What is a heterozygous organism in biology definition. Allelic genes, their properties
Variability is the ability of living organisms to acquire new characteristics and qualities. A distinction is made between non-hereditary and hereditary variability (Scheme 1).
TO non-hereditary variability include changes change external signs(phenotype) that do not persist in generations. These include modifications, which arise under the influence environment.
in insects and other animals → change in fur color in some mammals when weather conditions change (for example, in a hare) fig. 2,
in humans → an increase in the level of red blood cells when climbing mountains, an increase in skin pigmentation with intense exposure ultraviolet rays, development of the musculoskeletal system as a result of training (Fig. 3).
Rice. 3 Development of the musculoskeletal system as a result of training
Hereditary variability represents changes in the genotype that persist over a number of generations. These include combinations and mutations. Combination variability occurs when the genes of the father and mother are recombined (mixed).
Example: the manifestation of fruit flies with a dark body and long wings when crossing gray fruit flies with long wings with dark fruit flies with short wings (Fig. 4).
Rice. 4 Drosophila with dark body and long wings
the night beauty flower has petals pink color occur when a red and white gene are combined (Fig. 5).
Rice. 5 Formation of pink petals in the night beauty
Mutational variability- these are changes in the DNA of a cell (changes in the structure and number of chromosomes). Occur under the influence of ultraviolet radiation, radiation (X-rays), etc.
in humans → trisomy 21 ( Down syndrome),
in animals → biceps (Fig. 6).
Rice. 6 Two-headed turtle from China
GENOME
Genome - a collection of hereditary material found in a cell of an organism. Most genomes, including the human genome and the genomes of all other cellular life forms, are built from DNA.
Deoxyribonucleic acid (DNA)- a macromolecule that ensures the storage, transmission and implementation from generation to generation of the genetic program for the development and functioning of living organisms.
Genotype- the totality of genes of a given organism.
So, the genome is a characteristic of the species as a whole, and the genotype is a characteristic of an individual.
Gene - an elementary unit of heredity of living organisms. A gene is a section of DNA responsible for the manifestation of a trait.
Genes There is in the core each cells living organism Fig. 7.
Rice. 7 Gene location in the cell
As a result of the interaction of the genotype with environmental factors, phenotype , that is, the totality of all the signs and properties of an organism. Examples: height, body weight, eye color pic. 8, hair shape, blood type, left-handed, right-handed.
Rice. 8 Brown and blue eye colors Fig. 9 Genotype and phenotype in peas
TOf e n O T And n at include not only external signs, but also internal ones: anatomical, physiological, biochemical. Each individual has its own characteristics appearance, internal structure, the nature of metabolism, the functioning of organs, i.e. your phenotype, which was formed under certain environmental conditions.
CHROMOSOME STRUCTURE
CHROMOSOMES are a structural element of the nucleus in which all hereditary information is contained (Fig. 10, 11, 12).
Rice. 10 Schematic representation of a chromosome
CENTROMERE - a region of a chromosome that divides the chromosome into two arms.
Rice. 11 Image of a chromosome in an electron microscope
Rice. 12 Location of the chromosome in the cell
There is an X chromosome and a Y chromosome. 13.
X chromosome - sex chromosome Most mammals, including humans, determine the female sex of the organism.
Y chromosome - the sex chromosome of most mammals, including humans, determining the male sex of the organism.
Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
Rice. 13 X chromosome and Y chromosome
KARYOTYPE- a set of chromosomes characteristic of a given type of organism (chromosomal set) Fig. 14.
Rice. 14 Karyotype healthy person
Autosomes- these chromosomes are the same in both sexes. Genotype female body has 44 chromosomes (22 pairs), identical to male ones. They are called autosomes. 14.
Rice. 15 Karyotypes of plants and animals
Rice. 16 Image of plants and animals of the corresponding karyotype:
skerda, butterfly, fruit fly, grasshopper and rooster
Karyotype– a set of external characteristics of the chromosome set (number, shape, size of chromosomes) characteristic of a given species.
NITROGEN BASES
NITROGEN BASES - organic compounds included in nucleic acids(DNA and RNA) fig. 17.
Latin and Russian codes for nucleic bases (nitrogen base):
A - A: Adenine;
G - G: Guanine;
C - C: Cytosine;
T - T: Thymine, found in bacteriophages (bacterial viruses) in DNA, takes the place of uracil in RNA;
U - U: Uracil, found in RNA, takes the place of thymine in DNA.
Rice. 17 Nitrogen bases in DNA and RNA
Rice. 18 Location nitrogenous bases in a cage
Nucleotide built from a pentose sugar, a nitrogenous base and a phosphoric acid (PA) residue.
Hydrogen bond is the interaction between two electronegative atoms of the same or different molecules through a hydrogen atom: G−H ... C (a line indicates a covalent bond, three dots indicate a hydrogen bond) Fig. 19.
Rice. 19 Hydrogen bond
The principle of complementarity is used in DNA synthesis. This is a strict correspondence to the combination of nitrogenous bases connected by hydrogen bonds, in which: A-T (Adenine connects to Thymine) G-C (Guanine connects to Cytosine).
The principle of complementarity is also used in RNA synthesis, in which A-U (Adenine combines with Uracil) G-C (Guanine combines with Cytosine).
CROSSING
Crossbreeding - natural or artificial union of two hereditarily different genotypes through fertilization.
Fertilization – the process of fusion of female and male reproductive cells Fig. 20.
Rice. 20 Fusion of egg and spermatoroid
Gametes are the sex cells of animals and plants. Ensures the transmission of traits from parents to offspring. It has a halved (haploid) set of chromosomes compared to a somatic cell. Sex cells carrying hereditary information.
Zygote- diploid (containing a complete double set of chromosomes) cell formed as a result of fertilization of Fig. 20
Rice. 21 Zygote
The emergence of a new organism as a result of fertilization, the fusion of male and female gametes with a haploid (single) set of chromosomes. Biological significance: restoration of the diploid (double) set of chromosomes in the zygote (Fig. 21).
Rice. 22 Zygote is the result of fertilization
There are homozygotes and heterozygotes.
Homozygote- an organism (zygote) that has identical alleles of one gene on homologous chromosomes (AABB; AA).
Heterozygote- an individual that produces different types of gametes. Heterozygote– the content in body cells of different genes of a given allelic pair, for example Aa, resulting from the combination of gametes with different alleles, for example AaBb, even for one trait AABb.
Dominance is the predominance of the effect of a certain allele (gene) in the process of realizing the genotype in the phenotype, expressed in the fact that the dominant allele more or less suppresses the actions of another allele (recessive), and the trait in question “submits” to it.
The dominant gene manifests itself in both homozygous and heterozygous organisms.
The phenomenon of predominance of the parents' trait in a hybrid is called dominance.
Rice. 23 Dominance of red hair and freckles
Rice. 24 Dominance of farsightedness
Recessiveness- absence of phenotypic manifestation of one allele in a heterozygous individual (in an individual carrying two different alleles of one gene). A suppressed (outwardly disappearing) sign.
Paired genes located on homologous chromosomes and controlling the development of the same trait are called allelic Fig. 25.
Rice. 25 Allelic genes
Allelic genes– paired genes – various shapes of the same gene, responsible for the alternative (different) manifestations of the same trait. For example, two allelic genes located in identical loci (locations) are responsible for eye color. Only one of them can be responsible for the development of brown eyes, and the other for the development of blue eyes. In the case when both genes are responsible for the same development of a trait, the organism is said to be homozygous for this trait. If allelic genes determine different development trait, they speak of a heterozygous organism. In species with large in number of individuals, at least 30-40% of genes have two, three or more alleles. This supply of alleles ensures high adaptability of species to changing environmental conditions - this is material for natural selection and at the same time the key to the survival of the species. Genetic diversity within a species is determined by the number and distribution of alleles of different genes.
The crossing of a homozygous organism with a recessive homozygote is called analyzing.
Analysis cross - crossbreeding carried out to determine the genotype of an organism. To do this, the experimental organism is crossed with an organism that is recessive homozygous for the trait being studied. Let's say we need to find out the genotype of a pea plant that has yellow seeds. There are two possible genotype options for the experimental plant: it can be either a heterozygote (Aa) or a dominant homozygote (Aa). To establish its genotype, we will carry out an analytical cross with a recessive homozygote (aa) - a plant with green seeds.
Thus, if, as a result of an analysis cross, a 1:1 ratio is observed in F1, then the experimental organism was heterozygous; if no cleavage is observed and all organisms in F1 exhibit dominant characteristics, then the experimental organism was homozygous. 26.
Rice. 26 Analyzing crosses
Clean line is a group of genetically homogeneous (homozygous) organisms. Pure lines are formed only by homozygous plants, therefore, when self-pollinating, they always reproduce one variant of the manifestation of the rice trait. 27. Self-pollination- pollination on one flower.
Rice. 27 Self-pollination
INCOMPLETE DOMINANCE- one of the types of interaction of allelic genes, in which one of the alleles (dominant) in a heterozygote is not completely suppressed by the manifestation of another allele (recessive), and in the first generation the expression of the trait is intermediate in nature (Fig. 28.
Rice. 28 Incomplete Dominance
The intermediate nature of the inheritance of the trait manifests itself with incomplete dominance.
The suppression of the activity of another non-allelic dominant gene by one dominant gene is called EPISTASE.
Rice. 28 Epistasis
Nonallelic genes are genes located in various areas chromosomes.
MENDEL'S LAWS
6.1 Mendel's first law - Law of uniformity of first generation hybrids.
The law of uniformity of first generation hybrids (Mendel’s first law) - when crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative manifestations of a trait, the entire first generation of hybrids (F1) will be uniform and will carry a manifestation of the trait of one of parents.
This law is also known as the "law of trait dominance." Its formulation is based on the concept clean line relative to the characteristic being studied - on modern language this means that individuals are homozygous for this trait. When crossing pure lines of purple-flowered peas and white-flowered peas, Mendel noticed that the descendants of the plants that emerged were all purple-flowered, with not a single white one among them.
Mendel repeated the experiment more than once and used other signs. If he crossed peas with yellow and green seeds, all the offspring would have yellow seeds. 29.
Rice. 29 Crossing peas
If he crossed peas with smooth and wrinkled seeds, the offspring would have smooth seeds. The offspring from tall and short plants were tall.
So, first-generation hybrids are always uniform in this characteristic and acquire the characteristic of one of the parents. This trait is stronger, dominant (the term was introduced by Mendel from the Latin dominus), always suppressed the other, recessive rice. 30.
Rice. 30 First Law - Law of Uniformity of First Generation Hybrids
6.2 Mendel's second law - The law of splitting.
The law of segregation, or Mendel's second law. When two descendants of the first generation are crossed with each other (two heterozygous individuals), in the second generation F2, splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1. 25% of organisms obtained in the second generation F2 are homozygous dominant (AA), 50% are dominant (Aa) in phenotype and 25% are homozygous recessive (aa).
With incomplete dominance in the offspring of F2 hybrids, the splitting by phenotype and genotype is 1:2:1. The law of segregation (Mendel's second law) - when two heterozygous descendants of the first generation are crossed with each other, in the second generation a segregation is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1.
The crossing of organisms of two pure lines, differing in the manifestations of one studied trait, for which the alleles of one gene are responsible, is called monohybrid crossing.
The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. Recessive trait in hybrids of the first generation it does not disappear, but is only suppressed and appears in the second hybrid generation Fig. 31, 32.
Rice. 31 Law of splitting
Rice. 32 Second Law
HETEROSYGOTE HETEROSYGOTE
(from hetero... and zygote), an organism (cell) in which homologous chromosomes carry different. alleles (alternative forms) of a particular gene. 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. In experiments, G. is obtained by crossing homozygotes for various types with each other. alleles. The descendants of such a cross turn out to be heterozygous for this gene. Analysis of the characteristics of G. in comparison with the original homozygotes allows us to draw a conclusion about the nature of the interaction between the various. alleles of one gene (complete or incomplete dominance, coding, interallelic complementation). Certain alleles are defined. genes can only be in a heterozygous state (recessive lethal mutations, dominant mutations with a recessive lethal effect). Heterozygosity for various lethal factors in various. homologous chromosomes leads to the fact that G.’s offspring are represented by the same G. This phenomenon is so-called. balanced lethality can serve, in particular, as the basis for “consolidating” the effect of heterosis, which has great value in agriculture practice, but is “lost” in a number of generations due to the appearance of homozygotes. The average person has approx. 20% of genes are in a heterozygous state. Determination of heterozygosity for recessive alleles that cause hereditary diseases(i.e. identifying carriers of this disease) is an important medical problem. genetics. The term "G." They are also used for chromosomal rearrangements (they speak of G. by inversion, translocation, etc.). In the case of multiple allelism, the term “compound” is sometimes used for G. (from the English compound - complex, composite). For example, in the presence of a “normal” allele A and mutant a1 and a2, the heterozygote a1/a2 is called. compound, unlike heterozygotes A/a1 or A/a2. (see HOMOZYGOTE).
.(Source: “Biological Encyclopedic Dictionary.” Editor-in-chief M. S. Gilyarov; Editorial Board: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected . - M.: Sov.
heterozygoteA cell or individual in which two genes that determine a trait are different. That is, allelic genes ( alleles) – paternal and maternal – are not the same. For example, in the experiments of G. Mendel to cross pea varieties with different seed colors, homozygous individuals for the dominant yellow color gene were used as parents ( A) and homozygous individuals for the recessive green gene ( A). All first generation hybrids obtained had a hereditary structure Ahh, i.e. were heterozygotes. Their seeds were yellow, like homozygotes for the dominant gene.
Comparison of the characteristics of heterozygous individuals with the characteristics of homozygous parents allows us to study various forms of interaction between alleles of one gene (pattern of dominance, etc.). In general, heterozygosity provides organisms with greater viability and adaptability than homozygosity. Compare Homozygote.
.(Source: “Biology. Modern illustrated encyclopedia.” Chief editor A. P. Gorkin; M.: Rosman, 2006.)
Synonyms:
See what "HETEROSYGOTE" is in other dictionaries:
Heterozygous... Spelling dictionary-reference book
- (from hetero... and zygote), a cell or organism in which homologous (paired) chromosomes carry different forms (alleles) of a particular gene. As a rule, it is a consequence of the sexual process (one of the alleles is introduced by the egg, and the other ... ... Modern encyclopedia
- (from hetero... and zygote) a cell or organism in which homologous chromosomes carry different forms (alleles) of a particular gene. Wed. Homozygote... Big Encyclopedic Dictionary
HETEROSYGOTE, an organism that has two contrasting forms (ALLELES) of a GENE in a pair of CHROMOSOMES. In cases where one of the forms is DOMINANT and the other is only recessive, the dominant form is expressed in the PHENOTYPE. see also HOMOZYGOTE... Scientific and technical encyclopedic dictionary
Genetics- a science that studies genes, mechanisms of inheritance of traits and variability of organisms. During the process of reproduction, a number of traits are passed on to the offspring. It was noticed back in the nineteenth century that living organisms inherit the characteristics of their parents. The first to describe these patterns was G. Mendel.
Heredity- the property of individual individuals to transmit their characteristics to their offspring through reproduction (through sexual and somatic cells). This is how the characteristics of organisms are preserved over a number of generations. When transmitting hereditary information, its exact copying does not occur, but variability is always present.
Variability– the acquisition by individuals of new properties or the loss of old ones. This is an important link in the process of evolution and adaptation of living beings. The fact that there are no identical individuals in the world is due to variability.
Inheritance of characteristics is carried out using elementary units of inheritance - genes. The set of genes determines the genotype of an organism. Each gene carries encoded information and is located in certain place DNA.
Genes have a number of specific properties:
- Different traits are encoded by different genes;
- Constancy - in the absence of a mutating effect, hereditary material transmitted unchanged;
- Lability – the ability to succumb to mutations;
- Specificity - a gene carries special information;
- Pleiotropy – one gene encodes several traits;
Subject to conditions external environment genotype produces different phenotypes. The phenotype determines the degree to which the organism is influenced by environmental conditions.
Allelic genes
The cells of our body have a diploid set of chromosomes; they, in turn, consist of a pair of chromatids, divided into sections (genes). Different shapes identical genes (for example brown/ Blue eyes), located in the same loci of homologous chromosomes, are called allelic genes. In diploid cells, genes are represented by two alleles, one from the father and one from the mother.
Alleles are divided into dominant and recessive. The dominant allele determines which trait will be expressed in the phenotype, and the recessive allele is inherited, but does not manifest itself in a heterozygous organism.
There are alleles with partial dominance, such a condition is called codominance, in which case both traits will appear in the phenotype. For example, flowers with red and white inflorescences were crossed, resulting in red, pink and white flowers in the next generation (pink inflorescences are a manifestation of codominance). All alleles are designated by letters of the Latin alphabet: large - dominant (AA, BB), small - recessive (aa, bb).
Homozygotes and heterozygotes
Homozygote is an organism in which alleles are represented only by dominant or recessive genes.
Homozygosity means the presence of the same alleles on both chromosomes (AA, bb). In homozygous organisms they code for the same traits (e.g. white rose petals), in which case all offspring will receive the same genotype and phenotypic manifestations.
Heterozygote is an organism in which the alleles are both dominant and recessive gene s.
Heterozygosity is the presence of different allelic genes in homologous regions of chromosomes (Aa, Bb). The phenotype of heterozygous organisms will always be the same and is determined by the dominant gene.
For example, A - brown eyes, a – blue eyes, an individual with genotype Aa will have brown eyes.
Heterozygous forms are characterized by splitting, when when crossing two heterozygous organisms in the first generation we get the following result: by phenotype 3:1, by genotype 1:2:1.
An example would be the inheritance of dark and light hair if both parents have dark hair. A is a dominant allele for dark hair, and a is recessive ( blonde hair).
R: Aa x Aa
G: A, a, a, a
F: AA:2Aa:aa
*Where P – parents, G – gametes, F – offspring.
According to this diagram, you can see that the probability of inheriting a dominant trait (dark hair) from parents is three times higher than a recessive one.
Diheterozygote- a heterozygous individual that carries two pairs of alternative characteristics. For example, Mendel's study of the inheritance of traits using pea seeds. The dominant characteristics were yellow and smooth seed surface, and recessive - green and rough surface. As a result of the crossing, nine different genotypes and four phenotypes were obtained.
Hemizygote- this is an organism with one allelic gene, even if it is recessive, it will always manifest itself phenotypically. Normally they are present on sex chromosomes.
Difference between homozygote and heterozygote (table)
| Differences between homozygous and heterozygous organisms | ||
|---|---|---|
| Characteristic | Homozygote | Heterozygote |
| Alleles of homologous chromosomes | Identical | Different |
| Genotype | AA, aa | Aa |
| The phenotype is determined by the trait | By recessive or dominant | By dominant |
| First generation monotony | + | + |
| Split | Doesn't happen | From the second generation |
| Manifestation of a recessive gene | Characteristic | Suppressed |
Reproduction and crossing of homozygotes and heterozygotes leads to the formation of new characteristics that are necessary for living organisms to adapt to changing environmental conditions. Their properties are necessary when breeding crops and breeds with high quality indicators.
In genetics, like any other science, there is specific terminology designed to clarify key concepts. Back in school, many of us heard terms such as dominance, recessiveness, gene, allele, homozygosity and heterozygosity, but did not fully understand what was hidden behind them. Let us examine in more detail what a homozygote is, how it differs from a heterozygote, and what role allelic genes play in its formation.
A bit of general genetics
To answer the question of what a homozygote is, let us recall the experiments of Gregor Mendel. By crossing pea plants of different color and shape, he came to the conclusion that the resulting plant somehow inherited genetic information from its “ancestors.” Although the concept of "gene" did not yet exist, Mendel was able to general outline explain the mechanism of inheritance of traits. From discovered by Mendel in the middle of the 19th century, laws resulted in the following statement, later called the “hypothesis of gamete purity”: “When a gamete is formed, only one of the two allelic genes responsible for this sign"That is, from each of the parents we receive only one allelic gene responsible for a certain trait - height, hair color, eye color, nose shape, skin tone.
Allelic genes can be dominant or recessive. This brings us very close to defining what a homozygote is. Dominant alleles are able to mask a recessive so that it does not manifest itself in the phenotype. If both genes in a genotype are recessive or dominant, then it is a homozygous organism.
Types of homozygotes
From all of the above, we can answer the question of what a homozygote is: this is a cell in which the allelic genes responsible for a certain trait are the same. Allelic genes are located on homologous chromosomes and, in the case of a homozygote, can be either recessive (aa) or dominant (AA). If one allele is dominant and the other is not, then it is a heterozygote (Aa). In the case when the genotype of the cell is aa, then it is a recessive homozygote, if AA is dominant, since it carries alleles responsible for the dominant trait.
Features of crossing
When crossing two identical (recessive or dominant) homozygotes, a homozygote is also formed.
For example, there are two white rhododendron flowers with bb genotypes. After crossing them we also get white flower with the same genotype.
You can also give an example with eye color. If both parents have brown eyes and are homozygous for this trait, then their genotype is AA. Then all children will have brown eyes.
However, crossing homozygotes does not always lead to the formation of an organism homozygous for any trait. For example, crossing red (DD) and white (dd) carnations can result in a pink or red and white flower. The pink carnation, like the two-color carnation, is an example of incomplete dominance. In both cases, the resulting plants will be heterozygous with the Dd genotype.
Examples of homozygotes
There are quite a lot of examples of homozygotes in nature. White tulips, carnations, rhododendrons are all examples of recessive homozygotes.
In humans, as a result of the interaction of allelic genes, organisms that are homozygous for some trait are also often formed, be it very light skin, blue eyes, blond hair or color blindness.
Dominant homozygotes are also common, but due to the ability of dominant traits to mask recessive ones, it is impossible to immediately say whether a person is a carrier of a recessive allele or not. Most of the genes responsible for genetic diseases, caused gene mutations and are recessive, therefore they appear only if there is no normal, dominant allele on the homologous chromosomes.
One of the most significant properties of any living organism is heredity, which underlies evolutionary processes on the planet, as well as the preservation of species diversity on it. The smallest unit of heredity is the gene - structural element responsible for the transmission of hereditary information associated with a particular trait of the organism. Depending on the degree of manifestation, dominant and Characteristic feature dominant units is the ability to “suppress” recessive ones, having a decisive effect on the body, not allowing them to manifest themselves in the first generation. However, it is worth noting that along with incomplete, in which it is not able to completely suppress the manifestation of recessive and overdominance, which involves the manifestation of the corresponding characteristics in a form stronger than in homozygous organisms. Depending on which allelic (that is, responsible for the development of the same trait) genes it receives from the parental individuals, heterozygous and homozygous organisms are distinguished.
Determination of a homozygous organism
Homozygous organisms are objects of living nature that have two identical (dominant or recessive) genes for one or another trait. Distinctive feature subsequent generations of homozygous individuals is their lack of splitting of characters and their uniformity. This is explained mainly by the fact that the genotype of a homozygous organism contains only one type of gametes, designated if we're talking about o and lowercase when mentioning recessives. Heterozygous organisms differ in that they contain different allelic genes, and, in accordance with this, form two different types gametes. Homozygous organisms that are recessive for major alleles can be designated as aa, bb, aabb, etc. Accordingly, homozygous organisms with dominant alleles have the code AA, BB, AABB.
Patterns of inheritance
Crossing two heterozygous organisms, the genotypes of which can be conventionally designated as Aa (where A is a dominant and a is a recessive gene), provides the opportunity to obtain, with equal probability, four different combinations of gametes (genotype variant) with a 3:1 split in phenotype. Under the genotype in in this case refers to the set of genes that the diploid set of a particular cell contains; under the phenotype - a system of external, as well as internal signs the organism in question.
and its features
Let us consider the patterns associated with crossing processes in which homozygous organisms take part. In the same case, if a dihybrid or polyhybrid crossing takes place, regardless of the nature of the inherited traits, splitting occurs in a ratio of 3:1, and this law is valid for any number of them. Crossing of second generation individuals in this case forms four main types of phenotypes with a ratio of 9:3:3:1. It is worth noting that this law is valid for homologous pairs of chromosomes, the interaction of genes within which does not occur.