Mendel's 3rd law is stated briefly. Mendel's second law. What types of crossing did G. Mendel study?

Mendel's laws

Diagram of Mendel's first and second laws. 1) A plant with white flowers (two copies of the recessive allele w) is crossed with a plant with red flowers (two copies of the dominant allele R). 2) All descendant plants have red flowers and the same genotype Rw. 3) When self-fertilization occurs, 3/4 of the plants of the second generation have red flowers (genotypes RR + 2Rw) and 1/4 have white flowers (ww).

Mendel's laws- these are the principles of transmission of hereditary characteristics from parent organisms to their descendants, resulting from the experiments of Gregor Mendel. These principles formed the basis for classical genetics and were subsequently explained as a consequence of the molecular mechanisms of heredity. Although three laws are usually described in Russian-language textbooks, the “first law” was not discovered by Mendel. Of particular importance among the patterns discovered by Mendel is the “hypothesis of gamete purity.”

Story

At the beginning of the 19th century, J. Goss, experimenting with peas, showed that when crossing plants with greenish-blue peas and yellowish-white peas in the first generation, yellow-white ones were obtained. However, during the second generation, the traits that did not appear in the first generation hybrids and later called recessive by Mendel appeared again, and plants with them did not split during self-pollination.

O. Sarge, conducting experiments on melons, compared them according to individual characteristics (pulp, peel, etc.) and also established the absence of confusion of characteristics that did not disappear in the descendants, but were only redistributed among them. C. Nodin, crossing various types of datura, discovered the predominance of the characteristics of datura Datula tatula above Datura stramonium, and this did not depend on which plant was the mother and which was the father.

Thus, by the middle of the 19th century, the phenomenon of dominance was discovered, the uniformity of hybrids in the first generation (all hybrids of the first generation are similar to each other), splitting and combinatorics of characters in the second generation. However, Mendel, highly appreciating the work of his predecessors, pointed out that they had not found a universal law for the formation and development of hybrids, and their experiments did not have sufficient reliability to determine numerical ratios. The discovery of such a reliable method and mathematical analysis of the results, which helped create the theory of heredity, is the main merit of Mendel.

Mendel's methods and progress of work

• Mendel studied how individual traits are inherited.
• Mendel chose from all the characteristics only alternative ones - those that had two clearly different options in his varieties (the seeds are either smooth or wrinkled; there are no intermediate options). Such a conscious narrowing of the research problem made it possible to clearly establish the general patterns of inheritance.
• Mendel planned and carried out a large-scale experiment. He received 34 varieties of peas from seed-growing companies, from which he selected 22 “pure” varieties (which do not produce segregation according to the studied characteristics during self-pollination). Then he carried out artificial hybridization of varieties, and crossed the resulting hybrids with each other. He studied the inheritance of seven traits, studying a total of about 20,000 second-generation hybrids. The experiment was facilitated by a successful choice of object: peas are normally self-pollinating, but artificial hybridization is easy to carry out.
• Mendel was one of the first in biology to use precise quantitative methods to analyze data. Based on his knowledge of probability theory, he realized the need to analyze a large number of crosses to eliminate the role of random deviations.

Mendel called the manifestation of the trait of only one of the parents in hybrids as dominance.

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 the trait, the entire first generation of hybrids (F1) will be uniform and will carry the manifestation of the trait of one of the parents.

This law is also known as the "law of trait dominance." Its formulation is based on the concept clean line regarding the trait under study - in modern language this means homozygosity of individuals for this trait. Mendel formulated the purity of a character as the absence of manifestations of opposite characters in all descendants in several generations of a given individual during self-pollination.

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. 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 sign (stronger, dominant), always suppressed the other ( recessive).

Codominance and incomplete dominance

Some opposing characters are not in the relation of complete dominance (when one always suppresses the other in heterozygous individuals), but in the relation incomplete dominance. For example, when pure snapdragon lines with purple and white flowers are crossed, the first generation individuals have pink flowers. When pure lines of black and white Andalusian chickens are crossed, gray chickens are born in the first generation. With incomplete dominance, heterozygotes have characteristics intermediate between those of recessive and dominant homozygotes.

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. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation.

Explanation

Law of gamete purity: each gamete contains only one allele from a pair of alleles of a given gene of the parent individual.

Normally, the gamete is always pure from the second gene of the allelic pair. This fact, which could not be firmly established in Mendel's time, is also called the gamete purity hypothesis. This hypothesis was later confirmed by cytological observations. Of all the laws of inheritance established by Mendel, this “Law” is the most general in nature (it is fulfilled under the widest range of conditions).

Law of independent inheritance of characteristics

Illustration of independent inheritance of traits

Definition

Law of independent inheritance(Mendel’s third law) - when crossing two homozygous individuals that differ from each other in two (or more) pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations (as in monohybrid crossing). When plants differing in several characters, such as white and purple flowers and yellow or green peas, were crossed, the inheritance of each character followed the first two laws and in the offspring they were combined in such a way as if their inheritance occurred independently of each other. The first generation after crossing had a dominant phenotype for all traits. In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1, that is, 9:16 were with purple flowers and yellow peas, 3:16 were with white flowers and yellow peas, 3:16 were with purple flowers and green peas, 1 :16 with white flowers and green peas.

Explanation

Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes. During meiosis, homologous chromosomes of different pairs are randomly combined in gametes. If the paternal chromosome of the first pair gets into the gamete, then with equal probability both the paternal and maternal chromosomes of the second pair can get into this gamete. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. (It later turned out that of the seven pairs of characters studied by Mendel in the pea, which has a diploid number of chromosomes 2n=14, the genes responsible for one of the pairs of characters were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since as linkage between these genes was not observed due to the large distance between them).

Basic provisions of Mendel's theory of heredity

In modern interpretation, these provisions are as follows:

• Discrete (separate, non-mixable) hereditary factors - genes are responsible for hereditary traits (the term “gene” was proposed in 1909 by V. Johannsen)
• Each diploid organism contains a pair of alleles of a given gene responsible for a given trait; one of them is received from the father, the other from the mother.
• Hereditary factors are transmitted to descendants through germ cells. When gametes are formed, each of them contains only one allele from each pair (the gametes are “pure” in the sense that they do not contain the second allele).

Conditions for the fulfillment of Mendel's laws

According to Mendel's laws, only monogenic traits are inherited. If more than one gene is responsible for a phenotypic trait (and the absolute majority of such traits), it has a more complex pattern of inheritance.

Conditions for fulfilling the law of segregation during monohybrid crossing

Splitting 3:1 by phenotype and 1:2:1 by genotype is performed approximately and only under the following conditions.

Gregor Mendel is the founder of genetics! Brief History of Life.

July 22, 1822 – in a small village on the territory of the modern Czech Republic, the scientist G. Mendel was born, who was named Johann at baptism.

In 1843 Mendel was accepted into the Augustinian monastery of St. Thomas and chose the order name Gregorius.

In 1854 Mendel was given a plot of land (35x7 m), on which he sowed peas for the first time in the spring.

In 1865 Mendel outlined the results of his experiments in his work “Experiments on Plant Hybrids” and reported on it at a meeting of the Brunn Society for Natural Sciences.

Spring 1868 of the yearMendel was elected the new abbot of the Augustinian monastery of St. Thomas.

In January 1884 of the yearDue to severe heart and kidney disease, the founder of genetics, Johann Gregor Mendel, died.

Peas - as an object of genetics.

Mendel conducted his first experiments on a plant such as the pea. Why did he choose this particular object? Below are signs by which we can assume that the selected object was successful:

- Convenience in cultivating peas;

- Self-pollination;

- Clearly expressed signs;

- Large flowers that tolerate fire well and are protected from foreign pollen;

- Fertile hybrids.

Mendel identified 7 pairs of alternative characters:

• Seed shape,

Seed skin color

bean shape,

• Coloring of an unripe bean,

• Flower location,

• Stem length.

Mendel's hybridological method. Mendel's laws for monohybrid crossing.

Hybridological method is a system of crossings that allows one to trace patterns of inheritance and changes in characteristics over a number of generations.

Prerequisites for creating the method.

Monohybrid cross - This is the crossing of individuals that differ in one pair of contrasting alternative characters.

IMendel's law (law of uniformity of first generation hybrids, law of dominance):

When crossing two parental individuals belonging to different pure lines (HML) and differing in one pair of contrasting alternative traits, all first-generation hybrids will be uniform in both genotype and phenotype.

Consequences:

1. Domination- this is the phenomenon of predominance of the characteristics of one of the parents in first-generation hybrids. The trait that appears in the first generation hybrids is called dominant, and the suppressed trait is called recessive.

2. If, when crossing two parental individuals with opposite characteristics in the phenotype, all hybrids in their offspring are identical or uniform, then the original parental individuals were GMZ.

3. Gamete purity hypothesis:

Gametes are pure because they carry only 1 gene (hereditary factor) from the pair. Hybrids receive both hereditary factors - one from the mother, the other from the father.

IIMendel's law (law of character splitting):

The recessive trait does not disappear without a trace, but is suppressed in the first generation hybrids and appears in the second generation hybrids in a ratio of 3:1.

Consequences:

1. Feature splitting- this is the phenomenon of the appearance of different pheno- and genotypic classes in the offspring.

2. If, when crossing two parental individuals with the same characteristics in the phenotype, a split in the offspring occurred in a ratio of 3:1, then the original individuals were GTZ.

Cytological mechanism:

1. Somatic cells are diploid and contain paired allelic genes responsible for the development of each pair of contrasting characters.

2. as a result of meiosis, 1 gene from each pair enters the gametes, because gametes are haploid.

3. during fertilization, gametes merge and the diploid set of chromosomes is restored (gene pairing is restored)

Analyzing crossing.

This is a crossing carried out with the aim of establishing the genotype of the individual under study with dominant traits in the phenotype.

To do this, the studied individual is crossed with a recessive GMZ and the genotype of the studied individual is judged from the offspring:

INTERACTION OF ALLELIC GENES:

Complete dominance

Incomplete dominance

Overdominance

Co-dominance,

Multiple allelism.

Gene interaction– a phenomenon when several genes (alleles) are responsible for the development of a trait.

• If genes of one allelic pair interact, such interaction is called allelic, and if genes of different allelic pairs interact, it is called non-allelic.

• COMPLETE DOMINANCE – an interaction in which one gene completely suppresses (excludes) the effect of another trait.

Mechanism:

1. The dominant allele in the GTZ state ensures the synthesis of products sufficient for the manifestation of a trait of the same quality and intensity as in the dominant GMZ state in the parent form.

2. The recessive allele is either completely inactive, or the products of its activity do not interact with the products of the activity of the dominant allele.

• INCOMPLETE DOMINANCE - intermediate nature of inheritance. This is a type of interaction of allelic genes in which the dominant gene does not completely suppress the action of the recessive gene, as a result of which first generation hybrids (GTH) have a phenotypic variant intermediate between the parental forms.

Moreover, in the second generation, the splitting by genotype and phenotype coincides and is equal to 1:2:1.

Mechanism:

1. The recessive allele is not active.

2. The degree of activity of the dominant allele is sufficient to ensure the level of manifestation of the trait, as in the dominant GMZ.

• CODOMINATION - this is a phenomenon in which both genes find their manifestation in the phenotype of the offspring, while neither of them suppresses the action of the other gene. Codominant genes are equivalent. (For example, the roan coloring of cattle is formed with the simultaneous presence of red and white genes in the genotype; blood type in humans). When codominance is 1:2:1.

• OVERDOMINANCE – this is a type of interaction of allelic genes when the dominant gene in the HTG state demonstrates a more pronounced manifestation of the trait than the same gene in the GMZ state.

• MULTIPLE ALLELISM - this is an intra-allelic interaction of genes, in which not one allele, but several, is responsible for the development of one trait, and in addition to the main dominant and recessive alleles, intermediate ones appear, which are related to the other. behave as recessive, and in relation to recessive ones, as dominant.

(for example, in Siamese cats, in rabbits: C - wild type, C/ - Siamese, C// - albino; ​​blood groups in humans)

Multiple alleles are those that are represented in a population by more than two allelic states that arise as a result of multiple mutations of the same chromosomal locus.

Mendel's laws for dihybrid crossing.

Dihybrid crossing is the crossing of individuals that differ in two pairs of contrasting alternative traits.

Combinative variability is the emergence of new combinations of genes and traits as a result of crossing. Causes:

Conjugation and crossing over, random divergence of chromosomes and chromatids during anaphase of meiosis, random fusion of gametes during fertilization.

III Mendel's law (law of free independent combination of characteristics):

Individual pairs of traits during dihybrid crossing behave independently, freely combining with each other in all possible combinations.

INTERACTION OF NON-ALLELIC GENES:

Non-allelic interaction is the interaction of genes of different allelic pairs.

COMPLEMENTARY - this is a type of interaction of non-allelic genes in which they complement each other and, when found together in the genotype (A-B-), determine the development of a qualitatively new trait compared to the action of each gene separately (A-bb, aaB-).

Complementary genes are genes that complement each other.

EPISTASEis a type of interaction of non-allelic genes in which one non-allelic gene suppresses the action of another non-allelic gene.

The gene that is suppressed is called an epistatic gene, a suppressor gene, or an inhibitor.

The gene that is suppressed is called hypostatic.

POLYMERIA –this is the conditioning of the development of a certain, usually quantitative trait, by several equivalent polymer genes.

For example, a person’s skin color, height, body weight, blood pressure.

Dominant genes that equally influence the development of one trait are called genes with unambiguous actions (A1, A2, A3..), and traits are called polymeric.

The threshold effect is the minimum number of polymer genes at which a trait appears.

LINKED INHERITANCE OF GENES.

A linkage group is a set of genes localized on one chromosome and, as a rule, inherited together.

Complete linkage is a phenomenon in which the linkage group is not broken by crossing over and genes localized on the same chromosome are transmitted together.

The offspring exhibit only parental characteristics.

Incomplete linkage is a phenomenon in which a linkage group is disrupted by crossing over. Genes located on the same chromosome will not always be transmitted together. And new combinations of traits appear in the offspring, along with the known parental ones.

Introduction.

Genetics is a science that studies the patterns of heredity and variability of living organisms.

Man has long noted three phenomena related to heredity: first, the similarity of the characteristics of descendants and parents; secondly, the differences between some (sometimes many) characteristics of the descendants from the corresponding parental characteristics; thirdly, the appearance in the offspring of characteristics that were present only in distant ancestors. The continuity of characteristics between generations is ensured by the process of fertilization. Since time immemorial, man has spontaneously used the properties of heredity for practical purposes - to breed varieties of cultivated plants and breeds of domestic animals.

The first ideas about the mechanism of heredity were expressed by the ancient Greek scientists Democritus, Hippocrates, Plato, and Aristotle. The author of the first scientific theory of evolution, J.-B. Lamarck used the ideas of ancient Greek scientists to explain what he postulated at the turn of the 18th-19th centuries. the principle of transmitting new characteristics acquired during the life of an individual to offspring. Charles Darwin put forward the theory of pangenesis, which explained the inheritance of acquired characteristics

Charles Darwin defined heredity as the property of all living organisms to transmit their characteristics and properties from generation to generation, and variability as the property of all living organisms to acquire new characteristics in the process of individual development.

Inheritance of traits occurs through reproduction. In sexual reproduction, new generations arise as a result of fertilization. The material foundations of heredity are contained in the germ cells. With asexual or vegetative reproduction, a new generation develops either from unicellular spores or from multicellular formations. And with these forms of reproduction, the connection between generations is carried out through cells that contain the material foundations of heredity (elementary units of heredity) - genes - which are sections of DNA chromosomes.

The set of genes that an organism receives from its parents constitutes its genotype. The combination of external and internal characteristics is a phenotype. The phenotype develops as a result of the interaction of the genotype and environmental conditions. One way or another, the basis remains the characteristics that genes carry.

The patterns by which traits are passed on from generation to generation were first discovered by the great Czech scientist Gregor Mendel. He discovered and formulated three laws of inheritance, which formed the basis of modern genetics.

The Life and Scientific Research of Gregor Johann Mendel.

Moravian monk and plant geneticist. Johann Mendel was born in 1822 in the town of Heinzendorf (now Gincice in the Czech Republic), where his father owned a small peasant plot. Gregor Mendel, according to those who knew him, was truly a kind and pleasant person. After receiving primary education at the local village school and later, after graduating from the Piarist College in Leipnik, in 1834 he was accepted into the first grammar class at the Troppaun Imperial-Royal Gymnasium. Four years later, Johann's parents, as a result of a confluence of many unfortunate events that quickly followed each other, were completely deprived of the opportunity to reimburse the necessary expenses associated with his studies, and their son, being then only 16 years old, was forced to take care of his own maintenance completely independently. . In 1843, Mendel was admitted to the Augustinian monastery of St. Thomas in Altbrunn, where he took the name Gregor. In 1846, Mendel also attended lectures on housekeeping, gardening and viticulture at the Philosophical Institute in Brünn. In 1848, having completed his theological course, with deep respect, Mendel received permission to prepare for examinations for the degree of Doctor of Philosophy. When the following year he strengthened his intention to take the exam, he was given an order to take the place of supporter of the Imperial-Royal Gymnasium in Znaim, which he followed with joy.

In 1851, the abbot of the monastery sent Mendel to study at the University of Vienna, where he studied, among other things, botany. After graduating from university, Mendel taught natural sciences at a local school. Thanks to this step, his financial situation changed radically. In the beneficial well-being of physical existence, so necessary for every occupation, courage and strength returned to him, with deep reverence, and for a trial year he studied the prescribed classical subjects with great diligence and love. In his free hours, he studied the small botanical and mineralogical collection that was placed at his disposal in the monastery. His passion for the field of natural science became greater, the greater the opportunities he received to devote himself to it. Although the one mentioned in these studies was deprived of any guidance, and the path of the autodidact here, like in no other science, is difficult and leads to the goal slowly, nevertheless, during this time Mendel acquired such a love for the study of nature that he no longer spared effort to fill the gaps that have changed in him through self-education and following the advice of people with practical experience. On April 3, 1851, the “teacher corps” of the school decided to invite the canon of the Monastery of St. Thomas, Mr. Gregor Mendel, to temporarily fill the professorial position. Gregor Mendel's pomological successes gave him the right to a star title and a temporary position as a natural history supplanter in the preparatory class of the Technical School. In the first semester of his studies, he studied only ten hours a week and only with Doppler. In the second semester, he studied twenty hours a week. Of these, ten are in physics with Doppler, five a week are in zoology with Rudolf Kner. Eleven hours a week - botany with Professor Fenzl: in addition to lectures on morphology and systematics, he also took a special workshop on the description and identification of plants. In the third semester, he already signed up for thirty-two hours of classes a week: ten hours - physics with Doppler, ten - chemistry with Rottenbacher: general chemistry, medicinal chemistry, pharmacological chemistry and a workshop in analytical chemistry. Five for zoology with Kner. Six hours of lessons from Unger, one of the first cytologists in the world. In his laboratories he studied the anatomy and physiology of plants and took a workshop in microscopy techniques. And once a week at the mathematics department there is a workshop on logarithm and trigonometry.

1850, life was going well. Mendel could already support himself, and was greatly respected by his colleagues, because he handled his responsibilities well, and was very pleasant to talk to. His students loved him.

In 1851, Gregor Mendel took aim at the cardinal issue of biology - the problem of variability and heredity. It was then that he began to conduct experiments on the directed cultivation of plants. Mendel brought various plants from the distant and near surroundings of Brünn. He cultivated plants in groups in a part of the monastery garden specially designated for each of them under various external conditions. He was engaged in painstaking meteorological observations. Gregor carried out most of his experiments and observations with peas, which, starting from 1854, were sown every spring in a small garden under the windows of the prelature. It turned out to be not difficult to carry out a clear hybridization experiment on peas. To do this, you just need to open a large, although not yet ripe, flower with tweezers, tear off the anthers, and independently predetermine a “pair” for it to cross. Since self-pollination is excluded, pea varieties are, as a rule, “pure lines” with constant characteristics that do not change from generation to generation and are extremely clearly defined. Mendel identified the characteristics that determined intervarietal differences: the color of the skin of mature grains and, separately, of unripe grains, the shape of mature peas, the color of the “protein” (endosperm), the length of the stem axis, the location and color of the buds. He used more than thirty varieties in the experiment, and each of the varieties was previously subjected to a two-year test for “constancy”, for “constancy of characteristics”, for “purity of blood” - in 1854 and in 1855. Experiments with peas went on for eight years. Hundreds of times during eight flowerings, he carefully tore off the anthers with his own hands and, having collected pollen from the stamens of a flower of a different variety with tweezers, applied it to the stigma of the pistil. Ten thousand passports were issued for ten thousand plants obtained as a result of crossings and from self-pollinating hybrids. The records are neat: when the parent plant was grown, what flowers it had, whose pollen was fertilized, what peas - yellow or green, smooth or wrinkled - were produced, what flowers - color at the edges, color in the center - bloomed, when the seeds were received , how many of them are yellow, how many are green, round, wrinkled, how many of them are selected for planting, when they are planted, and so on.

The result of his research was the report “Experiments on plant hybrids,” which was read by the Brunn naturalist in 1865. The report says: “The reason for conducting the experiments to which this article is devoted was the artificial crossing of ornamental plants, carried out with the aim of obtaining new forms that differ in color. To carry out further experiments in order to trace the development of crossbreeds in their offspring, the impetus was given by the striking pattern with which hybrid forms constantly returned to their ancestral forms.” As often happens in the history of science, Mendel's work did not immediately receive due recognition from his contemporaries. The results of his experiments were published at a meeting of the Society of Natural Sciences of the city of Brünn, and then published in the journal of this Society, but Mendel’s ideas did not find support at that time. An issue of the journal describing Mendel's revolutionary work had been collecting dust in libraries for thirty years. Only at the end of the 19th century did scientists working on the problems of heredity discover the works of Mendel, and he was able to receive (posthumously) the recognition he deserved.

The patterns of distribution of hereditary characteristics in the offspring established by G. Mendel. The patterns were established by G. Mendel on the basis of many years (1856-1863) of experiments in crossing pea varieties that differed in some contrasting characteristics. G. Mendel's discovery did not receive recognition during his lifetime. In 1900, these patterns were rediscovered by three independent researchers (K. Correns, E. Chermak and H. De Vries). Many genetics textbooks mention Mendel's three laws:

1. The law of uniformity of first-generation hybrids - the offspring of the first generation from crossing stable forms that differ in one trait have the same phenotype.

2. The law of segregation states that when hybrids of the first generation are crossed with each other, among the hybrids of the second generation, individuals with the phenotype of the original parental forms and hybrids of the first generation appear in a certain ratio. In the case of complete dominance, 3/4 of the individuals have a dominant trait and 1/4 have a recessive trait.

3. The law of independent combination - each pair of alternative characteristics behaves independently of each other in a series of generations.

Mendel's first law.

Law of uniformity of the first generation of hybrids.

To illustrate Mendel's first law - the law of uniformity of the first generation - let us reproduce his experiments on montlhybrid crossing of pea plants. The crossing of two organisms is called hybridization, the offspring from the crossing of two individuals with different heredity is called hybrid, and an individual is called a hybrid, the site emphasizes. Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative (mutually exclusive) characteristics. Consequently, with such crossing, patterns of inheritance of only two traits can be traced, the development of which is determined by a pair of allelic genes. All other characteristics characteristic of these organisms are not taken into account.

If you cross pea plants with yellow and green seeds, then all the resulting hybrids will have yellow seeds. The same picture is observed when crossing plants with smooth and wrinkled seeds; all first generation offspring will have smooth seed shapes. Consequently, in the first generation hybrid, only one of each pair of alternative characters develops. The second sign seems to disappear and does not appear. G. Mendel called the phenomenon of predominance of the trait of one of the parents in a hybrid dominance. A trait that appears in a first-generation hybrid and suppresses the development of another trait was called dominant, and the opposite, i.e., suppressed, trait was called recessive. If the genotype of an organism (zygote) has two identical allelic genes - both dominant or both recessive (AA or aa), such an organism is called homozygous. If, of a pair of allelic genes, one is dominant and the other is recessive (Aa), then such an organism is called heterozygous.

The law of dominance - Mendel's first law - is also called the law of uniformity of first-generation hybrids, since all individuals of the first generation exhibit one trait.

Incomplete dominance.

A dominant gene in a heterozygous state does not always completely suppress a recessive gene. In some cases, the FI hybrid does not completely reproduce any of the parental characters and the trait is intermediate in nature with a greater or lesser bias towards a dominant or recessive state. But all individuals of this generation are uniform in this trait. Thus, when crossing a night beauty with red flower color (AA) with a plant with white flowers (aa), an intermediate pink flower color (Aa) is formed in FI. With incomplete dominance in the offspring of hybrids (Fi), the splitting by genotype and phenotype coincides (1:2:1).

Incomplete dominance is a widespread phenomenon. It was discovered when studying the inheritance of flower color in snapdragons, wool color in cattle and sheep, biochemical traits in humans, etc. Intermediate traits that arise as a result of incomplete dominance often represent aesthetic or material value for humans. The question arises: is it possible to develop, for example, a variety of night beauty with pink flower color through selection? Obviously not, because this trait develops only in heterozygotes and when they are crossed with each other, splitting always occurs:

Multiple allelism. So far, examples have been examined in which the same gene was represented by two alleles - dominant (A] and recessive (a). These two states of the gene arise in the process of mutation. However, mutation (replacement or loss of part of the nucleotides in the DNA molecule) can arise in different parts of one gene. In this way, several alleles of one gene are formed and, accordingly, several variants of one trait can mutate into the state a, a^, az, .... and gene B in another locus - into the state bi. , ir, b3, b*, ..., b„, etc. Let us give a few examples. In the Drosophila fly, a series of alleles for the eye color gene is known, consisting of 12 members: red, coral, cherry, apricot, etc. . to white, determined by a recessive gene. Rabbits have a series of multiple alleles for coat color: solid (chinchilla), Himalayan (ermine), and albinism. Himalayan rabbits have black tips of the ears, paws, tail and muzzle against the background of the overall white coat color. Albinos are completely devoid of pigment. Members of the same series of alleles may have different dominant-recessive relationships to each other. Thus, the solid color gene is dominant in relation to all members of the series. The Himalayan color gene is dominant to the white color gene, but recessive to the chinchilla color gene. The development of all three types of color is due to three different alleles localized at the same locus. In humans, the gene that determines blood type is represented by a series of multiple alleles. At the same time, the genes that determine blood groups A and B are not dominant in relation to each other and both are dominant in relation to the gene that determines blood group O. It should be remembered that the genotype of diploid organisms can contain only two genes from a series of alleles. The remaining alleles of this gene in different combinations are included in the genotype of other individuals of this species. Thus, multiple allelism characterizes the diversity of the gene pool of an entire species, i.e., it is a species and not an individual trait.

Mendel's second law.

Splitting of characters in second generation hybrids.

From hybrid pea seeds, G. Mendel grew plants that, through self-pollination, produced second-generation seeds. Among them were not only yellow seeds, but also green ones. In total he received 2001 green and 6022 yellow seeds. And what? the seeds of the second generation hybrids were yellow in color and? - green. Consequently, the ratio of the number of descendants of the second generation with a dominant trait to the number of descendants with a recessive trait turned out to be equal to 3:1. He called this phenomenon splitting of signs.

Numerous experiments on hybridological analysis of other pairs of characters gave similar results in the second generation. Based on the results obtained, G. Mendel formulated his second law - the law of splitting. In the offspring obtained from crossing first-generation hybrids, the phenomenon of splitting is observed: a quarter of individuals from second-generation hybrids carry a recessive trait, three quarters - a dominant one.

Homozygous and heterozygous individuals. In order to find out how the inheritance of traits would occur during self-pollination in the third generation, Mendel raised second-generation hybrids and analyzed the offspring obtained from self-pollination. He found that 1/3 of the second generation plants that grew from yellow seeds produced only yellow seeds when self-pollinated. Plants grown from green seeds produced only green seeds. The remaining 2/3 of the second generation plants, grown from yellow seeds, produced yellow and green seeds in a 3:1 ratio. Thus, these plants were similar to first generation hybrids.

So, 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 produce cleavage in the next generation are called homozygous (from the Greek “homo” - equal, “zygote” - fertilized egg). Individuals whose offspring exhibit clefting are called heterozygous (from the Greek “hetero” - different).

The reason for the splitting of characters in hybrids. What is the reason for the segregation of segregation traits in the offspring of hybrids? Why do individuals arise in the first, second and subsequent generations that, as a result of crossing, produce offspring with dominant and recessive traits? Let us turn to the diagram on which the results of the monohybrid crossing experiment are written with symbols. Symbols P, F1, F2, etc. denote the parental, first and second generations, respectively. The X symbol indicates crossing, the > symbol indicates the male gender (shield and spear of Mars), and the + symbol indicates the female gender (mirror of Venus).

The gene responsible for the dominant yellow color of seeds will be denoted by a capital letter, for example A; the gene responsible for the recessive green color - small letter a. Since each chromosome is represented in somatic cells by two homologues, each gene is also present in two copies, as geneticists say, in the form of two alleles. The letter A denotes a dominant allele, and a denotes a recessive allele.

The scheme for the formation of zygotes in a monohybrid cross is as follows:

where P are parents, F1 are first generation hybrids, F2 are second generation hybrids. For further discussion, it is necessary to recall the main phenomena occurring in meiosis. In the first division of meiosis, the formation of cells carrying a haploid set of chromosomes (n) occurs. Such cells contain only one chromosome from each pair of homologous chromosomes, and later gametes are formed from them. The fusion of haploid gametes during fertilization leads to the formation of a haploid (2n) zygote. The process of formation of haploid gametes and restoration of diploidity during fertilization necessarily occurs in each generation of organisms that reproduce sexually.

The original parental plants in the experiment under consideration were homozygous. Therefore, crossing can be written as follows: P (AA X aa). Obviously, both parents are capable of producing gametes of only one variety, and plants with two dominant AA genes produce only gametes carrying the A gene, and plants with two recessive aa genes form germ cells with the a gene. In the first F1 generation, all offspring are heterozygous Aa and have only yellow seeds, since the dominant gene A suppresses the action of the recessive gene a. Such heterozygous Aa plants are capable of producing gametes of two varieties, carrying the A and a genes.

During fertilization, four types of zygotes appear - AA + Aa + aA + aa, which can be written as AA + 2Aa + aa. Since in our experiment the heterozygous seeds of Aa are also colored yellow, in F2 the ratio of yellow to green seeds is equal to 3:1. It is clear that 1/3 of the plants that grew from yellow seeds having AA genes, when self-pollinated, again produce only yellow seeds. In the remaining 2/3 of plants with Aa genes, just like in hybrid plants from F1, two different types of gametes will be formed, and in the next generation, during self-pollination, the seed color trait will split into yellow and green in a 3:1 ratio.

Thus, it was established that the splitting of traits in the offspring of hybrid plants is the result of the presence of two genes in them - A and a, responsible for the development of one trait, for example, seed color.

Mendel's third law.

The law of independent combination, or Mendel's third law.

Mendel's study of the inheritance of one pair of alleles made it possible to establish a number of important genetic patterns: the phenomenon of dominance, the constancy of recessive alleles in hybrids, the splitting of the offspring of hybrids in a ratio of 3: 1, and also to assume that gametes are genetically pure, i.e. they contain only one gene from allelic pairs. However, organisms differ in many genes. The patterns of inheritance of two pairs of alternative characters or more can be established by dihybrid or polyhybrid crossing.

For dihybrid crosses, Mendel took homozygous pea plants that differed in two genes - seed color (yellow, green) and seed shape (smooth, wrinkled). Dominant characteristics are yellow color (A) and smooth shape (B) of the seeds. Each plant produces one variety of gametes according to the alleles studied:

When gametes merge, all offspring will be uniform: In cases where gametes are formed in a hybrid, from each pair of allelic genes, only one gets into the gamete, and due to the accidental divergence of the paternal and maternal chromosomes in the first division of meiosis, gene A can end up in the same gamete with gene B or c genome b. In the same way, gene a can appear in the same gamete with gene B or gene b. Therefore, the hybrid produces four types of gametes: AB, Av, aB, oa.

During fertilization, each of the four types of gametes from one organism encounters randomly any of the gametes from another organism. All possible combinations of male and female gametes can be easily established using a Punnett grid, in which the gametes of one parent are written horizontally and the gametes of the other parent vertically. The genotypes of zygotes formed during the fusion of gametes are entered into the squares.

It is easy to calculate that according to phenotype, the offspring are divided into 4 groups: 9 yellow smooth, 3 yellow wrinkled, 3 green smooth, 1 yellow wrinkled. If we take into account the results of splitting for each pair of characters separately, it turns out that the ratio of the number of yellow seeds to the number of green ones and the ratio of smooth seeds to wrinkled ones for each pair is 3:1. Thus, with a dihybrid crossing, each pair of characters, when split in the offspring, behaves in the same way as with a monohybrid crossing, i.e., independently of the other pair of characters.

During fertilization, gametes are combined according to the rules of random combinations, but with equal probability for each. In the resulting zygotes, various combinations of genes arise. Independent distribution of genes in the offspring and the occurrence of various combinations of these genes during dihybrid crossing is possible only if pairs of allelic genes are located in different pairs of homologous chromosomes.

Thus, Mendel's third law states: When crossing two homozygous individuals that differ from each other in two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations.

Mendel's first law. Law of uniformity of first generation hybrids

When crossing homozygous individuals that differ in one pair of alternative (mutually exclusive) characters, all offspring in first generation uniform in both phenotype and genotype.

Pea plants with yellow (dominant trait) and green (recessive trait) seeds were crossed. The formation of gametes is accompanied by meiosis. Each plant produces one type of gamete. From each homologous pair of chromosomes, one chromosome with one of the allelic genes (A or a) goes into gametes. After fertilization, the pairing of homologous chromosomes is restored and hybrids are formed. All plants will have only yellow seeds (phenotype), heterozygous for the Aa genotype. This happens when complete dominance.

Hybrid Aa has one gene A from one parent, and the second gene - a - from the other parent (Fig. 73).

Haploid gametes (G), unlike diploid organisms, are circled.

As a result of crossing, first generation hybrids are obtained, designated F 1.

To record crosses, a special table is used, proposed by the English geneticist Punnett and called the Punnett grid.

The gametes of the paternal individual are written out horizontally, and the gametes of the maternal individual vertically. Genotyping is recorded at intersections.

Rice. 73.Inheritance in monohybrid crosses.

I - crossing two varieties of peas with yellow and green seeds (P); II

Cytological foundations of Mendel's I and II laws.

F 1 - heterozygotes (Aa), F 2 - segregation according to genotype 1 AA: 2 Aa: 1 aa.

py descendants. In the table, the number of cells depends on the number of gamete types produced by the individuals being crossed.

Mendel's II law. The law of splitting of first generation hybrids

When hybrids of the first generation are crossed with each other, individuals with both dominant and recessive traits appear in the second generation and splitting occurs by phenotype in a ratio of 3:1 (three dominant phenotypes and one recessive) and 1:2:1 by genotype (see. Fig. 73). Such splitting is possible when complete dominance.

Hypothesis of “purity” of gametes

The law of splitting can be explained by the hypothesis of the “purity” of gametes.

Mendel called the phenomenon of non-mixing of alleles of alternative characters in the gametes of a heterozygous organism (hybrid) the hypothesis of “purity” of gametes. Two allelic genes (Aa) are responsible for each trait. When hybrids are formed, allelic genes are not mixed, but remain unchanged.

As a result of meiosis, Aa hybrids form two types of gametes. Each gamete contains one of a pair of homologous chromosomes with allelic gene A or allelic gene a. Gametes are pure from another allelic gene. During fertilization, the homology of chromosomes and allelicity of genes are restored, and a recessive trait (the green color of peas) appears, the gene of which did not show its effect in the hybrid organism. Traits develop through the interaction of genes.

Incomplete dominance

At incomplete dominance heterozygous individuals have their own phenotype, and the trait is intermediate.

When crossing night beauty plants with red and white flowers, pink-colored individuals appear in the first generation. When crossing first-generation hybrids (pink flowers), the cleavage in the offspring by genotype and phenotype coincides (Fig. 74).

Rice. 74.Inheritance with incomplete dominance in the night beauty plant.

The gene that causes sickle cell anemia in humans has the property of incomplete dominance.

Analysis cross

The recessive trait (green peas) appears only in the homozygous state. Homozygous (yellow peas) and heterozygous (yellow peas) individuals with dominant traits do not differ from each other in phenotype, but have different genotypes. Their genotypes can be determined by crossing with individuals with a known genotype. Such an individual may be green peas, which have a homozygous recessive trait. This cross is called an analyzed cross. If, as a result of crossing, all the offspring are uniform, then the individual under study is homozygous.

If splitting occurs, then the individual is heterozygous. The offspring of a heterozygous individual produces cleavage in a 1:1 ratio.

Mendel's III law. Law of independent combination of characteristics (Fig. 75). Organisms differ from each other in several ways.

The crossing of individuals that differ in two characteristics is called dihybrid, and in many respects - polyhybrid.

When crossing homozygous individuals that differ in two pairs of alternative characters, in the second generation occurs independent combination of features.

As a result of dihybrid crossing, the entire first generation is uniform. In the second generation, phenotypic cleavage occurs in a ratio of 9:3:3:1.

For example, if you cross a pea with yellow seeds and a smooth surface (dominant trait) with a pea with green seeds and a wrinkled surface (recessive trait), the entire first generation will be uniform (yellow and smooth seeds).

When hybrids were crossed with each other in the second generation, individuals appeared with characteristics that were not present in the original forms (yellow wrinkled and green smooth seeds). These traits are inherited regardless from each other.

A diheterozygous individual produced 4 types of gametes

For the convenience of counting individuals resulting in the second generation after crossing hybrids, the Punnett grid is used.

Rice. 75.Independent distribution of traits in dihybrid crosses. A, B, a, b - dominant and recessive alleles that control the development of two traits. G - germ cells of the parents; F 1 - first generation hybrids; F 2 - second generation hybrids.

As a result of meiosis, each gamete will receive one of the allelic genes from a homologous pair of chromosomes.

4 types of gametes are formed. Cleavage after crossing in the ratio 9:3:3:1 (9 individuals with two dominant traits, 1 individual with two recessive traits, 3 individuals with one dominant and the other recessive traits, 3 individuals with dominant and recessive traits).

The appearance of individuals with dominant and recessive traits is possible because the genes responsible for the color and shape of peas are located on various non-homologous chromosomes.

Each pair of allelic genes is distributed independently of the other pair, and therefore genes can be combined independently.

A heterozygous individual for “n” pairs of characteristics forms 2 n types of gametes.

Questions for self-control

1. How is Mendel’s first law formulated?

2. What seeds did Mendel cross with peas?

3. Plants with what seeds resulted from crossing?

4. How is Mendel’s II law formulated?

5. Plants with what characteristics were obtained as a result of crossing first generation hybrids?

6. In what numerical ratio does splitting occur?

7. How can the law of splitting be explained?

8. How to explain the hypothesis of “purity” of gametes?

9. How to explain the incomplete dominance of traits? 10.What kind of cleavage by phenotype and genotype occurs

after crossing first generation hybrids?

11.When is an analytical cross carried out?

12. How is an analytical cross carried out?

13.What kind of cross is called dihybrid?

14. On which chromosomes are the genes responsible for the color and shape of peas located?

15. How is Mendel’s III law formulated?

16. What phenotypic cleavage occurs in the first generation?

17. What kind of phenotypic cleavage occurs in the second generation?

18.What is used for the convenience of counting individuals resulting from crossing hybrids?

19.How can we explain the appearance of individuals with characteristics that were not there before?

Keywords of the topic “Mendel’s Laws”

allelicity anemia

interaction

gametes

gene

genotype

heterozygote

hybrid

hypothesis of "purity" of gametes

homozygote

homology

peas

pea

action

dihybrid

dominance

uniformity

law

meiosis

education coloring

fertilization

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pairing

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count

generation

polyhybrid

offspring

appearance

sign

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Punnett grid

parents

property

seeds

crossing

merger

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convenience

phenotype

form

character

color

flowers

Multiple allelism

Allelic genes may include not two, but a greater number of genes. These are multiple alleles. They arise as a result of mutation (replacement or loss of a nucleotide in a DNA molecule). An example of multiple alleles can be the genes responsible for human blood groups: I A, I B, I 0. Genes I A and I B are dominant to the I 0 gene. Only two genes from a series of alleles are always present in a genotype. Genes I 0 I 0 determine blood group I, genes I A I A, I A I O - group II, I B I B, I B I 0 - group III, I A I B - group IV.

Gene interaction

There is a complex relationship between a gene and a trait. One gene can be responsible for the development of one trait.

Genes are responsible for the synthesis of proteins that catalyze certain biochemical reactions, resulting in certain characteristics.

One gene can be responsible for the development of several traits, exhibiting pleiotropic effect. The severity of the pleiotropic effect of a gene depends on the biochemical reaction catalyzed by the enzyme synthesized under the control of this gene.

Several genes may be responsible for the development of one trait - this is polymer gene action.

The manifestation of symptoms is the result of the interaction of various biochemical reactions. These interactions can be associated with allelic and non-allelic genes.

Interaction of allelic genes.

The interaction of genes located in the same allelic pair occurs as follows:

. complete dominance;

. incomplete dominance;

. co-dominance;

. overdominance.

At complete In dominance, the action of one (dominant) gene completely suppresses the action of another (recessive). When crossing, a dominant trait appears in the first generation (for example, the yellow color of peas).

At incomplete dominance occurs when the effect of a dominant allele is weakened in the presence of a recessive one. Heterozygous individuals obtained as a result of crossing have their own genotype. For example, when crossing night beauty plants with red and white flowers, pink flowers appear.

At co-dominance The effect of both genes is manifested when they are present simultaneously. As a result, a new symptom appears.

For example, blood group IV (I A I B) in humans is formed by the interaction of genes I A and I B. Separately, the I A gene determines the II blood group, and the I B gene determines the III blood group.

At overdominance the dominant allele in the heterozygous state has a stronger manifestation of the trait than in the homozygous state.

Interaction of nonallelic genes

One trait of an organism can often be influenced by several pairs of non-allelic genes.

The interaction of non-allelic genes occurs as follows:

. complementarity;

. epistasis;

. polymers.

Complementary the effect manifests itself with the simultaneous presence of two dominant non-allelic genes in the genotype of organisms. Each of the dominant genes can manifest itself independently if the other is in a recessive state, but their joint presence in a dominant state in the zygote determines a new state of the trait.

Example. Two varieties of sweet peas with white flowers were crossed. All first generation hybrids had red flowers. Flower color depends on two interacting genes A and B.

Proteins (enzymes) synthesized on the basis of genes A and B catalyze biochemical reactions that lead to the manifestation of the trait (red color of flowers).

Epistasis- an interaction in which one of the dominant or recessive non-allelic genes suppresses the action of another non-allelic gene. A gene that suppresses the action of another is called an epistatic gene, or suppressor. The suppressed gene is called hypostatic. Epistasis can be dominant or recessive.

Dominant epistasis. An example of dominant epistasis would be the inheritance of plumage color in chickens. The dominant gene C is responsible for plumage color. The dominant non-allelic gene I suppresses the development of plumage color. As a result of this, chickens that have the C gene in the genotype, in the presence of the I gene, have white plumage: IICC; IICC; IiCc; Iicc. Hens with the iicc genotype will also be white because these genes are in a recessive state. The plumage of chickens with the iiCC, iiCc genotype will be colored. The white color of the plumage is due to the presence of a recessive allele of the i gene or the presence of the color suppressor gene I. The interaction of genes is based on biochemical connections between enzyme proteins, which are encoded by epistatic genes.

Recessive epistasis. Recessive epistasis explains the Bombay phenomenon - the unusual inheritance of antigens of the ABO blood group system. There are 4 known blood groups.

In the family of a woman with blood group I (I 0 I 0), a man with blood group II (I A I A) gave birth to a child with blood group IV (I A I B), which is impossible. It turned out that the woman inherited the I B gene from her mother and the I 0 gene from her father. Only the I 0 gene showed an effect, therefore

it was believed that the woman had blood type I. Gene I B was suppressed by the recessive gene x, which was in a homozygous state - xx.

In the child of this woman, the suppressed I B gene showed its effect. The child had IV blood group I A I B.

PolymerThe effect of genes is due to the fact that several non-allelic genes can be responsible for the same trait, enhancing its manifestation. Traits that depend on polymer genes are classified as quantitative. Genes responsible for the development of quantitative traits have a cumulative effect. For example, polymeric non-allelic genes S 1 and S 2 are responsible for skin pigmentation in humans. In the presence of dominant alleles of these genes, a lot of pigment is synthesized, in the presence of recessive ones - little. The intensity of skin color depends on the amount of pigment, which is determined by the number of dominant genes.

From a marriage between mulattoes S 1 s 1 S 2 s 2, children are born with skin pigmentation from light to dark, but the probability of having a child with white and black skin color is 1/16.

Many traits are inherited according to the polymeric principle.

Questions for self-control

1. What are multiple alleles?

2. What genes are responsible for human blood types?

3. What blood types does a person have?

4. What connections exist between a gene and a trait?

5. How do allelic genes interact?

6. How do non-allelic genes interact?

7. How can the complementary action of a gene be explained?

8. How can epistasis be explained?

9. How can the polymeric action of a gene be explained?

Keywords of the topic “Multiple alleles and gene interaction”

allelism allele antigens marriage

interaction

genotype

hybrid

peas

peas

blood type

action

children

dominance

woman

replacement

codominance

co-dominance

leather

chickens

mother

molecule

mulatto

mutation

Availability

inheritance

nucleotides

coloring

plumage

the basis

attitude

pigment

pigmentation

pleiotropy

suppressor

generation

polymerism

sign

example

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child

result

overdominance connection

protein synthesis system

crossing

state

degree

loss

phenomenon

enzymes

color

flowers

Human