Genetics- a science that studies genes, the mechanisms of inheritance of traits and the variability of organisms. During reproduction, a number of traits are passed on to offspring. It was noticed as early as 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 offspring through reproduction (through sex and somatic cells). Thus, the characteristics of organisms are preserved in a number of generations. During the transfer of 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 the merit of variability.
Traits are inherited using elementary units inheritance - genes. The totality 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;
- Persistence - in the absence of a mutating effect, the hereditary material is transmitted unchanged;
- Lability - the ability to succumb to mutations;
- Specificity - a gene carries specific information;
- Pleiotropy - one gene encodes several traits;
Subject to conditions external environment genotype produces different phenotypes. The phenotype determines the degree of influence on the body of 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 forms of the same 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, the other from the mother.
Alleles are divided into dominant and recessive. The dominant allele will determine which trait will be expressed in the phenotype, and the recessive allele is inherited, but does not appear in a heterozygous organism.
Exist alleles with partial dominance, such a condition is called codominance, in which case both traits will appear in the phenotype. For example, they crossed flowers with red and white inflorescences, as a result, in the next generation they received red, pink and white flowers (pink inflorescences are a manifestation of codominance). All alleles are labeled with letters Latin alphabet: large - dominant (AA, BB), small - recessive (aa, bb).
Homozygotes and heterozygotes
Homozygote An organism in which alleles are represented only by dominant or recessive genes.
Homozygosity means having the same alleles on both chromosomes (AA, bb). In homozygous organisms, they code for the same traits (for example, White color rose petals), in which case all offspring will receive the same genotype and phenotypic manifestations.
heterozygote is an organism in which alleles are both dominant and recessive gene s.
Heterozygosity - the presence of different allelic genes in homologous regions of chromosomes (Aa, Bb). The phenotype in 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 the Aa genotype will have brown eyes.
For heterozygous forms, splitting is characteristic, when when crossing two heterozygous organisms in the first generation, we get the following result: according to the phenotype 3:1, according to the genotype 1:2:1.
An example would be the inheritance of dark and light hair if both parents have dark hair. A - dominant allele on the basis of dark hair, and - recessive (light hair).
R: Aa x Aa
G: A, a, a, a
F: AA:2Aa:aa
*Where P - parents, G - gametes, F - offspring.
According to this scheme, you can see that the probability of inheriting a dominant trait (dark hair) from parents is three times higher than a recessive trait.
Diheterozygote- a heterozygous individual that carries two pairs of alternative traits. For example, Mendel's study of the inheritance of traits using pea seeds. The dominant traits were yellow color and smooth seed surface, while the recessive traits were green color and rough surface. As a result of 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 appear phenotypically. Normally, they are present on the sex chromosomes.
The difference between homozygotes and heterozygotes (table)
| Differences between homozygous and heterozygous organisms | ||
|---|---|---|
| Characteristic | Homozygote | heterozygote |
| Alleles of homologous chromosomes | The same | Various |
| Genotype | AA, aa | aa |
| The phenotype is determined by the trait | recessive or dominant | Dominant |
| Monotony of the first generation | + | + |
| Split | Not happening | From the second generation |
| Manifestation of a recessive gene | Characteristically | Suppressed |
Reproduction, crossing of homozygotes and heterozygotes leads to the formation of new traits that are necessary for living organisms to adapt to changing environmental conditions. Their properties are necessary when breeding crops, breeds with high quality indicators.
The genotype includes a large number of different genes, which in turn act as a whole. The founder of genetics, Mendel, described in his writings that he discovered only one possibility for the interaction of allelic genes - when there is absolute dominance (predominance) of one of the alleles, while the second remains completely recessive (passive, i.e. does not participate in interaction).
Allelic genes and the main types of their interaction
Each gene has two states - A and a, so they make up one pair, and each of the members of the pair is called an allele. Thus, genes located in the same loci (regions) of homologous chromosomes and determining alternative development of the same trait are called alleles.
In the very simple case the gene has two alleles. For example, the purple and white color of a pea flower is dominant and recessive, respectively, for two alleles of the same gene.
allelic genes
Most people on the globe know what genes pass on hereditary traits parents to their offspring, and this applies not only to humans, but to all living beings on the planet. These microscopic structural units are a segment of DNA that determines the sequence of polypeptides (chains of more than 20 amino acids that make up DNA). The nature and methods of interaction of genes are quite complex, and the slightest deviations from the norm can lead to genetic diseases.
Biology and medicine
Allele (allelomorph, allele): one of two (or more) forms of a particular gene that differ in sequence. Alternative forms sequences localized at the same loci (same regions) on homologous chromosomes. Humans have two sets of chromosomes. one from each parent. The equivalent sequences in these two sets may be different, for example, due to single nucleotide polymorphisms.
Therefore, the alleles are alternative options gene at each locus.
What is an allele
alleles (from Greek word allelon - mutually), or allelomorphs, - different forms the same gene (in singular- allele).
In the simplest case, a gene is represented by two alleles (for example, alleles that determine green and yellow colors peas in the experiments of G. Mendel). An example of a three-allelic gene is gene. which determines the AB0 blood group system in humans (read "A-B-zero"). At different combinations These alleles form the first blood group (00), the second (A0, AA), the third (B0, BB) and the fourth (AB).
What are allelic genes
An allele is one of the forms of a gene that determines one of the many options for the development of a particular trait. Usually, alleles are divided into dominant and recessive - the first fully corresponds to a healthy gene, while the recessive one includes various mutations of its gene, leading to a “malfunction” in its work. There is also multiple allelism, in which geneticists distinguish more than two alleles.
An organism with the same allelic genes is considered homozygous, and an organism with different alleles is considered heterozygous.
The genotype includes a large number of diverse genes, which in turn act as a whole. Mendel, in his writings, described that he discovered only one possibility for the interaction of allelic genes - when there is absolute dominance (predominance) of one of the alleles, while the second remains completely recessive (passive, that is, does not participate in the interaction). But let's say right away that the phenotypic manifestation of genes (external, noticeable to the eye) cannot depend only on one or a pair of genes, because it is a consequence of the interaction whole system.
In reality, proteins and enzymes interact, not genes.
Only 2 types are distinguished - the first is the interaction of allelic genes, the second, respectively, non-allelic. It is only necessary to understand the material side of this issue, because not some concepts from a textbook interact, but proteins that are synthesized according to a certain program in the cytoplasm of cells, and the number of these proteins is in the millions. The program itself, according to which proteins will be synthesized, and, as a result, their further interaction will develop, is embedded in the genes that give external commands while being in the cell chromosomes (ultrascopic cell organelles).
What genes are called allelic?
allelic genes- these are genes that occupy the same "places" (or loci) in the chromosomes. Every living organism has allelic genes in pairs. The interaction of allelic genes can occur in several ways, which are called: codominance, overdominance, complete and incomplete dominance.
Allelic genes interact according to the principle if the action of the dominant gene completely overlaps the action of the recessive one. Incomplete dominance can be called a relationship in which it is not completely suppressed and takes, albeit minimally, participation in the formation of phenotypic traits.
Codominance occurs when allelic genes exhibit their properties independently of each other. Probably the most good example codominance is the AB0 blood system, in which both A and B genes function independently of each other.
Overdominance is an increase in quality phenotypic manifestations dominant gene in the event that it is "in conjunction" with a recessive one. That is, if there are 2 in one allele, then they manifest themselves worse than dominant gene, which is "in conjunction" with the recessive.
Multiple allelism
As mentioned earlier, each living creature can only have 2 allelic genes, but there can be much more than two alleles - this phenomenon is called multiple allelism. Let's say right away that only one pair of alleles can show phenotypic features, that is, while some are working, others are resting.
Almost always, homologous (identical) alleles are responsible for the development and manifestation of the same trait, but differ in the quality of its manifestation. Also, multiple allelism is inherent various forms gene interactions. That is, although they are responsible for the same sign, but, firstly, they manifest it in different ways, and secondly, with the help of various methods(complete, incomplete dominance, and so on).
It would seem, why such confusion? It's simple - only one pair of homologous alleles can get into the reproductive cell of a living being, but which of all the available ones is decided by chance. It is thanks to this that the variability of species is achieved, which plays a major role in the evolution of living beings.
An allele is one of the forms of a gene that determines one of the many options for the development of a particular trait. Usually, alleles are divided into dominant and - the first fully corresponds to a healthy gene, then it includes various mutations of its gene, leading to a “malfunction” in its work. There is also multiple allelism, in which geneticists distinguish more than two alleles.
With multiple allelism, diploid organisms have two alleles inherited from parents in different combinations.
An organism with the same allelic genes is considered homozygous, and an organism with different alleles is considered heterozygous. A heterozygote is distinguished by the manifestation of a dominant trait in the phenotype and concealment. With complete dominance, the heterozygous organism has a dominant phenotype, while with incomplete dominance it is intermediate between the recessive and dominant allele. Thanks to a pair of homologous alleles that enter the germ cell of the organism, the species of living beings are changeable and capable of evolution.
Interaction of allelic genes
There is only one possibility of interaction of these genes - with the absolute dominance of one allele over the second, remaining in a recessive state. The basics of genetics include no more than two types of interaction of allelic genes - allelic and non-allelic. Since the allelic genes of each living organism are always present in a pair, their interaction can occur in the form of co-dominance, over-dominance, as well as complete and incomplete dominance.
Only one pair of allelic genes is capable of manifesting phenotypic traits - while some are resting, others are working.
The interaction of alleles with complete dominance occurs only when the dominant gene completely overlaps the recessive one. Interaction with incomplete dominance is carried out with incomplete suppression of the recessive gene, which is partially involved in the formation of traits.
Codominance occurs with a separate manifestation of the properties of allelic genes, while overdominance is an increase in the quality of the phenotypic traits of a dominant gene that is in conjunction with a recessive gene. Thus, two dominant genes that are in the same allele will manifest themselves worse than a dominant gene supplemented by a recessive one.
Most people around the world know that genes transmit the hereditary traits of parents to their offspring, and this applies not only to humans, but to all living beings on the planet. These microscopic structural units are a kind of DNA segment that determines the sequence of polypeptides (chains of more than 20 amino acids that make up DNA). The nature and methods of interaction of genes are quite complex, and the slightest deviations from the norm can lead to genetic diseases. Let's try to understand the essence of genes and the principles of their behavior.
The concept of "allelicity", according to Greek terminology, implies reciprocity. It was introduced by the Danish scientist Wilhelm Johansen at the beginning of the 20th century. The term "gene", as well as "genotype" and "phenotype" was coined by the same Johansen. In addition, he opened important law heredity "pure line".
Based on numerous experiments with plant material, it was found that the same genes within the locus (the same section of the chromosome) can take on various forms that have direct influence on the variety of variations of any parental trait. Such genes were called alleles, or alleles. In creatures whose organism is diploid, that is, it has paired sets of chromosomes, allelic genes can be present both in two identical and in two different ones. In the first case, they speak of a homozygous type, in which the inherited traits are identical. In the second case, the type is heterozygous. Its hereditary traits vary because the copies of the genes on the chromosomes differ from one another.
The dominant principle of heredity
The human body is diploid. The cells of our body (somatic) include two allelic genes.
Only gametes (sex cells) contain a single allele that determines the sex characteristic. When the male and female gametes merge, a zygote is obtained, in which there is a double set of chromosomes, that is, 46, including 23 maternal and 23 paternal. Of these, 22 pairs are homologous (identical) and 1 is sexual. If she received the XX chromosome set, a female develops, and if XY, then a male. In each chromosome, as noted above, there are 2 alleles. For convenience, they were divided into two types - dominant and recessive. The former are much stronger than the latter. Imprisoned in them hereditary information turns out to be dominant. What traits the nascent individual inherits from its parents depends on whose allelic genes (father or mother) were dominant. This is the simplest way for alleles to interact.
Other types of inheritance
Each of the parents can be a carrier of homozygous and heterozygous genes for dominant or recessive traits. A child who has received dominant and recessive allelic genes from homozygous parents will inherit only dominant traits.
In other words, if the dominant pair is dark color hair, and recessive - light, all children will be born only dark-haired. In the case when one of the parents has a dominant gene of a heterozygous type, and the other has a homozygous one, their children will be born with a dominant and recessive trait of about 50 X 50. In our example, a couple can have both dark-haired children and blondes. If both parents have heterozygous dominant and recessive genes, every fourth child will inherit recessive traits, that is, they will be fair-haired. This rule of inheritance is very important, since there are many diseases that are transmitted through genes, and one of the parents can be a carrier. These pathologies include dwarfism, hemochromatosis, hemophilia and others.
How are alleles designated?
In genetics, alleles are usually denoted by the first letters of the name of the gene of which they are forms. The dominant allele is spelled with capital letter. Near indicate serial number modified genetic form. The word "allele" in Russian can be used both in the feminine and in the masculine.
Types of allele interaction
The interaction of allelic genes can be divided into several types:
What is allelic exclusion
It happens that in homogametic individuals containing germ cells with the same set of chromosomes, one of them becomes little or completely inactive. With regard to humans, this condition is observed in women, while, say, in butterflies, on the contrary, in males. With allelic exclusion, only one of the two chromosomes is expressed, and the second becomes the so-called Barr body, that is, an inactive unit twisted into a spiral. Such a structure is called a mosaic. In medicine, this can be seen in B-lymphocytes, which can synthesize antibodies only to certain antigens. Each such lymphocyte chooses between the activity of either the paternal or maternal allele.
Multiple allelism
In nature, the phenomenon is widespread when the same gene has not two, but more forms. In plants, this is manifested by a variety of stripes on leaves and petals, in animals - by various combinations of colors. In people a prime example multiple allelism is the inheritance of a child's blood type. Its system is designated ABO and is controlled by a single gene. Its locus is designated I, and allelic genes - IA, IB, IO. Combinations IO IO give the first blood group, IA IO and IA IA - the second, IB IO and IB IB - the third, and IA IB - the fourth. In addition, Rhesus is determined in people. Positive give a combination of 2 allelic genes with the trait "+" or 1+ and 1-. Negative Rh give two allelic genes with the trait "-". The Rh system is controlled by the CDE genes, and the D gene often causes an Rh conflict between the fetus and the mother, if her blood is Rh-negative, and the fetus is Rh-positive. In such cases, in order to successfully complete the second and subsequent pregnancies, the woman is given special therapy.
Lethal allelic genes
Alleles whose carriers die due to genetic diseases caused by these genes are called lethal. They cause Huntington's disease in humans. In addition to lethal, there are also so-called semi-lethal. They can cause death, but only under certain conditions, such as when high temperatures environment. If these factors can be avoided, semi-lethal genes do not cause the death of an individual.
