The improvement of the hybridiological method allowed G. Mendel to identify a number of the most important patterns inheritance of traits in peas, which, as it turned out later, are true for all diploid organisms that reproduce sexually.
When describing the results of crossings, Mendel himself did not interpret the facts he established as certain laws. But after their rediscovery and confirmation on plant and animal objects, these phenomena, repeated under certain conditions, began to be called the laws of inheritance of characteristics in hybrids.
Some researchers distinguish not three, but two of Mendel's laws. At the same time, some scientists combine the first and second laws, believing that the first law is part of the second and describes the genotypes and phenotypes of the descendants of the first generation (F1). Other researchers combine the second and third laws into one, believing that the “law of independent combination” is in essence the “law of independence of segregation” that occurs simultaneously in different pairs of alleles. However, in Russian literature we're talking about about Mendel's three laws.
Mendel's great scientific success was that the seven traits he chose were determined by genes on different chromosomes, which excluded possible linked inheritance. He found that:
1) In first-generation hybrids, the trait of only one parental form is present, while the other “disappears.” This is the law of uniformity of first generation hybrids.
2) In the second generation, a split is observed: three quarters of the descendants have the trait of hybrids of the first generation, and a quarter have a trait that “disappeared” in the first generation. This is the law of splitting.
3) Each pair of traits is inherited independently of the other pair. This is the law of independent inheritance.
Of course, Mendel did not know that these provisions would eventually be called Mendel's first, second and third laws.
Modern wording of laws
Mendel's first law
The law of uniformity of first-generation hybrids - 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 the parents.
This law is also known as the "law of trait dominance." Its formulation is based on the concept of a pure line relative to the characteristic being studied - on modern language this means that individuals are homozygous for this trait.
Mendel's second law
The law of segregation - when two heterozygous descendants of the first generation are crossed with each other in the second generation, segregation is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1.
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 (recombination) 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.
The splitting of offspring when crossing heterozygous individuals is explained by the fact that the gametes are genetically pure, that is, they carry only one gene from an allelic pair. The law of gamete purity can be formulated in the following way: during the formation of germ cells, only one allele from a pair of alleles of a given gene enters each gamete. Cytological basis splitting of characters - divergence of homologous chromosomes and the formation of haploid germ cells in meiosis (Fig. 4).
Fig.4.
The example illustrates crossing plants with smooth and wrinkled seeds. Only two pairs of chromosomes are depicted; one of these pairs contains the gene responsible for the shape of the seeds. In plants with smooth seeds, meiosis leads to the formation of gametes with the smooth allele (R), and in plants with wrinkled seeds, gametes with the wrinkled allele (r). First generation F1 hybrids have one chromosome with the smooth allele and one chromosome with the wrinkled allele. Meiosis in F1 leads to the formation of equal number gametes with R and with r. The random pairwise combination of these gametes during fertilization leads in the F2 generation to the appearance of individuals with smooth and wrinkled peas in a ratio of 3:1.
Mendel's third law
The law of independent inheritance - when crossing two 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).
Mendeleev's law of independent inheritance can be explained by the movement of chromosomes during meiosis (Fig. 5). During the formation of gametes, the distribution of alleles from a given pair of homologous chromosomes between them occurs independently of the distribution of alleles from other pairs. It is the random arrangement of homologous chromosomes at the spindle equator in metaphase I of meiosis and their subsequent arrangement in anaphase I that leads to a variety of recombinations of alleles in gametes. The number of possible combinations of alleles in male or female gametes can be determined by general formula 2n, where n is the haploid number of chromosomes. A person has n=23, and the possible number various combinations is 223=8,388,608.
Fig.5. Mendelian Law Explained independent distribution factors (alleles) R, r, Y, y as a result of independent divergence of different pairs of homologous chromosomes in meiosis. Crossing plants that differ in the shape and color of seeds (smooth yellow and green wrinkled) produces hybrid plants in which the chromosomes of one homologous pair contain the R and r alleles, and the other homologous pair contains the Y and y alleles. In metaphase I of meiosis, chromosomes obtained from each parent can with equal probability go either to the same spindle pole (left picture) or to different ones (right picture). In the first case, gametes arise containing the same combinations of genes (YR and yr) as in the parents, in the second case, alternative combinations of genes (Yr and yR). As a result, with a probability of 1/4, four types of gametes are formed; a random combination of these types leads to the splitting of the offspring 9: 3: 3: 1, as was observed by Mendel.
Formulation 1 of Mendel's law The law of uniformity of the first generation of hybrids, or Mendel's first law. When crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative traits, the entire first generation of hybrids (F1) will be uniform and will carry the trait of one of the parents
Formulation of the 2nd law of Mendel The law of segregation, or the second law of Mendel Mendel When two heterozygous descendants of the first generation are crossed with each other in the second generation, segregation is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1.
Formulation 3 of Mendel's law 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 characteristics, genes and their corresponding characteristics are inherited independently of each other and are combined in all possible combinations (as and with monohybrid crossing). (The first generation after crossing had a dominant phenotype for all characteristics. In the second generation, a splitting of phenotypes was observed according to the formula 9: 3: 3: 1)
R AA BB aa bb x yellow, smooth seeds green, wrinkled seeds G (gametes) ABabab F1F1 Aa Bb yellow, smooth seeds 100% Mendel's 3rd law DIHYBRID CROSSING. For the experiments, peas with smooth yellow seeds were taken as the mother plant, and peas with green wrinkled seeds were taken as the father plant. In the first plant both characters were dominant (AB), and in the second plant both were recessive (ab
The first generation after crossing had a dominant phenotype for all traits. (yellow and smooth peas) In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1. 9/16 yellow smooth peas, 3/16 yellow wrinkled peas, 3/16 green smooth peas, 1/16 green wrinkled peas.
Task 1. In spaniels, black coat color dominates over coffee, and short hair dominates over long hair. The hunter bought a black dog with short hair and, to be sure that it was a purebred, he carried out an analytical crossbreeding. 4 puppies were born: 2 short-haired black, 2 short-haired coffee. What is the genotype of the dog purchased by the hunter? Dihybrid crossing problems.
Problem 2. In a tomato, the red color of the fruit dominates over the yellow color, and the high stem dominates over the low stem. By crossing a variety with red fruits and a high stem and a variety with yellow fruits and a low stem, 28 hybrids were obtained in the second generation. The first generation hybrids were crossed with each other, resulting in 160 second generation hybrid plants. How many types of gametes does a first generation plant produce? How many plants in the first generation have red fruit and a tall stem? How many different genotypes are there among the second generation plants with red fruit color and tall stem? How many plants in the second generation have yellow fruit and a tall stem? How many plants in the second generation have yellow fruit and a low stem?
Task 3 In a person, brown eye color dominates blue, and the ability to use the left hand is recessive in relation to right-handedness. From the marriage of a blue-eyed, right-handed man with a brown-eyed, left-handed woman, a blue-eyed, left-handed child was born. How many types of gametes does the mother produce? How many types of gametes does the father produce? How many different genotypes can there be among children? How many different phenotypes can there be among children? What is the probability of having a blue-eyed, left-handed child in this family (%)?
Task 4 Crestedness in chickens dominates over the absence of a crest, and black plumage color dominates over brown. From crossing a heterozygous black hen without a crest with a heterozygous brown crested rooster, 48 chickens were obtained. How many types of gametes does a chicken produce? How many types of gametes does a rooster produce? How many different genotypes will there be among the chickens? How many tufted black chickens will there be? How many black chickens will there be without a crest?
Task 5 In cats, the short hair of the Siamese breed is dominant over the long hair of the Persian breed, and the black coat color of the Persian breed is dominant over the fawn color of the Siamese breed. Siamese cats crossed with Persian cats. When crossing hybrids with each other in the second generation, 24 kittens were obtained. How many types of gametes are produced in a Siamese cat? How many different genotypes were produced in the second generation? How many different phenotypes were produced in the second generation? How many second generation kittens look like Siamese cats? How many second generation kittens look like Persians?
Solving problems at home Option 1 1) A blue-eyed right-hander married a brown-eyed right-hander. They had two children - a brown-eyed left-hander and a blue-eyed right-hander. From this man’s second marriage to another brown-eyed, right-handed woman, 8 brown-eyed children were born, all right-handed. What are the genotypes of all three parents? 2) In humans, the gene for protruding ears dominates over the gene for normal flat ears, and the gene for non-red hair dominates over the gene for red hair. What kind of offspring can be expected from the marriage of a floppy-eared red-haired man, heterozygous for the first sign, with a heterozygous red-haired woman with normal flat-back ears. Option 2 1) In humans, clubfoot (R) dominates over the normal structure of the foot (R) and normal carbohydrate metabolism (O) over diabetes. Woman having normal structure feet and normal metabolism, married a club-footed man. From this marriage two children were born, one of whom developed clubfoot and the other diabetes mellitus. Determine the genotype of parents from the phenotype of their children. What phenotypes and genotypes of children are possible in this family? 2) A person has a gene brown eyes dominates the gene for blue eyes, and the ability to own right hand over left-handedness. Both pairs of genes are located on different chromosomes. What kind of children can they be if: the father is left-handed, but heterozygous for eye color, and the mother is blue-eyed, but heterozygous for the ability to use her hands.
Let's solve problems 1. In humans, normal carbohydrate metabolism dominates over the recessive gene responsible for the development of diabetes mellitus. Daughter healthy parents sick. Determine whether a child could be born in this family healthy child and what is the probability of this event? 2. In people, brown eye color is dominant over blue. The ability to better use the right hand dominates over left-handedness; the genes for both traits are located on different chromosomes. A brown-eyed right-hander marries a blue-eyed left-hander. What kind of offspring should be expected in this pair?
The patterns of inheritance of characters during sexual reproduction were established by G. Mendel. It is necessary to have a clear understanding of genotype and phenotype, alleles, homo- and heterozygosity, dominance and its types, types of crosses, and also draw up diagrams.
Monohybrid called crossing, in which the parent forms differ from each other in one pair of contrasting, alternative characters.
Consequently, with such crossing, patterns of inheritance of only two variants of the trait can be traced, the development of which is determined by the pair allelic genes. Examples of monohybrid crossings carried out by G. Mendel include crossings of peas with such clearly visible alternative characters as purple and white flowers, yellow and green coloring of unripe fruits (beans), smooth and wrinkled surface of seeds, yellow and green coloring, etc.
Uniformity of first generation hybrids (Mendel's first law). When crossing peas with purple (AA) and white (aa) flowers, Mendel found that all first generation hybrid plants (F 1) had purple flowers (Fig. 2).
Figure 2 – Monohybrid crossing scheme
At the same time, the white color of the flower did not appear. When crossing plants with smooth and wrinkled seeds, the hybrids will have smooth seeds. G. Mendel also established that all F 1 hybrids turned out to be uniform (homogeneous) in each of the seven characters he studied. Consequently, in first-generation hybrids, out of a pair of parental alternative traits, only one appears, and the trait of the other parent seems to disappear.
Alternative signs are mutually exclusive and contrasting signs.
Mendel called the phenomenon of predominance of traits of one of the parents in F 1 hybrids dominance, and the corresponding trait - dominant. He called traits that do not appear in F 1 hybrids recessive. Since all first-generation hybrids are uniform, this phenomenon was called Mendel's first laws, or the law of uniformity of first-generation hybrids, as well as the rule of dominance.
It can be formulated as follows: when crossing two organisms belonging to different pure lines (two homozygous organisms), differing from each other in one pair of alternative traits, the entire first generation of hybrids will be uniform and will carry the trait of one of the parents.
Each gene has two states - “A” and “a”, so they form one pair, and each member of the pair is called an allele. Genes located in the same loci (sections) of homologous chromosomes and determining the alternative development of the same trait are called allelic.
For example, the purple and white color of a pea flower are dominant and recessive traits, respectively, for two alleles (A and a) of one gene. Due to the presence of two alleles, two states of the body are possible: homo- and heterozygous. If an organism contains identical alleles of a particular gene (AA or aa), then it is called homozygous for this gene (or trait), and if different (Aa) it is called heterozygous. Therefore, an allele is a form of existence of a gene. An example of a triallelic gene is the gene that determines the ABO blood group system in humans. There are even more alleles: for the gene that controls the synthesis of human hemoglobin, many dozens of them are known.
From hybrid pea seeds, Mendel grew plants that self-pollinated, and sowed the resulting seeds again. As a result, the second generation of hybrids, or F 2 hybrids, was obtained. Among the latter, a split in each pair of alternative characters was found in a ratio of approximately 3:1, i.e. three quarters of the plants had dominant characters (purple flowers, yellow seeds, smooth seeds, etc.) and one quarter had recessive characters (white flowers, green seeds, wrinkled seeds, etc.). Consequently, the recessive trait in the F 1 hybrid did not disappear, but was only suppressed and reappeared in the second generation. This generalization was later called Mendel's second law, or law of splitting.
Segregation is a phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some of which carry a recessive trait.
Mendel's second law: when two descendants of the first generation are crossed with each other (two heterozygous individuals), in the second generation a splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1 (Fig. 3).
Figure 3 – Character splitting scheme
when crossing F 1 hybrids
G. Mendel explained the splitting of characters in the offspring when crossing heterozygous individuals by the fact that gametes are genetically pure, that is, they carry only one gene from an allelic pair. The law of gamete purity can be formulated as follows: during the formation of germ cells, only one gene from an allelic pair ends up in each gamete.
It should be borne in mind that the use of the hybridological method for analyzing the inheritance of traits in any species of animals or plants involves the following crosses:
crossing parental forms (P) that differ in one (monohybrid crossing) or several pairs (polyhybrid crossing) of alternative characters and obtaining first-generation hybrids (F 1);
crossing F 1 hybrids with each other and obtaining second generation hybrids (F 2);
mathematical analysis of crossing results.
Subsequently, Mendel moved on to the study of dihybrid crossing.
Dihybrid cross- this is a crossing in which two pairs of alleles are involved (paired genes are allelic and are located only on homologous chromosomes).
In dihybrid crossing, G. Mendel studied the inheritance of traits for which genes lying in different pairs of homologous chromosomes are responsible. In this regard, each gamete must contain one gene from each allelic pair.
Hybrids that are heterozygous for two genes are called diheterozygous, and if they differ in three or many genes, they are called tri- and polyheterozygous, respectively.
More complex dihybrid crossing schemes, recording of F 2 genotypes and phenotypes are carried out using the Punnett lattice. Let's look at an example of such a crossing. For crossing, two original homozygous parent forms: the first form had yellow and smooth seeds; the second form had green and wrinkled seeds (Fig. 4).
Figure 4 – Dihybrid crossing of pea plants,
seeds differing in shape and color
Yellow color and smooth seeds are dominant characteristics; green color and wrinkled seeds are recessive traits. First generation hybrids crossed with each other. In the second generation, phenotypic cleavage was observed in the ratio 9:3:3:1, or (3+1) 2 , after self-pollination of the F 1 hybrids, wrinkled and green seeds reappeared in accordance with the law of cleavage.
The parent plants in this case have the genotypes AABB and aabb, and the genotype of the F 1 hybrids is AaBb, i.e. it is diheterozygous.
Thus, when crossing heterozygous individuals that differ in several pairs of alternative traits, the offspring exhibit phenotypic cleavage in the ratio (3+1) n, where n is the number of pairs of alternative traits.
Genes that determine the development of different pairs of traits are called non-allelic.
The results of dihybrid and polyhybrid crossings depend on whether the genes that determine the traits under consideration are located on the same chromosome or on different chromosomes. Mendel came across traits whose genes were in different couples 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. Subsequently, it turned out that of the seven pairs of traits studied by Mendel in peas, which have a diploid chromosome number of 2n = 14, the genes responsible for one of the pairs of traits were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since linkage between these genes was not observed due to the large distance between them).
Based on his research, Mendel derived the third law - the law of independent inheritance of traits, or independent combination of genes.
Each pair of allelic genes (and the alternative traits controlled by them) is inherited independently of each other.
The law of independent combination of genes forms the basis of combinative variability observed during crossing in all living organisms. Note also that, unlike Mendel’s first law, which is always valid, the second law is valid only for genes localized in different pairs of homologous chromosomes. This is due to the fact that non-homologous chromosomes are combined in the cell independently of each other, which was proven not only by studying the nature of inheritance of traits, but also by direct cytological methods.
When studying the material, pay attention to cases of violations of regular phenotypic cleavages caused by the lethal effect of individual genes.
Heredity and variability. Heredity and variability are the most important properties characteristic of all living organisms.
Hereditary, or genotypic, variability is divided into combinative and mutational.
Combinative variation is called variability, which is based on the formation of recombinations, i.e., such combinations of genes that the parents did not have.
The basis of combinative variability is the sexual reproduction of organisms, as a result of which a huge variety of genotypes arises. Three processes serve as virtually unlimited sources of genetic variation:
Independent segregation of homologous chromosomes in the first meiotic division. It is the independent combination of chromosomes during meiosis that is the basis of G. Mendel’s third law. The appearance of green smooth and yellow wrinkled pea seeds in the second generation from crossing plants with yellow smooth and green wrinkled seeds is an example of combinative variability.
Mutual exchange of sections of homologous chromosomes, or crossing over. It creates new linkage groups, i.e. it serves as an important source of genetic recombination of alleles. Recombinant chromosomes, once in the zygote, contribute to the appearance of characteristics that are atypical for each of the parents.
Random combination of gametes during fertilization.
These sources of combinative variability act independently and simultaneously, ensuring a constant “shuffling” of genes, which leads to the emergence of organisms with a different genotype and phenotype (the genes themselves do not change). However, new gene combinations break down quite easily when passed on from generation to generation.
An example of combinative variability. The night beauty flower has a gene for red petals A and a gene white A. Organism Aa has pink petals. Thus, the night beauty does not have the gene Pink colour, the color pink comes from the combination (combination) of the red and white genes.
The person has the hereditary disease sickle cell anemia. AA is the norm, aa is death, Aa is SKA. With SCD, a person cannot tolerate increased physical activity, and he does not suffer from malaria, i.e., the causative agent of malaria, Plasmodium falciparum, cannot feed on the wrong hemoglobin. This feature is useful in the equatorial zone; There is no gene for it, it arises from a combination of genes A and a.
Thus, hereditary variability is enhanced by combinative variability. Having arisen, individual mutations find themselves in the vicinity of other mutations and become part of new genotypes, i.e., many combinations of alleles arise. Any individual is genetically unique (with the exception of identical twins and individuals resulting from asexual reproduction of a clone with a single cell as its ancestor). So, if we assume that in each pair of homologous chromosomes there is only one pair of allelic genes, then for a person who has a haploid set of chromosomes equal to 23, the number of possible genotypes will be 3 to the 23 power. Such a huge number of genotypes is 20 times greater than the number of all people on Earth. However, in reality, homologous chromosomes differ in several genes and the phenomenon of crossing over is not taken into account in the calculation . Therefore, the number of possible genotypes is expressed in an astronomical number, and we can confidently say that the emergence of two identical people almost impossible (with the exception of identical twins arising from a single fertilized egg). This, in particular, implies the possibility of reliably determining identity from the remains of living tissue, confirming or excluding paternity.
Thus, the exchange of genes due to the crossing of chromosomes in the first division of meiosis, the independent and random recombination of chromosomes in meiosis and the randomness of the fusion of gametes during the sexual process are three factors that ensure the existence of combinative variability. Mutational variability of the genotype itself.
Mutations are sudden, inherited changes in genetic material that lead to changes in certain characteristics of an organism.
The main provisions of mutation theory were developed by the scientist G. De Vries in 1901 – 1903 and boil down to the following:
Mutations arise suddenly, spasmodically, as discrete changes in characteristics;
Distinguished from non-hereditary changes, mutations are qualitative changes that are passed on from generation to generation;
Mutations manifest themselves in different ways and can be both beneficial and harmful, both dominant and recessive;
The probability of detecting mutations depends on the number of individuals examined;
Similar mutations may occur repeatedly;
Mutations are undirected (spontaneous), i.e., any part of the chromosome can mutate, causing changes in both minor and vital signs.
Almost any change in the structure or number of chromosomes, in which the cell retains the ability to reproduce itself, causes a hereditary change in the characteristics of the organism.
According to the nature of the change in the genome, i.e., the set of genes contained in a haploid set of chromosomes, gene, chromosomal and genomic mutations are distinguished.
Gene, or point, mutations are the result of changes in the nucleotide sequence in a DNA molecule within one gene.
Such a change in the gene is reproduced during transcription in the structure of the mRNA; it leads to a change in the sequence of amino acids in the polypeptide chain formed during translation on ribosomes. As a result, another protein is synthesized, which leads to a change in the corresponding characteristic of the body. This is the most common type of mutation and the most important source of hereditary variability in organisms.
Chromosomal mutations (rearrangements, or aberrations) are changes in the structure of chromosomes that can be identified and studied under a light microscope.
Various types of rearrangements are known:
a lack of – loss of the terminal sections of a chromosome;
Deletion – loss of a section of a chromosome in its middle part;
Duplication – two-or repetition genes localized in a specific region of the chromosome;
Inversion – rotation of a section of a chromosome by 180°, as a result of which genes in this section are located in the reverse sequence compared to the usual one;
Translocation – change in the position of any part of a chromosome in the chromosome set. The most common type of translocations are reciprocal, in which regions are exchanged between two non-homologous chromosomes. A section of a chromosome can change its position without reciprocal exchange, remaining in the same chromosome or being included in some other one.
Genomic mutations are changes in the number of chromosomes in the genome of body cells. This phenomenon occurs in two directions: towards an increase in the number of entire haploid sets (polyploidy) and towards the loss or inclusion of individual chromosomes (aneuploidy).
Polyploidy – multiple increase in the haploid set of chromosomes. Cells with different numbers of haploid sets of chromosomes are called triploid (3 n), tetraploid (4 n), hexaploid (6 n), octaploid (8 n), etc. Most often, polyploids are formed when the order of chromosome divergence to the cell poles is disrupted during meiosis or mitosis. Polyploidy results in changes in the characteristics of an organism and is therefore an important source of variation in evolution and selection, especially in plants. This is due to the fact that hermaphroditism (self-pollination), apomixis (parthenogenesis) and vegetative propagation are very widespread in plant organisms. Therefore, about a third of plant species common on our planet – polyploids, and in the sharply continental conditions of the high-mountain Pamirs, up to 85% of polyploids grow. Almost all cultivated plants are also polyploids, which, unlike their wild relatives, have larger flowers, fruits and seeds, and more nutrients accumulate in storage organs (stems, tubers). Polyploids adapt more easily to unfavorable conditions life, easier to bear low temperatures and drought. That is why they are widespread in the northern and high mountain regions.
Gregor Mendel - an Austrian botanist who studied and described Mendel's Laws - these are still played to this day important role in the study of the influence of heredity and the transmission of hereditary characteristics.
In his experiments, the scientist crossed different kinds peas, differing in one alternative characteristic: color of flowers, smooth-wrinkled peas, stem height. Besides, distinctive feature Mendel's experiments began to use so-called "pure lines", i.e. offspring resulting from self-pollination of the parent plant. Mendel's laws, formulation and short description will be discussed below.
Having studied and meticulously prepared an experiment with peas for many years: using special bags to protect the flowers from external pollination, the Austrian scientist achieved incredible results at that time. A thorough and lengthy analysis of the data obtained allowed the researcher to deduce the laws of heredity, which were later called “Mendel’s Laws.”
Before we begin to describe the laws, we should introduce several concepts necessary for understanding this text:
Dominant gene- a gene whose trait is manifested in the body. Designated A, B. When crossed, such a trait is considered conditionally stronger, i.e. it will always appear if the second parent plant has conditionally weaker characteristics. This is what Mendel's laws prove.
Recessive gene - the gene is not expressed in the phenotype, although it is present in the genotype. Designated capital letter a,b.
Heterozygous - a hybrid whose genotype (set of genes) contains both a dominant and a certain trait. (Aa or Bb)
Homozygous - hybrid , possessing exclusively dominant or only recessive genes, responsible for a certain sign. (AA or bb)
Mendel's Laws, briefly formulated, will be discussed below.
Mendel's first law, also known as the law of hybrid uniformity, can be formulated as follows: the first generation of hybrids resulting from crossing pure lines of paternal and maternal plants has no phenotypic (i.e. external) differences in the trait being studied. In other words, all daughter plants have the same color of flowers, stem height, smoothness or roughness of peas. Moreover, the manifested trait phenotypically exactly corresponds to the original trait of one of the parents.
Mendel's second law or the law of segregation states: the offspring of heterozygous hybrids of the first generation during self-pollination or inbreeding have both recessive and dominant characters. Moreover, splitting occurs according to the following principle: 75% are plants with a dominant trait, the remaining 25% are with a recessive trait. Simply put, if the parent plants had red flowers (dominant trait) and yellow flowers (recessive trait), then the daughter plants will have 3/4 red flowers and the rest yellow.
Third And last Mendel's law, which is also called in general outline means the following: when crossing homozygous plants with 2 or more different signs(that is, for example, a tall plant with red flowers (AABB) and a short plant with yellow flowers(aabb), the studied characters (stem height and flower color) are inherited independently. In other words, the result of crossing can be tall plants with yellow flowers (Aabb) or low ones with red ones (aaBb).
Mendel's laws, discovered in the mid-19th century, gained recognition much later. On their basis the whole modern genetics, and after it - selection. In addition, Mendel's laws confirm the great diversity of species that exist today.
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 molecular mechanisms heredity. Although three laws are usually described in Russian-language textbooks, the “first law” was not discovered by Mendel. Special meaning One of the regularities discovered by Mendel is the “gamete purity hypothesis”.
Story
IN early XIX 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. Finding such a reliable method and mathematical analysis the results that helped create the theory of heredity are 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 general patterns 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 (not producing segregation according to the studied characteristics during self-pollination) varieties. Then he carried out artificial hybridization of varieties, and crossed the resulting hybrids with each other. He studied the inheritance of seven traits by studying total 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 for data analysis. Based on his knowledge of probability theory, he understood the need for analysis large number 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 being studied - 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. Offspring from tall and low plants was high. 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 opposite signs are not in relation complete dominance(when one always suppresses the other in heterozygous individuals), and in relation to 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 character(performed 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 combined into gametes randomly. 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 more complex nature 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.