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Lecture: Concepts: genotype, phenotype, trait. allelic and non-allelic genes, homozygous and heterozygous organisms, the concept of hemizygosity. Heterozygous and homozygous organisms

Homo is translated from Latin as the same; a homozygous trait is a trait that is inherited in the body by the same gene, which is in a paired state (AA). Therefore, a homozygous organism is an organism in which a trait is inherited by the same gene.

A dominant trait is denoted by the letter A, a recessive trait is denoted by the letter a.

Hetero is translated from Latin as different, this is when in an organism a trait can be inherited both by a dominant and a recessive, i.e. there may be inheritance of traits like AA, Aa and aa. In the first two cases, the trait is inherited as a dominant, and in the second case as a recessive. Therefore, a heterozygous organism is an organism in which a trait is inherited by different genes.

- Homozygous organism is an organism (animal or plant) that has two absolutely identical genes, for example, two dominant black genes (BB) or two recessive brown genes (bb). Based on this characteristic, this organism is called pure.
- Heterozygous organism is an organism containing one dominant and one recessive gene (for example, Bb). Such an organism is called a hybrid.

In order to understand what we are talking about in general, it is necessary to understand genes, or rather, their division into dominant and recessive...

Dominant genes are those genes that dominate others, fight for their victory...

Recessive genes are those genes that are suppressed and cannot fight dominant...

So homozygous organisms contain two dominant genes (from the word homo - identical)...

Heterozygous organisms contain different genes, one dominant, the other recessive (from the word hetero - different)...

So the fundamental difference is that genes can be either identical in power or different...

The medical encyclopedia has a definition

Homozygous sex is a sex that has 2 identical sex chromosomes. In a homozygous (from the Greek homos meaning same and zygote meaning paired) organism, there are 2 identical copies of a particular gene on homologous chromosomes.

A heterozygous sex is a sex that has different sex chromosomes or just one chromosome. In a heterozygous organism, also called a hybrid organism, by definition there are two different shapes a specific gene (different forms of a gene) on homologous chromosomes.

These are very complex definitions for those who have not encountered such concepts, but a very clear explanation is given by the biological encyclopedia, see the link here.

homo - homogeneous.

hetero - heterogeneous.

For organisms, this means that if the allelic genes are the same, then the organism is homozygous, and if they are different, then it is heterozygous, which can be used when crossing two organisms.

Homozygous and heterozygous organisms differ from each other by the presence or absence of two identical genes. U homozygous organisms or both traits are dominant or recessive (for example, dark hair and brown eyes). U heterozygous one of the traits is dominant and the other is recessive (for example, blond hair and brown eyes).

Homozygous (homo - identical) are those organisms in which two genes are equally dominant in the entire organism.

Hererozygous (hetero - different) - those organisms in which two genes are different, i.e. one is dominant and the other is suppressed.

Homozygous (homos - identical, zygote - paired) organism with identical structures of a given type. Both dominant or both recessive. And in heterozygous organisms both traits are present - dominant and recessive.

Homozygous organisms are those organisms that have two identical gene forms (either both dominant or both recessive);

Heterozygous organisms are those organisms that have both dominant and recessive forms of genes.

Homozygous organisms do not have segregation of characters, but heterozygous ones do.

There are dominant genes and recessive ones (weakly influencing).

Dominant genes are designated by a capital English letter, for example A, and recessive - lowercase a.

In heterozygous organisms, usually one gene is dominant and the second is recessive:

It is designated as follows: Aa.

When a given organism creates offspring, the dominant gene plays a decisive role in what kind of offspring it will be, that is A.

For example, if we consider mice. If the dominant gene A is a fluffy coat, and recessive a- is bald (there are bald albinos), then the dominant gene will win A and the descendant will be hairy. Moreover, this will also lead to an increase in the genus, since bald individuals are not protected from the cold and will most likely die, while hairy ones will be able to survive until they grow up and leave offspring.

Homozygous organisms are those organisms that have the same genes (alleles). Or two recessive aa, or two dominant A.A..

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.

At incomplete dominance an intermediate variant is observed (as in 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 are 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.

Homozygosity - this 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 for this gene only one type of gamete.

Heterozygosity - this 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 merging different quality genes or structural composition gametes into heterozygotes. 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 is usually - 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 breeding practice, the homozygous state of genes is called " correct". If both alleles controlling a characteristic are the same, then the animal is called homozygous, and in breeding, this characteristic will be inherited. If one allele is dominant and the other is recessive, then the animal is called heterozygous, and will outwardly demonstrate dominant characteristic, and to 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 different characteristics. When one characteristic predominates over another it is called dominance, and genes dominant. A trait whose manifestation is suppressed is called recessive. Homozygosity allele is called the presence in it of two identical genes (carriers of hereditary information): either two dominant or two recessive. Heterozygosity allele is called 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.

Genotype - the totality of all the genes of an organism. A genotype is a collection of genes that interact with each other and influence each other. Each gene is influenced by other genes of the genotype and itself influences them, so the same gene can manifest itself differently in different genotypes.

Phenotype – the totality of all properties and characteristics of an organism. The phenotype develops on the basis of a specific genotype as a result of the interaction of the organism with conditions environment. Organisms that have the same genotype may differ from each other depending on the conditions of development and existence.

Sign- a unit of morphological, physiological, biochemical, immunological, clinical and any other discreteness of organisms (cells), i.e. a separate quality or property by which they differ from each other.

The genotype is the genetic constitution of an organism, which is the totality of all the hereditary inclinations of its cells, contained in their chromosomal set - the karyotype.

Genotype(from gene and type), the totality of all genes localized in the chromosomes of a given organism.

Phenotype (Phenotype) - the totality of all signs and properties inherent in an individual that were formed in the process of his individual development.

Phenotype is the totality of all the characteristics of an organism, formed in the interaction of the genotype with the environment.

Homozygosity, state of the hereditary apparatus body, in which homologous chromosomes have the same form of a given gene.

Heterozygosity, a condition inherent in any hybrid organism in which its homologous chromosomes carry different forms (alleles) of a particular gene.

Hemizygosity(from the Greek hemi- - semi- and zygotós - joined together), a condition associated with the fact that the organism has one or more genes that are not paired, that is, they do not have allelic partners. (In sex-linked inheritance, Xr or XR - r – daltonzyme)

35. Patterns of inheritance during monohybrid crossing.

Monohybrid cross - crossing forms that differ from each other in one pair of alternative characteristics.

Mendel's 1st law: when crossing two homozygous organisms that differ from each other in one pair of alternative traits, uniformity in genotype and phenotype is observed in the first generation. (gingival fibromatosis - A, healthy gums - A, the child is sick in any case)

Mendel's 2nd law: when crossing 2 heterozygous organisms that differ in one pair of alternative traits (F1 hybrids) in their offspring (F2 hybrids), a 3:1 cleavage is observed in the phenotype, 1:2:1 in the genotype

Complete dominance is a phenomenon in which one of the allelic genes is predominant and manifests itself in both heterozygous and homozygous states.

36. Dihybrid and polyhybrid crossing. The law of independent combination of genes and its cytological basis. General formula splitting with independent inheritance.

Dihybrid crossing - crossing of forms that differ in two pairs of studied characteristics

Polyhybrid crossing - crossing forms that differ in many characteristics.

Law of independent inheritance of characteristics:

When crossing homozygous individuals that differ in two and big amount pairs of alternative characters, in the second hybrid generation (with inbreeding of 1st generation hybrids) independent inheritance is recorded for each pair of characters and individuals appear with new combinations of characters not characteristic of the parental and ancestral forms ( law of independent distribution, or Mendel's III law) (Brown eyes- B, blue - b, right-handed - A, left-handed - a). Cleavage in the ratio (3:1)n, and in phenotype 9:3:3:1. The task is in the album.

Obviously, this law must be obeyed primarily by non-allelic genes located on different (non-homologous) chromosomes. In this case, the independent nature of the inheritance of traits is explained by the patterns of behavior of non-homologous chromosomes in meiosis. These chromosomes form different pairs, or bivalents, with their homologues, which in metaphase I of meiosis are randomly aligned in the plane of the spindle equator. Then, in anaphase I of meiosis, the homologues of each pair diverge to different spindle poles independently of the other pairs. As a result, at each pole random combinations of paternal and maternal chromosomes arise in the haploid set (see Fig. 3.75). Consequently, different gametes contain different combinations of paternal and maternal alleles of non-allelic genes.

The variety of gamete types produced by an organism is determined by the degree of its heterozygosity and is expressed by formula 2 n, Where n- number of loci in the heterozygous state. In this regard, diheterozygous F1 hybrids form four types of gametes with equal probability. The implementation of all possible encounters of these gametes during fertilization leads to the appearance in F2 of four phenotypic groups of descendants in the ratio 9:3:3:1. Analysis of the F2 offspring for each pair of alternative traits separately reveals a 3:1 split.

37.Multiple alleles. Inheritance of human blood groups of the ABO system.

Multiple allelism is different states (three or more) of the same chromosomal locus resulting from mutations.

The presence of different alleles of a gene in the gene pool of a species at the same time is called multiple allelism. An example of this is the different eye color options in the fruit fly: white, cherry, red, apricot, eosin, caused by different alleles of the corresponding gene. In humans, as in other representatives organic world, multiple allelism is characteristic of many genes. Thus, three alleles of gene I determine the blood group according to the AB0 system (IA, IB, I0). The gene that determines Rh status has two alleles. More than a hundred alleles include genes for α- and β-polypeptides of hemoglobin.

The cause of multiple allelism is random changes in gene structure (mutations) that are maintained during the process. natural selection in the gene pool of the population. The diversity of alleles that recombine during sexual reproduction determines the degree of genotypic diversity among representatives of a given species, which is of great evolutionary significance, increasing the viability of populations in the changing conditions of their existence. In addition to evolutionary and environmental significance the allelic state of genes has big influence on the functioning of genetic material. In diploid somatic cells of eukaryotic organisms, most genes are represented by two alleles, which jointly influence the formation of traits. Tasks in the album.

38. Interaction of non-allelic genes: complementarity, epistasis, polymerization, modifying effect.

Complementarity is a type of interaction when 2 non-allelic genes, entering the genotype in a dominant state, jointly determine the appearance of a new character, which each of them does not determine separately. (R - rose-shaped comb, P - pea-shaped, rp - leaf-shaped, RP - nut-shaped )

If one of the pair is present, it manifests itself.

An example is human blood groups.

Complementarity can be dominant or recessive.

In order for a person to have normal hearing, many genes, both dominant and recessive, must work in concert. If he is homozygous recessive for at least one gene, his hearing will be weakened.

Epistasis is the masking of the genes of one allelic pair by the genes of another.

Epistasis (from the Greek epi - above + stasis - obstacle) is the interaction of non-allelic genes, in which the expression of one gene is suppressed by the action of another, non-allelic gene.

A gene that suppresses the phenotypic manifestations of another is called epistatic; a gene whose activity is altered or suppressed is called hypostatic.

This is due to the fact that enzymes catalyze different cellular processes when several genes act on one metabolic pathway. Their action must be coordinated in time.

Mechanism: if B turns off, it will mask the action of C

In some cases, the development of a trait in the presence of two non-allelic genes in a dominant state is considered as a complementary interaction, in others, the non-development of a trait determined by one of the genes in the absence of another gene in a dominant state is regarded as recessive epistasis; If a trait develops in the absence of a dominant allele of a non-allelic gene, but does not develop in its presence, we speak of dominant epistasis.

Polymeria is a phenomenon when different non-allelic genes can have a unique effect on the same trait, enhancing its manifestation.

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 (P1P1P2P2P3P3P4P4) to the minimum in homozygotes for recessive alleles (p1p1p2p2p3p3p4p4) (see Fig. 3.80). When two mulattoes are married, heterozygous for all four pairs, which form 24 = 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 (Fig. 6.16), 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.

If genes are located, each in its own separate locus, but their interaction manifests itself in the same direction - these are polygenes. One gene exhibits the trait slightly. Polygenes complement each other and have a powerful effect - a polygenic system arises - i.e. the system is the result of the action of identically directed genes. Genes are exposed significant influence there are more than 50 main genes. There are many known polygenic systems.

At diabetes mellitus mental retardation is observed.

Height and level of intelligence are determined by polygenic systems

Modifying action. Modifier genes themselves do not determine any trait, but can enhance or weaken the effect of the main genes, thus causing a change in phenotype. The inheritance of piebaldity in dogs and horses is usually cited as an example. Numerical splitting is never given, since the nature of inheritance is more reminiscent of polygenic inheritance of quantitative traits.

1919 Bridges coined the term modifier gene. Theoretically, any gene can interact with other genes, and therefore exhibit a modifying effect, but some genes are more modifiers. They often do not have their own trait, but are able to enhance or weaken the manifestation of a trait controlled by another gene. In the formation of a trait, in addition to the main genes, modifying genes also exert their effect.

Brachydactyly - can be severe or minor. In addition to the main gene, there is also a modifier that enhances the effect.

Coloring of mammals – white, black + modifiers.

39. Chromosomal theory of heredity. Linkage of genes. Clutch groups. Crossing over as a mechanism that determines gene linkage disorders.

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Lecture: Concepts: genotype, phenotype, trait. allelic and non-allelic genes, homozygous and heterozygous organisms, the concept of hemizygosity. Heterozygous and homozygous organisms

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Lecture: Concepts: genotype, phenotype, trait. allelic and non-allelic genes, homozygous and heterozygous organisms, the concept of hemizygosity. Heterozygous and homozygous organisms

see also

Kondylos

see also

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What else to read

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