How to determine whether an organism is heterozygous or homozygous. Allelic genes, their properties

Represented by different alleles. When they say that a given organism heterozygous(or heterozygous for gene X), this means that the copies of genes (or a given gene) on each of the homologous chromosomes are slightly different from each other.

In heterozygous individuals, based on each allele, slightly different variants of the protein (or transport or ribosomal RNA) encoded by this gene are synthesized. As a result, a mixture of these variants appears in the body. If the effect of only one of them is externally manifested, then such an allele is called dominant, and the one whose effect does not receive external expression is called recessive. Traditionally, when schematically depicting a cross, the dominant allele is denoted by a capital letter, and the recessive one by a lowercase letter (for example, A And a). Sometimes other designations are used, such as an abbreviated gene name with plus and minus signs.

With complete dominance (as in Mendel's classic experiments with the inheritance of the pea shape), a heterozygous individual looks like a dominant homozygote. When homozygous plants with smooth peas (AA) are crossed with homozygous plants with wrinkled peas (aa), the heterozygous offspring (Aa) have smooth peas.

With incomplete dominance, an intermediate variant is observed (as with the inheritance of the color of the corolla of flowers in many plants). For example, when crossing homozygous red carnations (RR) with homozygous white ones (rr), the heterozygous offspring (Rr) have pink corollas.

If external manifestations represent a mixture of the effects of both alleles, as in the inheritance of blood groups in humans, then they speak of codominance.

It should be noted that the concepts of dominance and recessiveness were formulated within the framework of classical genetics, and their explanation from the standpoint of molecular genetics encounters certain terminological and conceptual difficulties.

20. The concept of a gene. Gene properties. Gene functions. Types of genes

Gene- a structural and functional unit of heredity that controls the development of a certain trait or property. Parents pass on a set of genes to their offspring during reproduction.

Gene properties

Allelic existence – genes can exist in at least two different forms; Accordingly, paired genes are called allelic.

Allelic genes occupy identical places on homologous chromosomes. The location of a gene on a chromosome is called a locus. Allelic genes are designated by the same letter of the Latin alphabet.

Specificity of action - a certain gene ensures the development of not just any trait, but a strictly defined one.

Dosage of action - the gene ensures the development of the trait not indefinitely, but within certain limits.

Discreteness - since the genes on a chromosome do not overlap, in principle a gene develops a trait independently of other genes.

Stability - genes can be passed on without any changes over a number of generations, i.e. the gene does not change its structure when transmitted to subsequent generations.

Mobility - with mutations, a gene can change its structure.

Gene function, its manifestation lies in the formation of a specific characteristic of the organism. Removal of a gene or its qualitative change leads, respectively, to the loss or change of the trait controlled by this gene. At the same time, any sign of an organism is the result of the interaction of a gene with the surrounding and internal, genotypic environment. The same gene can take part in the formation of several characteristics of an organism (the phenomenon of so-called pleiotropy). The bulk of traits are formed as a result of the interaction of many genes (the phenomenon of polygeny). At the same time, even within a related group of individuals in similar living conditions, the manifestation of the same gene can vary in degree of expression (expressiveness, or expression). This indicates that in the formation of traits, genes act as an integral system that strictly functions in a certain genotypic and environmental environment.

Types of genes.

Structural genes - carry information about the first protein structure

Regulatory genes - do not carry information about the first structure of the protein, but regulate the process of protein biosynthesis

Modifiers – capable of changing the direction of protein synthesis

Homozygosity (from the Greek "homo" equal, "zygote" fertilized egg) is a diploid organism (or cell) that carries identical alleles on homologous chromosomes.

Gregor Mendel was the first to establish a fact indicating that plants that are similar in appearance can differ sharply in hereditary properties. Individuals that do not split in the next generation are called homozygous. Individuals whose offspring exhibit splitting of characters are called heterozygous.

Homozygosity is a state of the hereditary apparatus of an organism in which homologous chromosomes have the same form of a given gene. The transition of a gene to a homozygous state leads to the manifestation of recessive alleles in the structure and function of the body (phenotype), the effect of which, in heterozygosity, is suppressed by dominant alleles. The test for homozygosity is the absence of cleavage at certain types crossing. A homozygous organism produces only one type of gamete for a given gene.

Heterozygosity is a condition inherent in any hybrid organism, in which its homologous chromosomes carry different forms (alleles) of a particular gene or differ in the relative position of genes. The term “Heterozygosity” was first introduced by the English geneticist W. Bateson in 1902. Heterozygosity occurs when gametes of different genetic or structural composition merge into a heterozygote. Structural heterozygosity occurs when a chromosomal rearrangement of one of the homologous chromosomes occurs; it can be found in meiosis or mitosis. Heterozygosity is revealed using test crossing. Heterozygosity, as a rule, is a consequence of the sexual process, but can arise as a result of mutation. With heterozygosity, the effect of harmful and lethal recessive alleles is suppressed by the presence of the corresponding dominant allele and appears only when this gene transitions to a homozygous state. Therefore, heterozygosity is widespread in natural populations and is, apparently, one of the causes of heterosis. The masking effect of dominant alleles in heterozygosity is the reason for the persistence and spread of harmful recessive alleles in the population (the so-called heterozygous carriage). Their identification (for example, by testing sires by offspring) is carried out during any breeding and selection work, as well as when making medical and genetic forecasts.
In our own words, we can say that in breeding practice the homozygous state of genes is called “correct”. If both alleles that control a characteristic are the same, then the animal is called homozygous, and in breeding it will inherit this particular characteristic. If one allele is dominant and the other is recessive, then the animal is called heterozygous, and will outwardly demonstrate a dominant characteristic, but inherit either a dominant characteristic or a recessive one.

Any living organism has a section of DNA (deoxyribonucleic acid) molecules called chromosomes. During reproduction, germ cells copy hereditary information by their carriers (genes), which make up a section of chromosomes that have the shape of a spiral and are located inside the cells. Genes located in the same loci (strictly defined positions in the chromosome) of homologous chromosomes and determining the development of any trait are called allelic. In a diploid (double, somatic) set, two homologous (identical) chromosomes and, accordingly, two genes carry the development of these various signs. The predominance of one trait over another is called dominance, and genes are dominant. A trait whose manifestation is suppressed is called recessive. Homozygosity of an allele is the presence in it of two identical genes (carriers of hereditary information): either two dominant or two recessive. Heterozygosity of an allele is the presence of two different genes in it, i.e. one of them is dominant and the other is recessive. Alleles that in a heterozygote give the same manifestation of any hereditary trait as in a homozygote are called dominant. Alleles that manifest their effect only in a homozygote, but are invisible in a heterozygote, or are suppressed by the action of another dominant allele, are called recessive.

The principles of homozygosity, heterozygosity and other fundamentals of genetics were first formulated by the founder of genetics, Abbot Gregor Mendel in the form of three their laws of inheritance.

Mendel's first law: "The offspring from the crossing of individuals homozygous for different alleys of the same gene are uniform in phenotype and heterozygous in genotype."

Mendel's second law: "When heterozygous forms are crossed, a natural split in the offspring is observed in a ratio of 3:1 in phenotype and 1:2:1 in genotype."

Mendel's third law: "The alleles of each gene are inherited regardless of the body composition of the animal.
From the point of view of modern genetics, his hypotheses look like this:

1. Each trait of a given organism is controlled by a pair of alleles. An individual that has received the same alleles from both parents is called homozygous and is designated by two identical letters(for example, AA or aa), and if it receives different ones, then it is heterozygous (Aa).

2. If an organism contains two different alleles of a given trait, then one of them (dominant) can manifest itself, completely suppressing the manifestation of the other (recessive). (The principle of dominance or uniformity of the descendants of the first generation). As an example, let’s take monohybrid (only based on color) crossbreeding among cockers. Let's assume that both parents are homozygous for color, so a black dog will have a genotype, which we will denote as AA for example, and a fawn dog will have aa. Both individuals will produce only one type of gamete: the black only A, and the fawn only a. No matter how many puppies are born in such a litter, they will all be black, since black is the dominant color. On the other hand, they will all be carriers of the fawn gene, since their genotype is Aa. For those who are not too clear, note that the recessive trait (in this case, fawn color) appears only in the homozygous state!

3. Each sex cell(gamete) receives one of each pair of alleles. (The principle of splitting). If we cross the descendants of the first generation or any two cockers with the genotype Aa, in the offspring of the second generation a split will be observed: Aa + aa = AA, 2Aa, aa. Thus, the phenotypic split will look like 3:1, and the genotypic split will look like 1:2:1. That is, when mating two black heterozygous cockers, we can have a 1/4 chance of having black homozygous dogs (AA), 2/4 chance of having black heterozygotes (Aa) and 1/4 chance of having fawn dogs (aa). Life is not that simple. Sometimes two black heterozygous cockers can produce fawn puppies, or they can be all black. We simply calculate the likelihood of a given trait appearing in puppies, and whether it will manifest itself depends on which alleles ended up in the fertilized eggs.

4. During the formation of gametes, any allele from one pair can enter each of them along with any other from another pair. (Principle of independent distribution). Many traits are inherited independently, for example, while eye color may depend on the overall color of the dog, it has virtually nothing to do with the length of the ears. If we take a dihybrid cross (two different signs), then we can see the following ratio: 9: 3: 3: 1

5. Each allele is transmitted from generation to generation as a discrete, unchanging unit.

b. Each organism inherits one allele (for each trait) from each parent.

Dominance
For a specific gene, if two alleles carried by an individual are the same, which one will predominate? Because mutation of alleles often results in loss of function (empty alleles), an individual carrying only one such allele will also have a "normal" (wild type) allele for the same gene; a single normal copy will often be enough to support normal function. As an analogy, let us imagine that we are building a brick wall, but one of our two regular contractors goes on strike. As long as the remaining supplier can supply us sufficient quantity bricks, we can continue to build our wall. Geneticists call this phenomenon, when one of two genes can still provide normal function, dominance. The normal allele is determined to be dominant to the abnormal allele. (In other words, we can say that the incorrect allele is recessive to the normal one.)

When one speaks of a genetic abnormality being "carried" by an individual or lineage, the implication is that there is a mutated gene that is recessive. Unless we have sophisticated testing to directly detect this gene, we will not be able to visually identify the carrier from an individual with two normal copies (alleles) of the gene. Unfortunately, lacking such testing, the courier will not be detected in a timely manner and will inevitably pass on the mutation allele to some of its offspring. Each individual can be similarly “completed” and carry several of these dark secrets in its genetic baggage (genotype). However, we all have thousands of different genes for many different functions, and while these abnormalities are rare, the likelihood that two unrelated individuals carrying the same “abnormality” will meet to reproduce is very low.

Sometimes individuals with a single normal allele may have an "intermediate" phenotype. For example, the Basenji, who carries one allele for pyruvate kinase deficiency (an enzyme deficiency leading to mild anemia), average duration red life blood cell- 12 days. This intermediate type between a normal 16-day cycle and a 6.5-day cycle in a dog with two incorrect alleles. Although this is often called incomplete dominance, in this case it would be preferable to say that there is no dominance at all.

Let's take our brick wall analogy a little further. What if a single supply of bricks is not enough? We will be left with a wall that is lower (or shorter) than expected. Will it matter? It depends on what we want to do with the "wall" and possibly genetic factors. The result may not be the same for the two people who built the wall. (A low wall can keep out a flood, but not a flood!) If it is possible that an individual carrying only one copy of an incorrect allele will express it with the incorrect phenotype, then that allele should be regarded as dominant. Its refusal to always do so is defined by the term penetrance.

The third possibility is that one of the contractors supplies us with custom bricks. Not understanding this, we continue to work - eventually the wall falls. We could say that the defective bricks are the predominant factor. Advances in understanding several dominant genetic diseases in humans suggest that this is a reasonable analogy. Most dominant mutations affect proteins that are components of large macromolecular complexes. These mutations lead to changes in proteins that cannot interact properly with other components, leading to the failure of the entire complex (defective bricks - a fallen wall). Others are in regulatory sequences adjacent to genes and cause the gene to be transcribed at an inappropriate time and place.

Dominant mutations can persist in populations if the problems they cause are subtle and not always pronounced, or appear late in life, after the affected individual has participated in reproduction.

A recessive gene (i.e., the trait it determines) may not appear in one or many generations until two identical recessive genes from each parent are encountered (the sudden manifestation of such a trait in offspring should not be confused with a mutation).
Dogs that have only one recessive gene - the determinant of any trait - will not display this trait, since the effect of the recessive gene will be masked by the manifestation of the influence of its paired dominant gene. Such dogs (carriers of a recessive gene) can be dangerous for the breed if this gene determines the appearance of an undesirable trait, because it will pass it on to its descendants, and they will then preserve it in the breed. If you accidentally or thoughtlessly pair up two carriers of such a gene, they will produce some offspring with undesirable traits.

The presence of a dominant gene is always clearly and externally manifested by a corresponding sign. Therefore, dominant genes that carry an undesirable trait pose a much lesser danger to the breeder than recessive ones, since their presence always manifests itself, even if the dominant gene “works” without a partner (Aa).
But apparently, to complicate matters, not all genes are completely dominant or recessive. In other words, some are more dominant than others and vice versa. For example, some factors that determine coat color may be dominant, but still not appear outwardly unless they are supported by other genes, sometimes even recessive ones.
Matings do not always produce ratios in exact accordance with the expected average results and to obtain reliable result a given mating must produce a large litter or a large number of offspring in several litters.
Some external signs may be “dominant” in some breeds and “recessive” in others. Other traits may be due to multiple genes or half-genes that are not simple Mendelian dominants or recessives.

Diagnostics genetic disorders
Diagnosis of genetic disorders as a doctrine of recognition and designation of genetic diseases consists mainly of two parts
identification pathological signs, that is, phenotypic deviations in individual individuals; proof of heritability of detected deviations. The term “genetic health assessment” means testing a phenotypically normal individual to identify unfavorable recessive alleles (heterozygosity test). Along with genetic methods, methods are also used that exclude environmental influences. Routine research methods: assessment, laboratory diagnostics, methods pathological anatomy, histology and pathophysiology. Special methods having great importance- cytogenetic and immunogenetic methods. The cell culture method has contributed to major advances in diagnostics and genetic analysis hereditary diseases. Behind short term this method made it possible to study about 20 genetic defects found in humans (Rerabek and Rerabek, 1960; New, 1956; Rapoport, 1969) with its help, in many cases it is possible to differentiate homozygotes from heterozygotes with a recessive type of inheritance
Immunogenetic methods are used to study blood groups, serum and milk proteins, seminal fluid proteins, hemoglobin types, etc. The discovery of a large number of protein loci with multiple alleles led to a “renaissance era” in Mendelian genetics. Protein loci are used:
to establish the genotype of individual animals
while researching some specific defects(immunoparesis)
for linkage studies (marker genes)
for gene incompatibility analysis
to detect mosaicism and chimerism
The presence of a defect from the moment of birth, defects that emerge in certain lines and nurseries, the presence of a common ancestor in each anomalous case does not mean the heredity of a given condition and genetic nature. When a pathology is identified, it is necessary to obtain evidence of its genetic cause and determine the type of inheritance. Statistical processing of the material is also necessary. Two groups of data are subjected to genetic and statistical analysis:
Population data - frequency congenital anomalies in the total population, frequency of congenital anomalies in a subpopulation
Family data - evidence of genetic determination and determination of the type of inheritance, inbreeding coefficients and the degree of concentration of ancestors.
When studying genetic conditioning and the type of inheritance, the observed numerical ratios of normal and defective phenotypes in the offspring of a group of parents of the same (theoretically) genotype are compared with the segregation ratios calculated on the basis of binomial probabilities according to Mendel’s laws. To obtain statistical material, it is necessary to calculate the frequency of affected and healthy individuals among blood relatives proband over several generations, determine the numerical ratio by combining individual data, combine data on small families with respectively identical parental genotypes. Information about the size of the litter and the sex of the puppies is also important (to assess the possibility of linked or sex-limited heredity).
In this case, it is necessary to collect selection data:
Complex selection - random sampling of parents (used when checking a dominant trait)
Purposeful selection - all dogs with a “bad” trait in the population after a thorough examination of it
Individual selection - the probability of an anomaly occurring is so low that it occurs in one puppy from the litter
Multiple selection is intermediate between targeted and individual, when there is more than one affected puppy in the litter, but not all of them are probands.
All methods except the first exclude mating of dogs with the Nn genotype, which do not produce anomalies in litters. Exist various ways data correction: N.T.J. Bailey(79), L. L. Kawaii-Sforza and W. F. Bodme and K. Stehr.
Genetic characteristics population begins with an estimate of the prevalence of the disease or trait being studied. Based on these data, the frequencies of genes and corresponding genotypes in the population are determined. The population method allows you to study the distribution of individual genes or chromosomal abnormalities in populations. To analyze the genetic structure of a population, it is necessary to examine large group individuals, which must be representative, allowing one to judge the population as a whole. This method is informative when studying various forms hereditary pathology. The main method for determining the type of hereditary anomalies is the analysis of pedigrees within related groups of individuals in which cases of the disease under study were recorded according to the following algorithm:
Determining the origin of anomalous animals using breeding cards;
Compiling pedigrees for anomalous individuals in order to search for common ancestors;
Analysis of the type of inheritance of the anomaly;
Carrying out genetic and statistical calculations on the degree of randomness of the occurrence of an anomaly and the frequency of occurrence in the population.
The genealogical method of analyzing pedigrees occupies a leading place in genetic research slowly reproducing animals and humans. By studying the phenotypes of several generations of relatives, it is possible to establish the nature of inheritance of the trait and the genotypes of individual family members, determine the likelihood of manifestation and the degree of risk for offspring for a particular disease.
When determining a hereditary disease, pay attention to typical signs genetic predisposition. Pathology occurs more often in a group of related animals than in an entire population. This helps to distinguish a congenital disease from a breed predisposition. However, pedigree analysis shows that there are familial cases of the disease, which suggests the presence of a specific gene or group of genes responsible for it. Secondly, a hereditary defect often affects the same anatomical region in a group of related animals. Thirdly, with inbreeding, there are more cases of the disease. Fourth, hereditary diseases often manifest themselves early, and often have a constant age of onset.
Genetic diseases usually affect several animals in a litter, unlike intoxication and infectious diseases, which affect the entire litter. Congenital diseases very varied, from relatively benign to invariably lethal. Their diagnosis is usually based on anamnesis, clinical signs, a history of the disease in related animals, the results of testing crossbreeding and certain diagnostic studies.
A significant number of monogenic diseases are inherited in a recessive manner. This means that with autosomal localization of the corresponding gene, only homozygous mutation carriers are affected. Mutations are most often recessive and appear only in the homozygous state. Heterozygotes are clinically healthy, but are equally likely to pass on the mutant or normal variant of the gene to their children. Thus, over a long period of time, a latent mutation can be transmitted from generation to generation. With an autosomal recessive type of inheritance in the pedigrees of seriously ill patients who either do not survive to reproductive age, or have a sharply reduced potential for reproduction, it is rarely possible to identify sick relatives, especially in the ascending line. The exception is families with increased level inbreeding.
Dogs that have only one recessive gene - the determinant of any trait - will not display this trait, since the effect of the recessive gene will be masked by the manifestation of the influence of its paired dominant gene. Such dogs (carriers of a recessive gene) can be dangerous for the breed if this gene determines the appearance of an undesirable trait, because it will pass it on to its descendants. If two carriers of such a gene are accidentally or deliberately paired together, they will produce some offspring with undesirable traits.
The expected ratio of offspring splitting according to one or another trait is approximately justified with a litter of at least 16 puppies. For a litter of normal size - 6-8 puppies - we can only talk about a greater or lesser probability of the manifestation of a trait determined by a recessive gene for the descendants of a certain pair of sires with a known genotype.
Selection for recessive anomalies can be carried out in two ways. The first of them is to exclude from breeding dogs with manifestations of anomalies, i.e. homozygotes. The occurrence of an anomaly with such selection in the first generations decreases sharply, and then more slowly, remaining at a relatively low level. The reason for the incomplete elimination of some anomalies even during long and persistent selection is, firstly, much more slow contraction carriers of recessive genes than homozygotes. Secondly, in the case of mutations that deviate slightly from the norm, breeders do not always cull abnormal dogs and carriers.
With an autosomal recessive type of inheritance:
A trait can be transmitted through generations even with a sufficient number of descendants
The symptom may appear in children in the (apparent) absence of it in parents. It is then found in 25% of cases in children
The trait is inherited by all children if both parents are sick
The symptom develops in 50% of children if one of the parents is sick
Male and female offspring inherit this trait equally
Thus, absolutely complete elimination of the anomaly is fundamentally possible provided that all carriers are identified. Scheme for such detection: heterozygotes for recessive mutations can be detected in some cases laboratory methods research. However, for the genetic identification of heterozygous carriers, it is necessary to carry out analytical crosses - matings of a suspected carrier dog with a homozygous abnormal one (if the anomaly slightly affects the body) or with a previously established carrier. If, as a result of such crosses, abnormal puppies are born, among others, the tested sire is clearly identified as a carrier. However, if such puppies are not identified, then an unambiguous conclusion cannot be drawn from the limited sample of puppies obtained. The probability that such a sire is a carrier decreases with the expansion of the sample - the increase in the number of normal puppies born from matings with him.
At the department veterinary academy Petersburg, an analysis of the structure of the genetic load in dogs was carried out and it was found that the greatest specific gravity- 46.7% are anomalies inherited in a monogenic autosomal recessive manner; anomalies with complete dominance amounted to 14.5%; 2.7% of anomalies appeared as incomplete dominant traits; 6.5% of anomalies are inherited as sex-linked, 11.3% hereditary traits with a polygenic type of inheritance and 18%3% of the entire spectrum of hereditary anomalies, the type of inheritance has not been established. Total number Anomalies and diseases that have a hereditary basis in dogs amounted to 186 items.
Along with traditional methods The use of phenotypic markers of mutations is relevant for selection and genetic prevention.
Genetic disease monitoring is a direct method for assessing hereditary diseases in the offspring of unaffected parents. "Guard" phenotypes can be: cleft palate, cleft lip, inguinal and umbilical hernias, hydrops of newborns, convulsions in newborn puppies. In monogenic fixed diseases, it is possible to identify the actual carrier through the marker gene associated with it.
The existing breed diversity of dogs provides a unique opportunity to study the genetic control of numerous morphological traits, different combination which determines breed standards. This situation can be illustrated by two of the current existing breeds domestic dog, contrastingly different from each other at least in such morphological characteristics like height and weight. This is a breed english mastiff, on the one hand, whose representatives have a height at the withers of 80 cm and a body weight exceeding 100 kg, and the Chi Hua Hua breed, 30 cm and 2.5 kg.
The process of domestication involves the selection of animals for their most outstanding characteristics, from a human point of view. Over time, when the dog began to be kept as a companion and for its aesthetic appearance, the direction of selection changed to producing breeds that were poorly adapted to survive in nature, but well adapted to the human environment. There is an opinion that mongrels are healthier than purebred dogs. Indeed, hereditary diseases are probably more common in domestic animals than in wild animals.
“One of the most important goals is the development of methods for combining the tasks of improving animals according to selected traits and preserving their fitness at the required level - as opposed to unilateral selection for maximum (sometimes exaggerated, excessive) development of specific breed traits, which is dangerous for the biological well-being of domesticated organisms” - (Lerner, 1958).
The effectiveness of selection, in our opinion, should consist in diagnosing anomalies in affected animals and identifying carriers with defective heredity, but with a normal phenotype. Treatment of affected animals in order to correct their phenotypes can be considered not only as an event to improve the aesthetic appearance of animals (oligodontia), but also as a prevention cancer diseases(cryptorchidism), preservation of biological, full activity (hip dysplasia) and stabilization of health in general. In this regard, selection against anomalies is necessary in the joint activities of cynology and veterinary medicine.
The ability to test DNA for various dog diseases is very new thing in cynology, knowledge of this can warn breeders about what genetic diseases special attention should be paid when selecting pairs of manufacturers. Good genetic health is very important because it determines biologically full life dogs. Dr. Padgett's book, Controlling Inherited Diseases in Dogs, shows how to read a genetic pedigree for any abnormality. Genetic pedigrees will show whether the disease is gender-linked, whether inheritance is through a simple dominant gene, or through a recessive gene, or whether the disease is polygenic in origin. Unintentional genetic errors will occur from time to time, no matter how careful the breeder is. By using genetic pedigrees as a tool in knowledge sharing, it is possible to dilute harmful genes to such an extent that they are stopped from expressing themselves until a DNA marker can be found to test their transmission. Since the selection process involves improving the population in the next generation, it is not the phenotypic characteristics of the direct elements of the selection strategy (individuals or pairs of crossed individuals) that are taken into account, but the phenotypic characteristics of their descendants. It is in connection with this circumstance that the need arises to describe the inheritance of a trait for breeding tasks. A pair of crossing individuals differ from other similar individuals in their origin and phenotypic characteristics of the trait, both themselves and their relatives. Based on this data, if available ready description inheritance, it is possible to obtain the expected characteristics of the offspring and, consequently, estimates of the selection values of each element of the breeding strategy. For any measures directed against any genetic abnormality, the first step is to determine the relative importance of a “bad” feature compared to other features. If an undesirable trait has a high frequency of heritability and causes serious harm to the dog, you should act differently than if the trait is rare or of minor importance. A dog of excellent breed type that carries a faulty color remains a much more valuable sire than a mediocre dog with the correct color.

HOMO-HETEROSYGOTES, terms introduced into genetics by Bateson to denote the structure of organisms in relation to any hereditary predisposition (gene). If a gene is received from both parents, the organism will be homozygous for that gene. Eg. if the reb-. nok" received the gene for brown eye color from his father and mother, he is homozygous for brown eyes. If we designate this gene with the letter A, then the body formula will be AA. If the gene is received from only one parent, then the individual is heterozygous. For example, if one parent has brown eyes and the other has blue eyes, then the offspring will be heterozygous; by eye color. Denoting the dominant brown color gene through A, blue-through A, for the descendant we have the formula Ah. An individual may be homozygous for both dominant gene (AA), and recessive (aa). An organism can be homozygous for some genes and heterozygous for others. Eg. both parents can have Blue eyes, but one of them has curly hair, and the other has smooth hair. There will be a descendant Aab. Heterozygotes for two genes are called diheterozygotes. In appearance, homo- and heterozygotes are either clearly distinguishable - a case of incomplete dominance (curly-haired - homozygous for a dominant gene, wavy-haired - heterozygous, smooth-haired - homozygous for a recessive gene, or black, blue and Andalusian chickens) or distinguishable by microscopic and other studies (peas , heterozygous for wrinkled seeds, distinguishable by not quite round grains) or not distinguishable at all in the case of complete dominance. Similar phenomena have been noted in humans: for example. there is reason to believe that a mild degree of recessive myopia can also appear in a heterozygote; the same applies to Fried-Reich ataxia and others. The phenomenon of complete dominance makes possible distribution in a latent form of lethal or harmful recessive genes, because if two individuals, apparently healthy, but containing such a gene in a heterozygous state, marry, then 25% of non-viable or sick children will appear in the offspring (for example, iehthyosis congenita) . From the marriage of two individuals who are homozygous for any trait, all offspring also have that trait: for example, from the marriage of two genotinically deaf-mutes (the trait is recessive, therefore it has the structure aa) all children will be deaf and mute; from the marriage of a recessive homozygote and a heterozygote, half of the offspring inherit the dominant trait. The doctor most often has to deal with heterozygous-heterozygous marriages (with recessive painful factor) and homozygotes-heterozygotes (with a dominant disease factor). Homozygous is a sex that has two identical sex chromosomes (female in mammals, male in birds, etc.). Sex that has different sex chromosomes (g and y) or just one X, called heterozygous. The term hemizygous [introduced into genetics by Lippin-cott] is more convenient, since heterozygotes must have the structure Ah, and individuals with one chromosome cannot be Ah, but have structure A or A. Examples of hemizygous patients are men with hemophilia, color blindness and some other diseases whose genes are localized on the α chromosome. Lit.: Bateson W., Mendel's principles of heredity, Cambridge, 1913; see also the literature to Art. Genetics. A. Serebrovsviy.

See also:

HOMIOTHERMAL ANIMALS(from the Greek homoios - equal, identical and therme - warmth), or warm-blooded (syn. homeothermic and homothermic animals), those animals that have a regulatory apparatus that allows them to maintain body temperature approximately constant and almost independent ...
HOMOLOGICAL SERIES, groups organic compounds with the same chemical function, but differing from each other in one or more methylene (CH2) groups. If in the simplest compound of a number of saturated hydrocarbons - methane, CH4, one of...
HOMOLOGY ORGANS(from the Greek ho-mologos - consonant, corresponding), the name of morphologically similar organs, i.e. organs of the same origin, developing from the same rudiments and revealing similar morphol. ratio. The term “homology” was introduced by the English anatomist R. Owen for...
HOMOPLASTY, or homoyoplasty (from the Greek homoios-like), isoplasty, free transplantation of tissues or organs from one individual to another of the same species, including from one person to another. Start...
HOMOSEXUALITY, unnatural sexual attraction to persons of the same sex. G. was previously considered a purely psychopathological phenomenon (Krafft-Ebing), and G.’s issues were dealt with primarily by psychiatrists and forensic doctors. Only recently, thanks to the work...

HETEROSYGOTE - (from hetero... HETEROSYGOTE - HETEROSYGOTE, an organism that has two contrasting forms (ALLELES) of a GENE in a pair of CHROMOSOMES. Heterozygote is an organism that has allelic genes of different molecular form; in this case, one of the genes is dominant, the other is recessive. A recessive gene is an allele that determines the development of a trait only in the homozygous state; such a trait will be called recessive.

Heterozygosity, as a rule, determines the high viability of organisms and their good adaptability to changing environmental conditions and is therefore widespread in natural populations.

The average person has approx. 20% of genes are in a heterozygous state. That is, the allelic genes (alleles) - paternal and maternal - are not the same. If we designate this gene with the letter A, then the body’s formula will be AA. If the gene is received from only one parent, then the individual is heterozygous. The development of a trait depends both on the presence of other genes and on environmental conditions; the formation of traits occurs during individual development individuals.

Mendel called the trait manifested in first-generation hybrids dominant, and the suppressed trait recessive. Based on this, Mendel made another conclusion: when crossing hybrids of the first generation, the characteristics in the offspring are split in a certain numerical ratio. In 1909, V. Johansen called these hereditary factors genes, and in 1912 T. Morgan will show that they are located in chromosomes.

HETEROSYGOTE is:

During fertilization, male and female gametes fuse and their chromosomes are combined into one zygote. From self-pollination of 15 first-generation hybrids, 556 seeds were obtained, of which 315 were yellow smooth, 101 yellow wrinkled, 108 green smooth and 32 green wrinkled (splitting 9:3:3:1). Mendel's third law is valid only for those cases when the genes of the analyzed traits are in different couples homologous chromosomes.

As a rule, it is a consequence of the sexual process (one of the alleles is introduced by the egg, and the other by the sperm). Heterozygosity maintains a certain level of genotypic variability in a population. Wed. Homozygote. In experiments, G. is obtained by crossing homozygotes for various types with each other. alleles.

Source: “Biological Encyclopedic Dictionary.” Ch. ed. M. S. Gilyarov; Editorial team: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected. Eg. Both parents may have blue eyes, but one of them has curly hair and the other has smooth hair. Lit.: Bateson W., Mendel’s principles of heredity, Cambridge, 1913; see also literature to Art. Genetics.A.

Genetics is the science of the laws of heredity and variability. Heredity is the property of organisms to transmit their characteristics from one generation to another. Variability is the property of organisms to acquire new characteristics compared to their parents.

The main one is the hybridological method - a system of crossings that allows one to trace the patterns of inheritance of traits in a series of generations. First developed and used by G. Mendel. Crossing, in which the inheritance of one pair of alternative characters is analyzed, is called monohybrid, two pairs - dihybrid, several pairs - polyhybrid. Mendel came to the conclusion that in first-generation hybrids, of each pair of alternative characters, only one appears, and the second seems to disappear.

In a monohybrid crossing of homozygous individuals having different meanings alternative traits, hybrids are uniform in genotype and phenotype. The experimental results are shown in the table. The phenomenon in which part of the second generation hybrids carries a dominant trait, and part - a recessive one, is called segregation.

From 1854, for eight years, Mendel conducted experiments on crossing pea plants. To explain this phenomenon, Mendel made a number of assumptions, which were called the “gamete purity hypothesis”, or the “gamete purity law”. At the time of Mendel, the structure and development of germ cells had not been studied, so his hypothesis of the purity of gametes is an example of brilliant foresight, which later found scientific confirmation.

Organisms differ from each other in many ways. Therefore, having established the patterns of inheritance of one pair of traits, G. Mendel moved on to studying the inheritance of two (or more) pairs of alternative traits. As a result of fertilization, nine genotypic classes may appear, which will give rise to four phenotypic classes.

Certain alleles are defined. Determination of heterozygosity for recessive alleles that cause hereditary diseases (i.e., identification of carriers of this disease) - important problem honey. genetics.

HOMOLOGICAL SERIES, groups of organic compounds with the same chemical. function, but differing from each other in one or more methylene (CH2) groups. HOMOLOGICAL ORGANS (from the Greek ho-mologos - consonant, corresponding), the name of morphologically similar organs, i.e. Alternative characteristics are understood as different meanings any sign, for example, the sign - the color of peas, alternative signs - yellow, green color peas

For example, in the presence of a “normal” allele A and mutant a1 and a2, the a1/a2 heterozygote is called. compound, unlike heterozygotes A/a1 or A/a2. (see HOMOZYGOTE). However, when breeding heterozygotes in the offspring, the valuable properties of varieties and breeds are lost precisely because their germ cells are heterogeneous. The yellow color (A) and smooth shape (B) of the seeds are dominant traits, the green color (a) and wrinkled shape (b) are recessive traits.

How to determine whether an organism is heterozygous or homozygous. Allelic genes, their properties

see also

See what “Heterozygote” is in other dictionaries:

20. The concept of a gene. Gene properties. Gene functions. Types of genes

HETEROSYGOTE is: