 P 4) to the minimum in homozygotes for recessive ones alleles (p 1 p 1 p 2 p 2 p 3 p 3 p 4 p 4). When two mulattoes are married, heterozygous for all four pairs, which form 2 4 = 16 types of gametes, the offspring is obtained, 1/256 of which have maximum skin pigmentation, 1/256 - minimum, and the rest are characterized by intermediate indicators of the expressiveness of this trait. In the example discussed, dominant alleles of polygenes determine the synthesis of pigment, while recessive alleles practically do not provide this trait. Skin cells of organisms homozygous for recessive alleles of all genes contain a minimal amount of pigment granules.

In some cases, dominant and recessive alleles of polygenes can provide the development of different variants of traits. For example, in the shepherd's purse plant, two genes have the same effect on determining the shape of the pod. Their dominant alleles produce one and their recessive alleles produce a different pod shape. When crossing two diheterozygotes for these genes, a 15:1 split is observed in the offspring, where 15/16 offspring have from 1 to 4 dominant alleles, and 1/16 have no dominant alleles in the genotype.

Pleiotropic action of genes. Examples.

Pleiotropic action of genes- this is the dependence of several traits on one gene, that is, the multiple effects of one gene. In Drosophila, the gene for white eye color simultaneously affects the color of the body, length, wings, structure of the reproductive apparatus, reduces fertility, and reduces life expectancy. A hereditary disease is known in humans - arachnodactyly ("spider fingers" - very thin and long fingers), or Marfan's disease. The gene responsible for this disease causes a disorder in the development of connective tissue and simultaneously affects the development of several signs: disruption of the structure of the eye lens, abnormalities in the cardiovascular system. The pleiotropic effect of a gene can be primary or secondary. With primary pleiotropy the gene exhibits its multiple effects. For example, in Hartnup disease, a gene mutation leads to impaired absorption of the amino acid tryptophan in the intestine and its reabsorption in the renal tubules. In this case, the membranes of intestinal epithelial cells and renal tubules are simultaneously affected, with disorders of the digestive and excretory systems. With secondary pleiotropy there is one primary phenotypic manifestation of a gene, followed by a stepwise process of secondary changes leading to multiple effects. Thus, with sickle cell anemia, homozygotes exhibit several pathological signs: anemia, an enlarged spleen, damage to the skin, heart, kidneys and brain. Therefore, homozygotes with the sickle cell anemia gene usually die in childhood. All these phenotypic manifestations of the gene constitute a hierarchy of secondary manifestations. The root cause, the direct phenotypic manifestation of the defective gene, is abnormal hemoglobin and sickle-shaped red blood cells. As a result, other pathological processes occur successively: adhesion and destruction of red blood cells, anemia, defects in the kidneys, heart, brain - these pathological signs are secondary. With pleiotropy, a gene, acting on one basic trait, can also change and modify the expression of other genes, and therefore the concept of modifier genes has been introduced. The latter enhance or weaken the development of traits encoded by the “main” gene.

Name the main biometric characteristics used in the genetic and mathematical analysis of quantitative traits.

Biometric data can be divided into two main classes:

Physiological- relate to the shape of the body. Examples include: fingerprints, facial recognition, DNA, palm of hand, retina, smell, voice.

Behavioral- related to human behavior. For example, gait and speech. Sometimes the term English is used. behaviometrics for this biometrics class.

The concept of variant and variation series.

Variation series- these are numerical values of a characteristic, presented in rank order with frequencies corresponding to these values.

Basic designations of the variation series

V - variant, a separate numerical expression of the characteristic being studied;

p - frequency (“weight”) of variants, the number of its repetitions in the variation series;

n is the total number of observations (i.e. the sum of all frequencies, n = Σр);

Vmax and Vmin are extreme options that limit the variation series (series limits);

A - series amplitude (i.e. the difference between the maximum and minimum options, A = Vmax - Vmin)

Types of variations:

a) simple - this is a series in which each variation occurs once (p = 1);

6) weighted - a series in which individual options occur repeatedly (with different frequencies).

Purpose variation series: necessary to determine the average value (M) and criteria for the diversity of the trait to be studied (σ, Cv).

The essence of the arithmetic mean, standard deviation, dispersion and methods of their calculation.

average value- this is a general characteristic of the size of the trait being studied. It allows one number to quantitatively characterize a qualitatively homogeneous population.

Application of averages

to assess health status - for example, physical development parameters (average height, average body weight, average lung capacity, etc.), somatic indicators (average blood sugar level, average pulse value, average ESR, etc.);

to assess the organization of the work of treatment-and-prophylactic and sanitary-anti-epidemic institutions, as well as the activities of individual doctors and other medical workers (the average length of stay of a patient in a bed, the average number of visits per 1 hour of admission to the clinic, etc.);

to assess the condition environment.

Method for calculating simple arithmetic average

Sum up the options: V1+V2+V3+...+Vn = Σ V;

The sum of the option is divided by the total number of observations: M = Σ V / n

Methodology for calculating the weighted arithmetic average

Get the product of each option and its frequency - Vp

Find the sum of the products of the variant by frequencies: V1p1 + V2p2+ V3p3 +...+ Vnpn = Σ Vp

Divide the resulting amount by the total number of observations: M = Σ Vp / n

Standard deviation is defined as a generalizing characteristic of the size of variation of a trait in the aggregate. It is equal to the square root of the average square deviation of individual values of the attribute from the arithmetic mean, i.e. root of the variance and can be found like this:

1. For the primary row:

2. For the variation series:

Transforming the standard deviation formula brings it to a form more convenient for practical calculations:

Standard deviation determines how much on average specific options deviate from their average value, and is also an absolute measure of the variability of a characteristic and is expressed in the same units as the options, and therefore is well interpreted.

Methodology for calculating standard deviation

Find the deviation (difference) of each option from the arithmetic mean value of the series (d = V - M);

Square each of these deviations (d2);

Obtain the product of the square of each deviation and the frequency (d2р);

Find the sum of these deviations: d21p1 + d22p2 + d23p3 +...+ d2npn = Σ d2р;

Divide the resulting amount by the total number of observations (for n< 30 в знаменателе n-1): Σ d2р / n

Take the square root: σ = √Σ d2р / n

at n< 30 σ = √Σ d2р / n-1

Application of standard deviation

for judging the variability of variation series and comparative assessment of the typicality (representativeness) of arithmetic averages. This is necessary in differential diagnosis when determining the stability of symptoms;

for reconstruction of the variation series, i.e. restoring its frequency response based on the “three sigma” rule. In the interval M±3σ there are 99.7% of all variants of the series, in the interval M±2σ - 95.5% and in the interval M±1σ - 68.3% of the variants of the series;

to identify “popping up” variants (when comparing real and reconstructed variation series);

to determine normal and pathological parameters using sigma estimates;

to calculate the coefficient of variation;

to calculate the average error of the arithmetic mean.

The concept of penetrance and expressivity of genes.

Indicators of the dependence of the functioning of hereditary inclinations on the characteristics of the genotype are penetrance and expressivity. Penentrance – the probability of gene expression, the phenomenon of the appearance or absence of a trait in organisms of the same genotype.

Penetrance varies significantly among both dominant and recessive genes. Along with genes whose phenotype appears only under a combination of certain conditions and fairly rare external conditions (high penetrance), humans have genes whose phenotypic manifestation occurs under any combination of external conditions (low penetrance). Penetrance is measured by the percentage of organisms with a phenotypic trait from the total number of examined carriers of the corresponding alleles. If a gene completely determines phenotypic expression, regardless of the environment, then it has 100 percent penetrance. However, some dominant genes are expressed less regularly. Thus, polydactyly has a clear vertical inheritance, but there are generation gaps. Dominant anomaly- premature puberty is characteristic only of men, but sometimes the disease can be transmitted from a person who has not suffered from this pathology. Penetrance indicates what percentage of gene carriers exhibit the corresponding phenotype. So penetrance depends on genes, on environment, on both. Thus, this is not a constant property of a gene, but a function of genes under specific environmental conditions. Penetrance calculation = number of individuals with phenotypic expression of the trait: total number of individuals with the gene.

Penetrance of congenital hip dislocation 25%

Expressiveness – degree of manifestation (degeneracy) of the characteristic.

change in the quantitative manifestation of a trait in different individuals carrying the corresponding alleles. With dominant hereditary diseases, expressivity may fluctuate. In the same family, hereditary diseases can manifest themselves from mild, barely noticeable, to severe: various shapes hypertension, schizophrenia, diabetes, etc. Recessive hereditary diseases within a family manifest themselves in the same way and have slight fluctuations in expressivity.

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Allelic genes, alleles (lat. allelos - opposite) - different shapes of the same gene, they occupy the same place (locus) of homologous chromosomes and determine alternative states of the same trait. Genes, like chromosomes, are paired. In each cell of a diploid organism, any gene is represented by two allelic genes (alleles), one of which the organism received from the father, the second from the mother. The exception is sex cells - gametes, which contain only one allele of a given gene. Allelic genes are paired genes, or genes of one allelic pair. Non-allelic genes are genes of different allelic pairs; they are located in different loci of chromosomes.

Allelic genes are dominant and recessive. A dominant gene (allele) is a gene that determines the phenotype of a heterozygous organism. A recessive gene (allele) is a gene that does not manifest itself in the phenotype of a heterozygous organism. Dominant and recessive alleles of the same gene are designated by the same letter of the Latin alphabet; the dominant allele is designated by a capital letter, and the recessive by a small letter. For example, in humans, normal skin pigmentation is determined by the dominant allele A, and its absence (albinism) is determined by the recessive allele of the same gene a.

According to modern genetic terminology, the patterns of inheritance of traits established by G. Mendel are based on the following provisions:

1. Each trait in the body is controlled by a pair of alleles of a specific gene. allelic rhesus gene cumulative

2. During meiosis, each pair of alleles is split and each gamete receives one allele from each pair.

3. When male and female gametes are formed, any allele from one pair can enter each of them along with any allele from the other pair.

4. Each allele is passed on from generation to generation as a discrete, immutable unit of heredity.

5. Maternal and parental organisms equally take part in the transmission of their hereditary factors to descendants. The new generation does not receive ready-made traits, but only material factors - one allele (for each trait) from each parent individual.

Mendelian traits in humans and types of their inheritance

Traits whose inheritance obeys the laws established by G. Mendel are called Mendelian.

All Mendelian traits are discrete and controlled by a single gene (monogenic inheritance). The following types of inheritance of Mendelian traits are distinguished: autosomal dominant, autosomal recessive, X-linked (dominant and recessive), Y-linked. With autosomal inheritance, the gene for the trait under study is located on an autosome (non-sex chromosome), with sex-linked inheritance - on the sex chromosomes (X, Y).

Multiple alleles

In Mendel's experiments, genes existed in only two forms - dominant and recessive. But most genes are represented not by two, but by a large number of alleles. In addition to the main alleles (dominant and recessive), there are also intermediate alleles. A series of alleles (three or more) of one gene are called multiple alleles, and this phenomenon is called multiple allelism. Multiple alleles arise from multiple mutations at the same chromosomal locus. In the genotype of a diploid organism there are only two alleles of one gene; in the population their number is practically unlimited. The peculiarity of interactions between multiple alleles is that they can be placed in one sequential row, in which each allele will be dominant in relation to all subsequent ones and recessive in relation to the previous ones.

Meaning. Multiple allelism increases the gene pool of a population, its genotypic and phenotypic polymorphism, which is important for evolution.

Inheritance of ABO and Rh factor blood groups

The ABO blood group system in humans is inherited by multiple alleles of one autosomal gene, the locus of which is designated by the letter I (from the word isohemaglutinogen). There are three multiple alleles: ІА, ІВ, і (allele і is designated by І0). Alleles ІА, ІВ dominate over allele і, and they are codominant among themselves. The IA allele controls the synthesis of antigen A, the IV allele controls the synthesis of antigen B, and the i allele controls none. Antigens are contained on the surface of red blood cells and other cells (leukocytes, platelets, tissue cells). Each person can inherit any of the three possible alleles, but no more than two. Depending on their combination, there are 4 blood groups (4 phenotypes), the differences between which are associated with the presence or absence of special substances: agglutinogens (antigens) A and B on the surface of red blood cells and agglutinins (antibodies) a and b in the blood plasma. Six genotypes correspond to four phenotypes.

Antigen A and antibody a are never contained together, just like antigen B and antibody b. When antigens interact with antibodies of the same name, red blood cells stick together and precipitate (agglutination), which indicates the incompatibility of the blood of the donor and recipient. When transfusing blood, it is necessary that the donor's antigens do not meet the recipient's antibodies of the same name. Since the first group does not have antigens, people with such blood are called universal donors, and people with the fourth group are called universal recipients.

The inheritance of two alleles out of three possible obeys Mendelian laws. Blood groups I (A) and II (B) are inherited according to an autosomal dominant type, group I (0) - according to an autosomal recessive type. If parents have blood group II (A), then their children may have II (A) and I (0), but not III (B) and not IV (AB). The fourth blood group (AB) is inherited not according to the rules of G. Mendel, but according to the type of codominance. Since blood groups are genetically determined and do not change throughout life, their determination can help in cases of disputed paternity. It must be remembered that the blood type cannot determine that this particular man is the father of the child. We can only say that he is the possible father of the child or paternity is excluded.

In people with IV (AB) blood group, in 0.1-0.2% of cases, a special position of genes is observed - cis-position, when both genes IA and IV are located on the same chromosome. Then, in the marriage of such a person with a person who has I (0) blood group, the possible birth of children with I (0) blood type, which must be taken into account during medical genetic counseling and forensic medical examination.

Inheritance of the Rh factor. Rh factor is a protein (antigen), so named because it was first isolated (1940) from the erythrocytes of the rhesus monkey (Macacus resus), and then from humans. About 85% of Europeans are able to synthesize it and form the Rh-positive group (Rh+), 15% are unable to synthesize it and are called Rh-negative (Rh-). The Rh factor is caused by three dominant closely linked genes (C, D, E) located on the first chromosome. They are inherited as in a monohybrid cross. The main role belongs to the D antigen; if it is detected, then the blood is classified as Rh-positive (DD or Dd), if it is not detected, it is classified as Rh-negative (dd). The Rh factor must be taken into account during blood transfusions and transplantations, since the body produces antibodies against it. The Rh factor can cause Rh conflict between mother and fetus. When a woman who has Rh-negative blood marries a man who is Rh-positive homozygote, all the children will be Rh-positive, and if he is heterozygous, 50% will be Rh-positive and 50% will be Rh-negative.

A conflict arises if a woman has Rh-negative blood, and the child received the dominant D allele from the father and is Rh-positive. The blood of mother and fetus does not mix. Therefore, the first pregnancy ends normally. But during the birth of the first child, when the placenta detaches, the baby’s red blood cells enter the mother’s body, where antibodies are formed against the Rh antigen. During the next pregnancy, these antibodies penetrate through the placental barrier into the blood of the fetus, combine with the Rh antigen, causing red blood cells to stick together and lyse (erythroblastosis, or hemolytic disease of the newborn). Moreover, with each subsequent birth, the disease in children becomes more severe. If a Rh-negative girl received a transfusion of Rh-positive blood before pregnancy, then the first child (if he is Rh-positive) will not be viable. Therefore, even a one-time transfusion of Rh-positive blood to girls with Rh-negative blood is absolutely unacceptable.

Hemolytic disease of the newborn was described more than 400 years ago. It occurs when there is incompatibility not only with the Rh system, but also with the ABO system: most often this happens when the mother has group I (O), and the child has group II (A) or III (B).

The genotype functions as a single integral system of interacting genes. A distinction is made between the interaction of allelic genes (genes of one allelic pair) and the interaction of non-allelic genes (genes of different allelic pairs).

Cumulative polymer. A significant part of the traits in eukaryotes that are inherited polygenically are under the control of not two or three, but more genes (their number is still difficult to determine). With a monogenic type of inheritance in a monohybrid cross, one gene appears in two alternative states without transitional forms. Such signs are qualitative; as a rule, no measurements are taken during their analysis. In the case of non-allelic interaction of two unlinked genes, even if the Mendelian ratio of 9:3:3:1 is maintained, the phenotype of the first generation of hybrids depends on the action of both genes. However, the inheritance of qualitative traits can be determined by the interaction of three or more genes. Moreover, each of these genes has its share of influence on the development of the trait. An example is the inheritance of red and white colors of wheat grains in the experiments of the Swedish geneticist Nilsson-Ehle. The results of these experiments were published in 1909. When crossing a wheat variety whose grains were dark red in color with a variety having white grains, the first generation hybrids had a lighter red color. In the second generation, the following phenotypic ratio was obtained: for 63 colored grains with various shades of red, there was 1 white grain (uncolored). These results were explained by Nilsson-Ehle as follows. The dark red color of wheat grains is due to the action of three pairs of dominant genes, and the white color is due to three pairs of recessive genes, and as the number of dominant genes increases, the color becomes more intense. Let us denote the dominant alleles of three genes localized on different chromosomes, in capital letters A1 A2 A3 and recessive - lowercase a1 a1 a3, then the genotypes of the original forms will be: A1A1 A2A2 A3A3 x a1я1 a2a2 a33a. The color of grains in the first generation hybrids A1a1 A2a2 A3a3 in the presence of three dominant alleles will be intermediate light red. When crossing hybrids of the first generation A1a1 A2a2 A3a3 x A1a1 A2a2 A3a3, each hybrid produces 8 types of gametes, therefore, in the second generation, splitting in 64 shares (8 x 8) is expected. Among the 63/64 plants with colored grains, the color intensity increases as the number of dominant alleles of various genes in the genotype increases. Apparently, each dominant gene contributes to an increase in the amount of synthesized pigment, and in this sense, such a trait can be classified as quantitative. The type of additive action of genes, each of which has its own, often small, share of influence on a trait, is called cumulative polymerization. Using the Punnett grid, the frequencies of dominant genes among second generation genotypes can be calculated. To do this, in each of the 64 cells, instead of the genotype, the number of dominant alleles present in it is recorded. Having determined the frequencies of dominant alleles, we can verify that genotypes with the number of dominant genes 6,5,4,3, 2, 1.0 occur 1,6,15,20,15,6,1 times, respectively. These data are presented in the form of a graph in the figure. The horizontal axis indicates the number of dominant genes in the genotype, and the vertical axis indicates their frequency of occurrence. As the number of genes that determine a single trait increases, this graph approaches an ideal normal distribution. This type of graph is typical for quantitative traits such as height, weight, lifespan, egg production and other traits that can be measured. Quantitative traits include those that vary more or less continuously from one individual to another, which makes it possible to distribute individuals into classes in accordance with the degree of expression of the trait. The figure shows an example of the distribution by height for men. This sample is divided into 7 classes with 5 cm intervals. Men with average height (171-175 cm) make up the majority of the sample. With the lowest frequency there are men who are included in the class with a height of 156--160 cm and 186--190 cm. With an increase in the sample and a decrease in the class interval, the graph can approach the normal distribution of height. Phenotypic variability without gaps in expression, plotted normal distribution characteristic is called continuous. Continuous variability of quantitative traits depends on two reasons: 1) from genetic splitting over a large number of genes, 2) from the influence of the environment as the cause of modification variability. First Danish geneticist Johansen showed that the continuous variability of such a quantitative trait as the mass of bean beans Phaseolus vulgaris depends on both genetic and environmental factors. By inbreeding over a number of generations, he developed several pure (homozygous) lines that differed in the average weight of the beans. For example, the average weight of beans in line 1 was 642 mg, in line 13 - 454 mg, in line 19 - 351 mg. Next, Johann Sen carried out the selection of large and small beans in each line from 1902 to 1907. Regardless of the weight of the parent seeds, the average weight of beans after 6 years of selection was the same as in the original line. Thus, in line No. 13, with the weight of parental seeds ranging from 275 mg to 575 mg, the average weight of seeds in the offspring remained at the same level of ±450 mg. Moreover, in each line the weight of beans varied from minimum to maximum values, and the most numerous was the class with average weight, which is typical for quantitative traits. Selection in pure lines turned out to be impossible. Another example, in 1977 D.S. Bileva, L.N. Zimina, A.A. Malinovsky studied the influence of genotype and environment on the lifespan of two inbred lines of Drosophila melanogaster. Through inbreeding and selection, two lines No. 5 and No. 3 were developed, clearly differing in life expectancy. Life expectancy was determined on three food options: complete (yeast, semolina, sugar, agar-agar), depleted (semolina, sugar, agar-agar) and sugar (sugar, agar-agar). Depletion of feed composition led to a decrease in life expectancy. Life expectancy of females of the 5th line on sugar food (in days) decreased from 58+2.1 to 27.2±1.8, and for males from 63.7±2.9 to 34.8±1.5, t .e. turned out to be approximately 2 times less than on full-fledged food. The same pattern was typical for females and males of the 3rd line. The lifespan of females of this line decreased from 50.7±],9 to 24.3±1.2, and for males from 32.9±2.9 to 21.6±1.5 days. At the same time, the histogram reflecting the variability for this trait on a complete feed is close to the histogram presented in Figure I, while on the depleted and sugar feeds an asymmetric distribution is observed with a shift in the average value towards a decrease in life expectancy. Non-cumulative polymer. Along with cumulative (additive) polymerization, cases of inheritance according to the type of non-cumulative (non-additive) polymerization are known, when the nature of the manifestation of the trait does not change depending on the number of dominant polymer genes. Thus, in chickens, the feathering of the legs is determined by the dominant alleles of two genes A1 and A2: P A1A1 A2A2 x a1a1a2a2 feathered unfeathered feathered F2 9 A1_A2_; 3 A1_ a2a2:; 3 a1a1 A2_; 1 a1a1 a2a2 feathered (15) unfeathered (1) In F2, among the 15/16 hybrids with feathered legs, there are those that have four dominant alleles (A1A1 A2A2), three (A1A"1 A2a2), two (A1a1 A2a2) or just one (A1a1 a2a2), the nature of the feathering of the legs in these cases is the same. The main genes in the polygene system. Among the genes that influence a quantitative trait, there may be a “strong” or main gene, and “weaker” genes. The action of the main gene is sometimes so much more significant than the action of other genes that the trait encoded by it is inherited according to Meckdelian laws. Variability of the same trait can be under the control of both one main gene and polygenes. For example, dwarfism in humans in the case of achondroplasia is caused by a specific major gene, while variation in height in a normal population of individuals is an example of polygenic variation. Genes whose effect is noticeably stronger than the effect of other genes on this trait can be studied separately from the effect of other genes. On the other hand, the same gene, due to its pleiotropic effect, can have a strong effect on one trait and a less significant effect on another trait. In addition, the main genes can include those that determine traits inherited according to Mendelian laws, without their relation to the polygene system. The division of genes into major and non-major is not always justified, although it is undeniable that their role in determining a trait may be different. Widespread human diseases, for example, arterial hypertension, coronary heart disease, bronchial asthma, peptic ulcer stomach, are inherited polygenically. Moreover, the severity of the disease depends not only on the combined action of many genes, but also on provoking environmental factors.

Cumulative polymer. A significant part of the traits in eukaryotes that are inherited polygenically are under the control of not two or three, but a larger number of genes (their number is still difficult to determine). With a monogenic type of inheritance in a monohybrid cross, one gene appears in two alternative states without transitional forms. Such signs are qualitative; as a rule, no measurements are taken during their analysis. In the case of non-allelic interaction of two unlinked genes, even if the Mendelian ratio of 9:3:3:1 is maintained, the phenotype of the first generation of hybrids depends on the action of both genes. However, the inheritance of qualitative traits can be determined by the interaction of three or more genes. Moreover, each of these genes has its share of influence on the development of the trait. An example is the inheritance of red and white colors of wheat grains in the experiments of the Swedish geneticist Nilsson-Ehle. The results of these experiments were published in 1909.

When crossing a wheat variety whose grains were dark red in color with a variety having white grains, the first generation hybrids had a lighter red color. In the second generation, the following phenotypic ratio was obtained: for 63 colored grains with various shades of red, there was 1 white grain (uncolored). These results were explained by Nilsson-Ehle as follows. The dark red color of wheat grains is due to the action of three pairs of dominant genes, and the white color is due to three pairs of recessive genes, and as the number of dominant genes increases, the color becomes more intense. Let us denote the dominant alleles of three genes localized on different chromosomes by capital letters A1 A2 A3 and recessive alleles by lowercase letters a1 a1 a3, then the genotypes of the original forms will be: A1A1 A2A2 A3A3 x a1ya1 a2a2 a33a.

The color of grains in the first generation hybrids A1a1 A2a2 A3a3 in the presence of three dominant alleles will be intermediate light red. When crossing hybrids of the first generation A1a1 A2a2 A3a3 x A1a1 A2a2 A3a3, each hybrid produces 8 types of gametes, therefore, in the second generation, splitting in 64 shares (8 x 8) is expected. Among the 63/64 plants with colored grains, the color intensity increases as the number of dominant alleles of various genes in the genotype increases. Apparently, each dominant gene contributes to an increase in the amount of synthesized pigment, and in this sense, such a trait can be classified as quantitative.

The type of additive action of genes, each of which has its own, often small, share of influence on a trait, is called cumulative polymerization. Using the Punnett grid, the frequencies of dominant genes among second generation genotypes can be calculated. To do this, in each of the 64 cells, instead of the genotype, the number of dominant alleles present in it is recorded. Having determined the frequencies of dominant alleles, we can verify that genotypes with the number of dominant genes 6,5,4,3, 2, 1.0 occur 1,6,15,20,15,6,1 times, respectively. These data are presented in the form of a graph in the figure. The horizontal axis indicates the number of dominant genes in the genotype, and the vertical axis indicates their frequency of occurrence. As the number of genes that determine a single trait increases, this graph approaches an ideal normal distribution.

This type of graph is typical for quantitative traits such as height, weight, lifespan, egg production and other traits that can be measured.

Quantitative traits include those that vary more or less continuously from one individual to another, which makes it possible to distribute individuals into classes in accordance with the degree of expression of the trait. The figure shows an example of the distribution by height for men. This sample is divided into 7 classes with 5 cm intervals. Men with average height (171-175 cm) make up the majority of the sample. With the lowest frequency there are men who are included in the class with a height of 156--160 cm and 186--190 cm. With an increase in the sample and a decrease in the class interval, the graph can approach the normal distribution of height.

Phenotypic variability without breaks in expression, presented on a graph of the normal distribution of a trait, is called continuous. Continuous variability of quantitative traits depends on two reasons: 1) from genetic splitting over a large number of genes, 2) from the influence of the environment as the cause of modification variability.

For the first time, the Danish geneticist Johansen showed that the continuous variability of such a quantitative trait as the mass of beans of Phaseolus vulgaris depends on both genetic and environmental factors. By inbreeding over a number of generations, he developed several pure (homozygous) lines that differed in the average weight of the beans. For example, the average weight of beans in line 1 was 642 mg, in line 13 - 454 mg, in line 19 - 351 mg. Next, Johann Sen carried out the selection of large and small beans in each line from 1902 to 1907. Regardless of the weight of the parent seeds, the average weight of beans after 6 years of selection was the same as in the original line. Thus, in line No. 13, with the weight of parental seeds ranging from 275 mg to 575 mg, the average weight of seeds in the offspring remained at the same level of ±450 mg. Moreover, in each line the weight of beans varied from minimum to maximum values, and the most numerous was the class with average weight, which is typical for quantitative traits. Selection in pure lines turned out to be impossible.

Another example, in 1977 D.S. Bileva, L.N. Zimina, A.A. Malinovsky studied the influence of genotype and environment on the lifespan of two inbred lines of Drosophila melanogaster. Through inbreeding and selection, two lines No. 5 and No. 3 were developed, clearly differing in life expectancy. Life expectancy was determined on three food options: complete (yeast, semolina, sugar, agar-agar), depleted (semolina, sugar, agar-agar) and sugar (sugar, agar-agar). Depletion of feed composition led to a decrease in life expectancy. Life expectancy of females of the 5th line on sugar food (in days) decreased from 58+2.1 to 27.2±1.8, and for males from 63.7±2.9 to 34.8±1.5, t .e. turned out to be approximately 2 times less than on full-fledged food. The same pattern was typical for females and males of the 3rd line. The lifespan of females of this line decreased from 50.7±],9 to 24.3±1.2, and for males from 32.9±2.9 to 21.6±1.5 days. At the same time, the histogram reflecting the variability for this trait on a complete feed is close to the histogram presented in Figure I, while on the depleted and sugar feeds an asymmetric distribution is observed with a shift in the average value towards a decrease in life expectancy.

Non-cumulative polymer. Along with cumulative (additive) polymerization, cases of inheritance according to the type of non-cumulative (non-additive) polymerization are known, when the nature of the manifestation of the trait does not change depending on the number of dominant polymer genes. Thus, in chickens, the feathering of the legs is determined by the dominant alleles of two genes A1 and A2: P A1A1 A2A2 x a1a1a2a2 feathered unfeathered feathered F2 9 A1_A2_; 3 A1_ a2a2:; 3 a1a1 A2_; 1 a1a1 a2a2 feathered (15) unfeathered (1) In F2, among the 15/16 hybrids with feathered legs, there are those that have four dominant alleles (A1A1 A2A2), three (A1A"1 A2a2), two (A1a1 A2a2) or just one (A1a1 a2a2), the nature of the feathering of the legs in these cases is the same.

The main genes in the polygene system. Among the genes that influence a quantitative trait, there may be a “strong” or main gene, and “weaker” genes. The action of the main gene is sometimes so much more significant than the action of other genes that the trait encoded by it is inherited according to Meckdelian laws. Variability of the same trait can be under the control of both one main gene and polygenes. For example, dwarfism in humans in the case of achondroplasia is caused by a specific major gene, while variation in height in a normal population of individuals is an example of polygenic variation. Genes whose effect is noticeably stronger than the effect of other genes on this trait can be studied separately from the effect of other genes. On the other hand, the same gene, due to its pleiotropic effect, can have a strong effect on one trait and a less significant effect on another trait. In addition, the main genes can include those that determine traits inherited according to Mendelian laws, without their relation to the polygene system. The division of genes into major and non-major is not always justified, although it is undeniable that their role in determining a trait may be different.

Widespread human diseases, for example, arterial hypertension, coronary heart disease, bronchial asthma, and gastric ulcers, are inherited polygenically. Moreover, the severity of the disease depends not only on the combined action of many genes, but also on provoking environmental factors.

Interaction of allelic genes

The main forms of interaction of allelic genes are complete and incomplete dominance, overdominance and codominance

Complete dominance (dominance) is the complete predominance in the phenotype of a heterozygous organism of one allele (dominant) over another (recessive) allele of the same gene. Recessiveness is the suppression in the phenotype of a heterozygous organism of one allele (recessive) by another allele (dominant) of the same gene. Dominance can be complete or incomplete. In the case of complete dominance, the dominant homozygote (AA) and heterozygote (Aa) have the same phenotype. The phenomenon of complete dominance was observed in the experiments of G. Mendel, where one allelic gene was always dominant, the other was recessive. Therefore, pea seeds were always either yellow or green in color and did not have another color, for example, blue. With complete dominance in the crossing of heterozygotes (Aa x Aa), the split for phenotype was 3:1, for genotype - 1:2:1.

According to the type of complete dominance, a person inherits Mendelian traits (monogenic inheritance): dimples on the cheeks, the ability to roll the tongue into a tube, the ability to bend the tongue back, a free earlobe, as well as many hereditary diseases: polydactyly, polydactyly, myopathy, cystic adenoid epithelioma, achondroplasia , etc.

Incomplete dominance is the interaction of allelic genes, in which in a heterozygous organism the dominant allele does not fully demonstrate its dominance, and the recessive allele of the same gene does not fully demonstrate its recessivity. With incomplete dominance, the phenotype of heterozygote Aa is intermediate between the phenotype of dominant AA and recessive aa homozygotes. Thus, in crossing a night beauty with red flowers (AA) and a night beauty with white flowers (aa), all hybrids (Aa) of the first generation F1 had pink flowers. In the crossing of hybrids of the first generation F1 with each other (Aa x Aa), in the second generation F2 there is a splitting of the phenotype in the ratio 1:2:1, which coincides with the corresponding genotype 1AA:2Aa:1aa, but differs from the splitting of the phenotype with complete dominance (3:1).

By the type of incomplete dominance, cystinuria, Pilgerian anemia, thalassemia, Friedreich's ataxia, etc. are inherited in humans. In homozygotes for the recessive cystinuria gene aa, cystine stones are formed in the kidneys, in heterozygotes Aa, only an increased content of cystine in the urine is observed, homozygotes AA are healthy.

Overdominance is the interaction of allelic genes, in which the dominant allele in the heterozygous state manifests itself in the phenotype more strongly than in the homozygous state (Aa > AA). In this type, the action of lethal genes takes place. In humans, for example, shortened fingers - brachydactyly - are an autosomal dominant trait. Moreover, dominant homozygotes die in the early stages of embryogenesis. We follow that heterozygotes are patients with brachydactyly, and dominant homozygotes have a normal hand structure. As a result of marriage, parents suffering from brachydactyly may have children with this disease and healthy ones in a ratio of 2:1.

Codominance is the interaction of allelic genes, in which both alleles of the same gene appear in the phenotype of a heterozygous organism. According to the type of codominance, a person inherits the fourth blood group (genotype ІАІВ). In people with this group, their red blood cells simultaneously contain antigen A, which is controlled by the IA allele, and antigen B, a product of the expression of the IV allele. Alleles IA and IV are codominant.

Interaction of nonallelic genes

The main forms of interaction of non-alelic genes are complementarity, epistasis and polymerization. They predominantly modify the classical formula for segregation by phenotype, established by G. Mendel for dihybrid crossing (9: 3: 3: 1).

Complementarity (lat. complementum - additions). Complementary, or complementary, are non-allelic genes that do not act individually, but when simultaneously present in the genotype, predetermine the development of a new trait. In sweet peas, flower color is determined by two dominant non-allelic genes, of which one gene (A) provides the synthesis of a colorless substrate, the other (B) provides the synthesis of pigment. Therefore, when crossing plants with white flowers (AAbb x aaBB), all plants in the first generation F1 (AaBb) have colored flowers, and in the second generation F2, the phenotype is split in a ratio of 9:7, where 9/16 plants have colored flowers and 7 /16 - unpainted.

In humans, normal hearing is due to the complementary interaction of two dominant non-allelic genes D and E, one of which determines the development of the helix, the other - the auditory nerve. People with genotypes D-E- have normal hearing, while people with genotypes D-ee and ddE- are deaf. In a marriage where the parents are deaf (DDee ґ ddEE), all children will have normal hearing (DdEe).

Epistasis is the interaction of non-allelic genes, in which one gene suppresses the action of another, non-allelic gene. The first gene is called epistatic, or suppressor (inhibitor), the other, non-allelic, gene is called hypostatic. If the epistatic gene is dominant, epistasis is called dominant (A>B). And, conversely, if the epistatic gene is recessive, epistasis is recessive (aa>B or aa>bb). The interaction of genes during epistasis is the opposite of complementarity.

An example of dominant epistasis. In chickens, the dominant allele C of one gene determines the development of feather color, but the dominant allele I of another gene is its suppressor. Therefore, chickens with the І-С- genotype are white, and those with the ііСС and ііСс genotypes are colored. In the crossing of white chickens (ІІСС x ііСС), the hybrids of the first generation F1 will turn out to be white, but when crossing F1 with each other in the second generation F2, there will be a splitting of the phenotype in a ratio of 13:3. Of the 16 individuals, 3 will be colored (ЖіСС and ііСС), since they lack a dominant suppressor gene and have a dominant color gene. The other 13 individuals will be white.

An example of recessive epistasis can be the Bombay phenomenon - the unusual inheritance of ABO blood groups, first identified in one Indian family. In a family where the father had blood type I (O) and the father had blood type III (B), a girl was born with type I (O), she married a man with blood type II (A) and they had two girls: one with blood group IV (AB), the other with I (O). The birth of a girl with IV (AB) blood group in a family where the father had II (A) and the mother had I (O) was unusual. Genetics explained this phenomenon as follows: a girl with group IV (AB) inherited the IA allele from her father, and the IV allele from her mother, but the IV allele was not phenotypically manifested in her mother, since her genotype contained a rare recessive epistatic gene s in a homozygous state, which provoked the phenotypic manifestation of the IV allele.

Hypostasis is an interaction of non-allelic genes in which the dominant gene of one allelic pair is suppressed by an epistatic gene from another allelic pair. If gene A suppresses gene B (A>B), then in relation to gene B, the interaction of non-allelic genes is called hypostasis, and in relation to gene A - epistasis.

Polymerism is the interaction of non-allelic genes, in which the same trait is controlled by several dominant non-allelic genes, which act on this trait uniquely, equally, enhancing its manifestation. Such unambiguous genes are called polymeric (multiple, polygenes) and are designated by one letter of the Latin alphabet, but with different digital indices. For example, dominant polymer genes are A1, A2, A3, etc., recessive genes are a1, a2, a3, etc. Accordingly, the genotypes are designated A1A1A2A2A3A3, a1a1a2a2a3a3. Traits that are controlled by polygenes are called polygenic, and the inheritance of these traits is polygenic, in contrast to monogenic, where the trait is controlled by a single gene. The phenomenon of polymerization was first described in 1908 by the Swedish geneticist G. Nilsson-Ehle while studying the inheritance of wheat grain color.

Polymeria can be cumulative or non-cumulative. With cumulative polymerization, each gene individually has a weak effect (weak dose), but the number of doses of all genes is summed up in the final result, so that the degree of expression of the trait depends on the number of dominant alleles. The type of polymer in a person is inherited by height, body weight, skin color, mental abilities, size blood pressure. Thus, human skin pigmentation is determined by 4-6 pairs of polymer genes. In the genotype of indigenous Africans there are predominantly dominant alleles (P1P1P2P2P3P3P4P4), while representatives of the Caucasian race have recessive alleles (p1p1p2p2p3p3p4p4). From the marriage of a dark-skinned man and a white woman, children with intermediate skin color are born - mulattoes (P1p1P2p2P3p3P4p4). If the spouses are mulattoes, then the possible birth of children with skin pigmentation from the lightest to the darkest.

In typical cases, quantitative traits are inherited polygenically. Nevertheless, in nature there are examples of polygenic inheritance of qualitative traits, when the final result does not depend on the number of dominant alleles in the genotype - the trait either manifests itself or does not manifest itself (non-cumulative polymery).

Pleiotropy is the ability of one gene to control several traits (multiple gene action). Thus, Marfan syndrome in typical cases is characterized by a triad of signs: subluxation of the lens of the eye, heart defects, elongation of the bones of the fingers and toes (arachnodactyly - spider fingers). This complex of traits is controlled by one autosomal dominant gene, which causes disorders in the development of connective tissue.

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Dominant principle of heredity

The human body is diploid. The cells of our body (somatic) include two allelic genes.

Only gametes (sex cells) contain a single allele that determines the sex characteristic. When male and female gametes fuse, a zygote is obtained in which there is a double set of chromosomes, that is, 46, including 23 maternal and 23 paternal. Of these, 22 pairs are homological (identical) and 1 is sexual. If she received the XX chromosome set, a female individual develops, and if XY, then a male. Each chromosome, as noted above, contains 2 alleles. For convenience, they were divided into two types - dominant and recessive. The former are much stronger than the latter. The hereditary information contained in them turns out to be prevalent. What characteristics a nascent individual will inherit from its parents depends on whose allelic genes (father or mother) were dominant. This is the simplest way for alleles to interact.

Other types of inheritance

Each parent can be a carrier of homozygous or heterozygous genes for dominant or recessive traits. A child who has received dominant and recessive allelic genes from homozygous parents will inherit only dominant traits.

Simply put, if the dominant person in a couple is dark color hair, and recessive - light, all children will be born only dark-haired. In the case when one of the parents has a dominant gene of the heterozygous type, and the other - homozygous, their children will be born with a dominant and recessive trait of approximately 50 X 50. In our example, the couple may have both dark-haired and blond children. If both parents have heterozygous dominant and recessive genes, every fourth child will inherit recessive traits, that is, will be fair-haired. This rule of inheritance is very important, since there are many diseases transmitted through genes, and one of the parents may be the carrier. Such pathologies include dwarfism, hemochromatosis, hemophilia and others.

How are alleles designated?

In genetics, alleles are usually denoted by the first letters of the name of the gene of which they are forms. The dominant allele is written with a capital letter. Nearby indicate serial number modified gene form. The word “allele” in Russian can be used in both the feminine and masculine gender.

Types of allelic interactions

The interaction of allelic genes can be divided into several types:

What is allelic exclusion

It happens that in homogametic individuals containing germ cells with the same set of chromosomes, one of them becomes little or completely inactive. Regarding people, this condition is observed in women, while, say, in butterflies, on the contrary, in males. With allelic exclusion, only one of the two chromosomes is expressed, and the second becomes a so-called Barr body, that is, an inactive unit twisted into a spiral. This structure is called mosaic. In medicine, this can be seen in B lymphocytes, which can synthesize antibodies only to certain antigens. Each such lymphocyte chooses between the activity of either the paternal allele or the maternal one.

Multiple allelism

In nature, a widespread phenomenon is when the same gene has not two, but more forms. In plants this is manifested by a variety of stripes on leaves and petals, in animals - various combinations colors In humans, a striking example of multiple allelism is the inheritance of a child's blood type. Its system is designated ABO and is controlled by a single gene. Its locus is designated I, and allelic genes are designated IA, IB, IO. Combinations of IO IO give the first blood group, IA IO and IA IA - the second, IB IO and IB IB - the third, and IA IB - the fourth. In addition, Rh is determined in humans. Positive is given by combinations of 2 allelic genes with the sign “+” or 1+ and 1-. Rh negative is produced by two allelic genes with the “-” trait. The Rh system is controlled by CDE genes, and the D gene often causes Rh conflict between the fetus and the mother if her blood is Rh negative and the fetus is Rh positive. In such cases, in order for the second and subsequent pregnancies to be successfully completed, the woman is given special therapy.

Lethal allelic genes

Alleles whose carriers die due to genetic diseases caused by these genes are called lethal. In humans they cause Huntington's disease. In addition to lethal ones, there are also so-called semi-lethal ones. They can cause death, but only under certain conditions, e.g. high temperatures environment. If these factors can be avoided, semi-lethal genes do not cause the death of the individual.

Most people on the globe know that genes transmit the hereditary characteristics of parents to their offspring, and this applies not only to humans, but to all living beings on the planet. These microscopic structural units represent a certain segment of DNA that determines the sequence of polypeptides (chains of more than 20 amino acids that make up DNA). The nature and methods of interaction of genes are quite complex, and the slightest deviations from the norm can lead to genetic diseases. Let's try to understand the essence of genes and the principles of their behavior.

Dominant principle of heredity

The human body is diploid. The cells of our body (somatic) include two allelic genes.

Other types of inheritance

Simply put, if a couple has dark hair color as dominant and light hair color as recessive, all children will be born with only dark hair. In the case when one of the parents has a dominant gene of the heterozygous type, and the other - homozygous, their children will be born with a dominant and recessive trait of approximately 50 X 50. In our example, the couple may have both dark-haired and blond children. If both parents have heterozygous dominant and recessive genes, every fourth child will inherit recessive traits, that is, will be fair-haired. This rule of inheritance is very important, since there are many diseases transmitted through genes, and one of the parents may be the carrier. Such pathologies include dwarfism, hemochromatosis, hemophilia and others.

How are alleles designated?

In genetics, alleles are usually denoted by the first letters of the name of the gene of which they are forms. The dominant allele is written with a capital letter. The serial number of the modified gene form is indicated next to it. The word “allele” in Russian can be used in both the feminine and masculine gender.

Types of allelic interactions

The interaction of allelic genes can be divided into several types:

What is allelic exclusion

Multiple allelism

In nature, a widespread phenomenon is when the same gene has not two, but more forms. In plants this is manifested by a variety of stripes on leaves and petals, in animals - by various combinations of colors. In humans, a striking example of multiple allelism is the inheritance of a child's blood type. Its system is designated ABO and is controlled by a single gene. Its locus is designated I, and allelic genes are designated IA, IB, IO. Combinations of IO IO give the first blood group, IA IO and IA IA - the second, IB IO and IB IB - the third, and IA IB - the fourth. In addition, Rh is determined in humans. Positive is given by combinations of 2 allelic genes with the sign “+” or 1+ and 1-. Rh negative is produced by two allelic genes with the “-” trait. The Rh system is controlled by CDE genes, and the D gene often causes Rh conflict between the fetus and the mother if her blood is Rh negative and the fetus is Rh positive. In such cases, in order for the second and subsequent pregnancies to be successfully completed, the woman is given special therapy.

Lethal allelic genes

Alleles whose carriers die due to genetic diseases caused by these genes are called lethal. In humans they cause Huntington's disease. In addition to lethal ones, there are also so-called semi-lethal ones. They can cause death, but only under certain conditions, such as high ambient temperatures. If these factors can be avoided, semi-lethal genes do not cause the death of the individual.

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