The patterns of inheritance of traits during sexual reproduction were established by G. Mendel. It is necessary to have a clear understanding of the 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 parental forms differ from each other in one pair of contrasting, alternative characters.
Consequently, with such crossing, the patterns of inheritance of only two variants of the trait are traced, the development of which is due to a pair of allelic genes. Examples of monohybrid crossing carried out by G. Mendel are crossings of peas with such clearly visible alternative traits as purple and white flowers, yellow and green color of unripe fruits (beans), smooth and wrinkled surface of seeds, their yellow and green color, etc.
Uniformity of hybrids of the first generation (Mendel's first law). When crossing peas with purple (AA) and white (aa) flowers, Mendel found that all hybrid plants of the first generation (F 1) had purple flowers (Fig. 2).
Figure 2 – Monohybrid cross scheme
In this case, the white color of the flower did not appear. When crossing plants that have a smooth and wrinkled seed shape, hybrids will have smooth seeds. G. Mendel also established that all F 1 hybrids turned out to be uniform (homogeneous) for each of the seven traits he studied. Consequently, in hybrids of the first generation, out of a pair of parental alternative traits, only one appears, and the trait of the other parent, as it were, disappears.
Alternative signs are signs that are mutually exclusive, contrasting.
The phenomenon of predominance in F 1 hybrids of the traits of one of the parents Mendel called dominance, and the corresponding trait - dominant. He called the traits that do not appear in F 1 hybrids recessive. Since all first-generation hybrids are uniform, this phenomenon has been called Mendel's first laws, or the law of uniformity of first-generation hybrids, and also 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 make up one pair, and each of the members 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 is dominant and recessive, 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 the same alleles of a particular gene (AA or aa), then it is called homozygous for this gene (or trait), and if different (Aa), then heterozygous. Therefore, an allele is a form of existence of a gene. An example of a three-allelic 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 seeds of peas, Mendel grew plants that he self-pollinated, and the resulting seeds were sown again. The result was a second generation of hybrids, or F 2 hybrids. Among the latter, splitting was found for each pair of alternative traits in a ratio of approximately 3: 1, i.e. three-quarters of the plants had dominant traits (purple flowers, yellow seeds, smooth seeds, etc.) and one quarter were recessive (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 splitting law.
Splitting 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 are recessive.
Mendel's second law: when two descendants of the first generation are crossed with each other (two heterozygous individuals), in the second generation, splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1 (Fig. 3).
Figure 3 - Feature splitting scheme
when crossing hybrids F 1
G. Mendel explained the splitting of traits in the offspring when heterozygous individuals were crossed 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 enters each gamete.
It should be borne in mind that the use of the hybridological method for the analysis of the inheritance of traits in any animal or plant species involves the following crosses:
crossing parental forms (P) that differ in one (monohybrid crossing) or several pairs (polyhybrid crossing) of alternative traits and obtaining first-generation hybrids (F 1);
crossing F 1 hybrids with each other and obtaining second generation hybrids (F 2);
mathematical analysis of the results of crossing.
Later, Mendel moved on to the study of dihybrid crossing.
Dihybrid cross- this is a crossing in which two pairs of alleles participate (paired genes are allelic and are located only on homologous chromosomes).
With dihybrid crossing, G. Mendel studied the inheritance of traits for which genes located 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 more genes, they are called tri- and polyheterozygous, respectively.
More complex schemes of dihybrid crossings, recording of F 2 genotypes and phenotypes are carried out using the Punnett lattice. Consider an example of such a crossing. Two initial homozygous parental forms were taken for crossing: 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 that differ in shape and color
Yellow color and smooth seeds are dominant characters; green color and wrinkled seeds are recessive traits. Hybrids of the first generation - crossed with each other. In the second generation, phenotypic splitting 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 splitting law.
In this case, the parent plants have the genotypes AABB and aabb, and the genotype of the F 1 hybrids is AaBb, i.e., it is diheterozygous.
Thus, when heterozygous individuals that differ in several pairs of alternative traits are crossed, phenotypic splitting is observed in the offspring 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 crosses depend on whether the genes that determine the traits considered are located on the same or on different chromosomes. Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes.
During meiosis, homologous chromosomes of different pairs combine in gametes randomly. If the paternal chromosome of the first pair got into the gamete, then both the paternal and maternal chromosomes of the second pair can get into this gamete with equal probability. 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, in which the diploid number of chromosomes is 2 n = 14, the genes responsible for one of the pairs of traits were located on the same chromosome. However, Mendel did not find a violation of the law of independent inheritance, since the linkage between these genes was not observed due to the large distance between them).
On the basis of 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 alternative traits controlled by them) is inherited independently of each other.
The law of independent combination of genes is 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 located in different pairs of homologous chromosomes. This is due to the fact that non-homologous chromosomes are combined in a cell independently of each other, which was proved not only when studying the nature of the inheritance of traits, but also by direct cytological method.
When studying the material, pay attention to cases of violations of the regular splitting of the phenotype caused by the lethal action 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.
Variability is called combinative, which is based on the formation of recombinations, i.e. such combinations of genes that the parents did not have.
Combinative variability is based on sexual reproduction of organisms, as a result of which a huge variety of genotypes arises. Three processes serve as almost unlimited sources of genetic variability:
Independent divergence 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, that is, it serves as an important source of genetic recombination of alleles. Recombinant chromosomes, once in the zygote, contribute to the appearance of signs that are atypical for each of the parents.
Random combination of gametes during fertilization.
These sources of combinative variability act independently and simultaneously, while providing 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 combinations of genes fall apart quite easily when passed from generation to generation.
An example of combinative variability. The night beauty flower has the gene for the red color of the petals A and the gene white color a. Aa's organism has pink petals. Thus, the nocturnal beauty does not have a pink color gene, pink color occurs when a combination (combination) of a red and white gene.
A person has an inherited disease of sickle cell anemia. AA - the norm, aa - death, Aa - SKA. With SKA, a person cannot tolerate increased physical exertion, while he does not suffer from malaria, that is, the causative agent of malaria, Plasmodium malaria, cannot feed on the wrong hemoglobin. Such a feature is useful in the equatorial belt; there is no gene for it, it occurs when genes A and a are combined.
Thus, hereditary variability is enhanced by combinative variability. Having arisen, individual mutations are in the vicinity of other mutations, are part of new genotypes, i.e., many combinations of alleles arise. Any individual is genetically unique (with the exception of identical twins and individuals that have arisen due to the asexual reproduction of a clone that has a single cell ancestor). So, if we assume that in each pair of homologous chromosomes there is only one pair of allelic genes, then for a person whose haploid set of chromosomes is 23, the number of possible genotypes will be 3 to the 23rd 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 calculation does not take into account the phenomenon of crossing over. . Therefore, the number of possible genotypes is expressed by an astronomical number, and it is safe to say that the occurrence of two identical people is almost improbable (with the exception of identical twins that arose from the same fertilized egg). From this, in particular, follows the possibility of a reliable determination of personality by the remnants of living tissues, confirmation or exclusion of paternity.
Thus, gene exchange due to chromosome crossing in the first division of meiosis, independent and random recombination of chromosomes in meiosis, and random fusion of gametes in 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 the genetic material, leading to a change in certain characteristics of the organism.
The main provisions of the mutation theory were developed by the scientist G. De Vries in 1901 – 1903 and boil down to the following:
Mutations occur suddenly, abruptly, as discrete changes in traits;
Unlike non-hereditary changes, mutations are qualitative changes that are passed down from generation to generation;
Mutations manifest themselves in many ways and can be both beneficial and harmful, both dominant and recessive;
The probability of detecting mutations depends on the number of individuals studied;
Similar mutations can occur repeatedly;
Mutations are non-directional (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 totality of genes contained in the haploid set of chromosomes, gene, chromosomal and genomic mutations are distinguished.
Gene, or point, mutations are the result of a change in the nucleotide sequence in a DNA molecule within a single gene.
Such a change in the gene is reproduced during transcription in the structure of 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 feature of the organism. 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 terminal sections of the chromosome;
deletion – loss of a chromosome segment in its middle part;
duplication – two- or multiple repetition of genes localized in a certain region of the chromosome;
Inversion – rotation of a chromosome segment by 180°, as a result of which the genes in this region are located in the reverse order compared to the usual one;
Translocation – change in the position of any part of the chromosome in the chromosome set. The most common type of translocations are reciprocal, in which regions are exchanged between two non-homologous chromosomes. A segment of a chromosome can change its position even 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 an organism's cells. This phenomenon occurs in two directions: towards an increase in the number of whole haploid sets (polyploidy) and towards the loss or inclusion of individual chromosomes (aneuploidy).
Polyploidy – fold increase in the haploid set of chromosomes. Cells with a different number 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 divergence of chromosomes to the poles of the cell is violated during meiosis or mitosis. Polyploidy results in a change in the traits of an organism and is therefore an important source of variability in evolution and selection, especially in plants. This is due to the fact that hermaphroditism (self-pollination), apomixis (parthenogenesis) and vegetative reproduction are very widespread in plant organisms. Therefore, about a third of the plant species common on our planet, – polyploids, and in the sharply continental conditions of the high-mountainous Pamir grows up to 85% of polyploids. Almost all cultivated plants are also polyploids, which, unlike their wild relatives, have larger flowers, fruits and seeds, and more nutrients accumulate in the storage organs (stem, tubers). Polyploids adapt more easily to adverse living conditions, more easily tolerate low temperatures and drought. That is why they are widespread in the northern and high mountain regions.
Formula 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 2 of Mendel's law The law of splitting, or the second law of Mendel Mendel When two heterozygous offspring of the first generation are crossed among themselves in the second generation, splitting is observed in a certain numerical ratio: according to the phenotype 3:1, according to the genotype 1:2:1.
Formulation 3 of Mendel's law The law of independent inheritance (Mendel's third law) When two homozygous individuals that differ from each other in two (or more) pairs of alternative traits are crossed, genes and their corresponding traits 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 in all respects. In the second generation, splitting of phenotypes according to the formula 9: 3: 3: 1 was observed)
P AA BB aa bb x yellow, smooth seeds, green, wrinkled seeds G (gametes) ABabab F1F1 Aa Bb yellow, smooth seeds 100% 3 Mendel's law DIGIBRID CROSSING. For experiments, peas with smooth yellow seeds were taken as the mother plant, and green wrinkled seeds were taken as the father plant. In the first plant, both traits were dominant (AB), and in the second, both traits were recessive (ab
The first generation after crossing had a dominant phenotype in all respects. (yellow and smooth peas) In the second generation, splitting of phenotypes according to the formula 9:3:3:1 was observed. 9/16 yellow smooth peas, 3/16 yellow wrinkled peas, 3/16 green smooth peas, 1/16 green wrinkled peas.
Task 1. In spaniels, the black color of the coat dominates over the coffee, and the short coat dominates over the long one. The hunter bought a short-haired black dog and, in order to be sure that it was a purebred, conducted an analysis cross. 4 puppies were born: 2 short-haired black, 2 short-haired coffee. What is the genotype of the dog bought by the hunter? Problems for dihybrid crossing.
Problem 2. In a tomato, the red color of the fruit dominates over the yellow color, and the tall 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. Hybrids of the first generation crossed with each other, received 160 hybrid plants of the second generation. How many types of gametes does a plant of the first generation form? How many plants in the first generation have a red fruit color and a tall stem? How many different genotypes are there among second generation plants with red fruits and tall stems? How many plants in the second generation have yellow fruits and tall stems? How many plants in the second generation have yellow fruits and low stems?
Task 3 In humans, brown eyes dominate over 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 the birth of a blue-eyed left-handed child in this family (%)?
Task 4 Crestedness in chickens dominates over the absence of a crest, and the black color of plumage dominates over brown. 48 chickens were obtained from crossing a heterozygous black hen without a crest with a heterozygous brown crested rooster. 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 crested black chickens will there be? How many black chickens will there be without a crest?
Problem 5 In cats, the short hair of the Siamese breed dominates over the long hair of the Persian breed, and the black color of the Persian breed is dominant in relation to the fawn color of the Siamese. Siamese cats were crossed with Persians. 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 are there in the second generation? How many different phenotypes were produced in the second generation? How many kittens in the second generation look like Siamese cats? How many kittens in the second generation 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 the second marriage of this man with another brown-eyed right-handed man, 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 the gene for normal flattened ears, and the gene for non-red hair dominates the gene for redheads. What kind of offspring can be expected from the marriage of a lop-eared redhead, heterozygous for the first trait, a man with a heterozygous non-redhead woman with normal flattened ears. Option 2 1) In humans, clubfoot (P) dominates over the normal structure of the foot (P) and normal carbohydrate metabolism (O) over diabetes mellitus. A woman with a normal foot structure and a normal metabolism married a clubfoot man. From this marriage, two children were born, one of whom developed a clubfoot, and the other had diabetes. Determine the genotype of the parents from the phenotype of their children. What phenotypes and genotypes of children are possible in this family? 2) In humans, the brown eye gene dominates the blue eye gene, and the ability to own right hand over left-handedness. Both pairs of genes are located on different chromosomes. What kind of children can 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 hands.
Let's solve problems 1. In humans, normal carbohydrate metabolism dominates over the recessive gene responsible for the development diabetes. The daughter of healthy parents is sick. Determine whether a healthy child can be born in this family and what is the probability of this event? 2. In humans, brown eyes dominate over blue. The ability to use the right hand better 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 offspring should be expected in this pair?
The law of splitting Mendel planted the hybrids of the first generation of peas (which were all yellow) and allowed them to self-pollinate. As a result, seeds were obtained, which are hybrids of the second generation (F2). Among them, not only yellow, but also green seeds were already encountered, that is, splitting occurred. At the same time, the ratio of yellow to green seeds was 3:1. The appearance of green seeds in the second generation proved that this trait did not disappear or dissolve in hybrids of the first generation, but existed in a discrete state, but was simply suppressed. The concepts of the dominant and recessive allele of a gene were introduced into science (Mendel called them differently). The dominant allele overrides the recessive one. A pure line of yellow peas has two dominant alleles, AA. A pure line of green peas has two recessive alleles - aa. In meiosis, only one allele enters each gamete.
Laws of Mendel. fundamentals of genetics
Gregor Mendel in the 19th century, conducting research on peas, identified three main patterns of inheritance of traits, which are called the three laws of Mendel.
The first two laws relate to monohybrid crossing (when parental forms are taken that differ in only one trait), the third law was revealed during dihybrid crossing (parental forms are examined according to two different traits).
Attention
Mendel's first law. The law of uniformity of hybrids of the first generation Mendel took for crossing pea plants that differ in one trait (for example, in seed color).
Some had yellow seeds, others green. After cross-pollination, hybrids of the first generation (F1) are obtained.
All of them had yellow seeds, that is, they were uniform.
The phenotypic trait that determines the green color of the seeds has disappeared.
Mendel's second law.
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Gregor Mendel is an Austrian botanist who studied and described the pattern of inheritance of traits.
Mendel's laws are the basis of genetics, to this day playing important role in the study of the influence of heredity and the transmission of hereditary traits.
In his experiments, the scientist crossed different kinds peas that differ in one alternative feature: shade of flowers, smooth-wrinkled peas, stem height.
Moreover, distinctive feature Mendel's experiments was the use of the so-called "clean lines", i.e.
offspring resulting from self-pollination of the parent plant. Mendel's laws, formulation and short description will be discussed below.
For many years, studying and meticulously preparing an experiment with peas: protecting flowers from external pollination with special bags, the Austrian scientist achieved incredible results at that time.
Lecture No. 17. Basic concepts of genetics. laws of mendel
The expression of some genes can be highly dependent on environmental conditions. For example, some alleles appear phenotypically only at a certain temperature at a certain phase of an organism's development. This can also lead to violations of the Mendelian splitting.
Modifier genes and polygenes. In addition to the main gene that controls this trait, the genotype may contain several more modifier genes that modify the manifestation of the main gene.
Important
Some traits may be determined not by one gene, but by a whole complex of genes, each of which contributes to the manifestation of a trait.
Such a trait is called polygenic. All this also introduces violations in the splitting of 3:1.
Mendel's laws
The state (allele) of a trait that appears in the first generation is called dominant, and the state (allele) that does not appear in the first generation of hybrids is called recessive. "Inclinations" of signs (according to modern terminology - genes) G.
Mendel proposed to denote by the letters of the Latin alphabet.
Conditions belonging to the same pair of traits are designated by the same letter, but the dominant allele is large, and the recessive allele is small.
Mendel's second law. When heterozygous hybrids of the first generation are crossed with each other (self-pollination or inbreeding), individuals with both dominant and recessive states of traits appear in the second generation, i.e. there is a split that occurs in certain relationships. So, in Mendel's experiments on 929 plants of the second generation, 705 with purple flowers and 224 with white flowers turned out to be.
one more step
Thus, peas with yellow seeds form only gametes containing the A allele.
Peas with green seeds form gametes containing the allele a.
When crossed, they produce Aa hybrids (first generation).
Since the dominant allele in this case completely suppresses the recessive one, the yellow color of the seeds was observed in all hybrids of the first generation.
First generation hybrids already produce gametes A and a. During self-pollination, randomly combining with each other, they form the genotypes AA, Aa, aa.
Moreover, the heterozygous Aa genotype will occur twice as often (since Aa and aA) than each homozygous one (AA and aa).
Thus we get 1AA: 2Aa: 1aa. Since Aa produces yellow seeds like AA, it turns out that for 3 yellows there is 1 green.
Mendel's third law. The Law of Independent Inheritance of Different Traits Mendel carried out a dihybrid cross, that is,
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All possible combinations of male and female gametes can be easily identified using the Punnett lattice, in which the gametes of one parent are written horizontally, and the gametes of the other parent are written vertically. The genotypes of the zygotes formed by the fusion of gametes are entered into the squares.
If we take into account the results of splitting for each pair of traits 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, in a dihybrid cross, each pair of traits, when split in the offspring, behaves in the same way as in a monohybrid cross, i.e.
i.e. regardless of the other pair of features.
One pure line of peas had yellow and smooth seeds, while the second line had green and wrinkled ones.
All of their first generation hybrids had yellow and smooth seeds. In the second generation, as expected, splitting occurred (a part of the seeds showed a green color and wrinkling). However, plants were observed not only with yellow smooth and green wrinkled seeds, but also with yellow wrinkled and green smooth ones.
In other words, there was a recombination of characters, indicating that the inheritance of the color and shape of the seeds occurs independently of each other.
Indeed, if the genes for seed color are located in one pair of homologous chromosomes, and the genes that determine the shape are in the other, then during meiosis they can be combined independently of each other.
The laws of mendel are short and clear
The rediscovery of Mendel's laws by Hugh de Vries in Holland, Carl Correns in Germany and Erich Tschermak in Austria did not occur until 1900. At the same time, the archives were raised and the old works of Mendel were found.
At this time, the scientific world was already ready to accept genetics.
Her triumphal procession began. They checked the validity of Mendelian inheritance laws (Mendelization) on more and more new plants and animals and received invariable confirmations. All exceptions to the rules quickly developed into new phenomena of the general theory of heredity. At present, the three fundamental laws of genetics, the three laws of Mendel, are formulated as follows. Mendel's first law. Uniformity of hybrids of the first generation.
All signs of an organism can be in their dominant or recessive manifestation, which depends on the presence of alleles of a given gene.
A thorough and lengthy analysis of the data obtained allowed the researcher to derive the laws of heredity, which later became known as Mendel's Laws.
Before proceeding to the description of the laws, it is necessary to introduce several concepts necessary for understanding this text: Dominant gene - a gene whose trait is manifested in the body.
It is denoted by a capital letter: A, B. When crossing, such a trait is considered conditionally stronger, i.e.
it will always appear if the second parent plant has conditionally less weak signs. This is what Mendel's laws prove. Recessive gene - a gene in the phenotype is not manifested, although it is present in the genotype. Capitalized letter a,b. Heterozygous - a hybrid in whose genotype (set of genes) there is both a dominant and a recessive gene for some trait.
During fertilization, gametes are combined according to the rules of random combinations, but with equally likely for each. In the resulting zygotes, various combinations of genes arise. An independent distribution of genes in the offspring and the emergence of various combinations of these genes during dihybrid crossing is possible only if the pairs of allelic genes are located in different pairs of homologous chromosomes. Thus, Mendel's third law is formulated as follows: when two homozygous individuals are crossed, differing from each other in two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other. Recessives flew. Mendel obtained the same numerical ratios when splitting the alleles of many pairs of traits. This in particular implied the same survival of individuals of all genotypes, but this may not be the case.
Lesson Plan #18
1 Educational:
2 Developing:
During the classes:
II main part
1 Checking homework
.
What is a genotype, phenotype?
♂ ,♀ ?
2 Explanation of new material
D) What is the purity of gametes?
III Summing up the lesson
IV Homework
1 Entries in a notebook
Lesson #18
Topic:
MONOHYBRID CROSSING
hybridization, hybrid, and a separate individual hybridoma.
dominance.
In the offspring obtained from crossing hybrids of the first generation, the phenomenon of splitting is observed: a quarter of individuals from hybrids of the second generation carry a recessive trait, three-quarters - a dominant one.
When two offspring of the first generation are crossed with each other (two heterozygous individuals), in the second generation, splitting is observed in a certain numerical ratio: according to the phenotype 3:1, according to the genotype 1:2:1
(25% homozygous dominant, 50% heterozygous, 25% homozygous recessive)
The law of purity of gametes
What is the reason for the split? Why do individuals appear in the first, second and subsequent generations that, as a result of crossing, give offspring with dominant and recessive traits?
From 1854, for eight years, Mendel conducted experiments on crossing pea plants. He found that as a result of crossing different varieties of peas with each other, hybrids of the first generation have the same phenotype, and hybrids of the second generation have splitting of characters in certain ratios. To explain this phenomenon, Mendel made a series of assumptions, which are called the "gamete purity hypothesis", or "the law of gamete purity".
Communication between generations during sexual reproduction is carried out through germ cells (gametes). Obviously, gametes carry material hereditary factors - genes that determine the development of a particular trait.
Let's turn to the diagram on which the results are written in symbols:
The gene responsible for the dominant yellow color of the seeds will be denoted by a capital letter, for example A ; gene responsible for recessive green color - small letter a . Let us denote the combination of gametes carrying the genes A and a by the multiplication sign: A X a=Ah. As can be seen, the resulting heterozygous form (F1) has both genes, Aa. The hypothesis of gamete purity states that in a hybrid (heterozygous) individual, germ cells are pure, i.e. they have one gene from a given pair. This means that the Aa hybrid will have an equal number of gametes with the A gene and with the a gene. What combinations are possible between them? Obviously, four combinations are equally likely:
| ♂ ♀ | A | a |
| A | AA | Ah |
| a | aa | aa |
As a result of 4 combinations, combinations of AA, 2Aa and aa will be obtained. The first three will give individuals with a dominant trait, the fourth - with a recessive one. The hypothesis of gamete purity explains the cause of splitting and the observed numerical ratios. At the same time, the reasons for the difference in relation to the further splitting of individuals with dominant traits in subsequent generations of hybrids are also clear. Individuals with dominant traits are heterogeneous in their hereditary nature. One of the three (AA) will produce gametes of only one variety (A) and will not split when self-pollinated or crossed with its own kind. The other two (Aa) will give gametes of 2 varieties, splitting will occur in their offspring in the same numerical ratios as in hybrids of the second generation. The hypothesis of gamete purity establishes that the law of splitting is the result of a random combination of gametes carrying different genes (Aa ). Whether a gamete carrying an A gene will unite with another gamete carrying an A or a gene, provided that the viability of gametes is equal and their number is equal, is equally likely.
With a random nature of the connection of gametes, the overall result turns out to be statistically regular.
Thus, it was found that the splitting of traits in the offspring of hybrid plants is the result of their having two genes, A and a, responsible for the development of one trait, for example, seed color.
Mendel suggested that hereditary factors in the formation of hybrids do not mix, but remain unchanged. In the body of the F1 hybrid from crossing parents that distinguish by alternative traits, both factors are present - the dominant gene and the recessive one, but the recessive gene is suppressed. Communication between generations during sexual reproduction is carried out through germ cells - gametes. Therefore, it must be assumed that each gamete carries only one factor of the pair. Then, during fertilization, the fusion of two gametes, each of which carries a recessive gene, leads to the formation of an organism with a recessive trait that manifests itself phenotypically. The fusion of gametes carrying a dominant gene, or two gametes, one of which contains a dominant and the other a recessive gene, will lead to the development of an organism with a dominant trait.
Thus, the appearance in the second generation (F 2) of a recessive trait of one of the parents (P) can take place only if two conditions are met: 1) if the hereditary factors remain unchanged in hybrids, 2) if the germ cells contain only one hereditary factor from an allelic pair. Mendel explained the splitting of traits in offspring when heterozygous individuals were crossed by the fact that gametes are genetically pure, i.e. carry only one gene from an allelic pair.
The gamete frequency law can be formulated as follows: during the formation of germ cells, only one gene from an allelic pair enters each gamete.
Why and how does this happen? It is known that in every cell of the body there is exactly the same diploid set of chromosomes. Two homologous chromosomes contain two identical allelic genes. Two varieties of gametes are formed for this allelic pair. At fertilization, gametes carrying the same or different alleles randomly meet each other. Due to the statistical probability, with a sufficiently large number of gametes in the offspring, 25% of the genotypes will be homozygous dominant, 50% - heterozygous, 25% - homozygous recessive, i.e. the ratio is set: 1AA:2Aa:1aa. Accordingly, according to the phenotype, the offspring of the second generation during monohybrid crossing is distributed in the ratio of 3 / 4 individuals with a dominant trait, / 4 individuals with a recessive trait (3: 1).
Thus, the cytological basis for the splitting of traits in offspring during monohybrid crossing is the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis.
Analyzing cross
The hybridological method developed by Mendel for studying heredity makes it possible to establish whether an organism is homozygous or heterozygous if it has a dominant phenotype for the gene (or genes) under study. To do this, an individual with an unknown genotype and an organism homozygous for the recessive alley (s) with a recessive phenotype are crossed.
If the dominant individual is homozygous, then the offspring from such a cross will be uniform and splitting will not occur (AAhaa \u003d Aa). If the dominant individual is heterozygous, then splitting will occur in a ratio of 1: 1 according to the phenotype (Aa x aa \u003d Aa, aa). This result of crossing is a direct proof of the formation at one of the parents of two varieties of gametes, i.e. his heterozygosity.
In dihybrid crosses, splitting for each trait occurs independently of the other trait. A dihybrid cross is two independently running monohybrid crosses, the results of which seem to overlap each other.
When two homozygous individuals are crossed, differing from each other in two or more pairs of alternative traits, the genes and their corresponding traits are inherited independently of each other and combined in all possible combinations.
The analysis of splitting is based on Mendel's laws and, in more complex cases, when individuals differ in three, four or more pairs of signs.
Lesson Plan #18
TOPIC: Monohybrid and dihybrid crossing. Mendel's laws
1 Educational:
To form knowledge about monohybrid crossing, the first law of Mendel
Show the role of Mendel's research in understanding the essence of trait inheritance
Reveal the wording of the splitting law, Mendel's second law
To reveal the essence of the gamete purity hypothesis
To form knowledge about dihybrid crossing as a method of studying heredity
Use the example of di- and polyhybrid crossing to reveal the manifestation of Mendel's third law
2 Developing:
Develop memory, expand horizons
To promote the development of the skill of using genetic symbols in solving genetic problems
During the classes:
I organizational moment
1 Familiarize students with the topic and purpose of the lesson
2 Students are given a number of tasks to complete during the lesson:
Know the formulation of Mendel's laws
Learn the patterns of inheritance of traits established by Mendel
Learn the essence of the gamete purity hypothesis
Learn the essence of dihybrid crossing
II main part
1 Checking homework
What does genetics study? What problems does genetics solve?
Define heredity and variability.
What are the stages of the embryonic period?
Explain the terms: gene, dominant and recessive genes . What development is called direct?
What genes are called allelic? What is multiple allelism?
What is a genotype, phenotype?
What is the peculiarity of the hybridological method?
What does genetic symbolism mean: P, F1, F2, ♂ ,♀ ?
2 Explanation of new material
Monohybrid cross; Mendel's first law
Mendel's second law; gamete frequency law
The essence of dihybrid crossing; Mendel's third law
3 Fixing new material
a) Formulate 1 Mendel's law.
b) What kind of cross is called monohybrid?
C) Formulate the second law of Mendel
D) What is the purity of gametes?
E) What rules and patterns are manifested in dihybrid crossing?
E) How is the third law of Mendel formulated?
III Summing up the lesson
IV Homework
1 Entries in a notebook
2 Textbook by V.B. Zakharov, S.T. Mamontov "Biology" (pp. 266-277)
3 Textbook by Yu.I. Polyansky " General biology» (pp. 210-217)
Lesson #18
Topic: Monohybrid and dihybrid crosses. Laws of Mendel.
1. Monohybrid crossing. The rule of uniformity of hybrids of the first generation is the first law of heredity established by G. Mendel.
2. Mendel's second law - the law of splitting. Gamete purity hypothesis
3. Dihybrid and polyhybrid crossing. Mendel's third law is the law of independent combination of features.
MONOHYBRID CROSSING
To illustrate Mendel's first law, let us recall his experiments on monohybrid crossing of pea plants. The crossing of two organisms is called hybridization, offspring from crossing two individuals with different heredity is called hybrid, and a separate individual hybridoma.
Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative (mutually exclusive) traits.
For example, when crossing peas with yellow (dominant trait) and green seeds (recessive trait), all hybrids will have yellow seeds. The same picture is observed when crossing plants that have a smooth and wrinkled seed shape; all first-generation offspring will have a smooth seed shape. Consequently, in a hybrid, the first generation, only one of each pair of alternative traits appears. The second sign, as it were, disappears, does not appear. The predominance of the trait of one of the parents in a hybrid Mendel called dominance. According to the phenotype, all hybrids have yellow seeds, and according to the genotype they are heterozygous (Aa). Thus, the whole generation is uniform.
Mendel's first law is the law of dominance.
The law of uniformity of the first generation of hybrids, or Mendel's first law- also called the law of dominance, since all individuals of the first generation have the same manifestation of the trait. 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 (F 1) will be uniform and will carry the trait of one of the parents.
Such a pattern will be observed in all cases when two organisms belonging to two pure lines are crossed, when there is a phenomenon of complete dominance of a trait (i.e., one trait completely suppresses the development of the other).
When considering the basic laws of genetics, it should be noted that they are of a statistical nature, i.e. these laws can be discovered when studying a very large number of objects. So, having studied 10 individuals of a given species, it is impossible to detect one or another law - there are too few parallel observations. The more parallel observations are made, the more clearly and vividly this or that genetic law will manifest itself.
Overview of the laws of genetics discovered by G. Mendel
Using the hybridological method of research, G. Mendel discovered the laws of independent inheritance of traits. These laws were discovered when studying the patterns of inheritance in pea plants, using monohybrid and dihybrid crosses.
1. Mendel's first law - the law of uniformity of all individuals of the first generation for any type of crossing (both mono- and polyhybrid crossing): for any crossing, all individuals of the first generation (F 1) are characterized by the same phenotype for the crossed trait.
This phenotype is determined either by a dominant trait, or intermediate traits appear, or new traits appear as a result of gene interaction. So, when crossing peas with yellow and green seeds in the first generation, all plants have yellow seeds (dominant-recessive inheritance). When crossing Violets Night Beauty with white and red flowers, all plants of the first generation have pink flowers(intermediate nature of inheritance).
For crossing take homozygous organisms. For example, the maternal organism has the genes for the yellow color of the seed (denoted as AA), and the paternal organism has the genes for the green color of the seed (denoted as aa). Then the mother's gametes (eggs) contain one gene for the yellow color of the seed (A) and the father's gametes (sperms) contain one gene for the green color of the seed (a). During fertilization, a heterozygote is formed containing the genes of the alternative traits discussed above; marked Aa. Since in this case a dominant-recessive nature of the inheritance of traits is observed, all individuals of the first generation (F 1) have yellow seeds, that is, they are characterized by the same phenotype for this trait.
In the case of an equivalent nature of the interaction of genes, an intermediate nature of inheritance is observed. In this case, heterozygous organisms also arise (designated A 1 A 2) with the same phenotype for a specific trait. So, when crossing Violets Night Beauty with white and red flowers in F 1, all plants have pink flowers.
2. Mendel's second law - the law of splitting of signs (the law of monohybrid crossing) - sometimes it is called the rule of splitting signs. This law is valid for monohybrid crossing and manifests itself when crossing different individuals obtained by monohybrid crossing in the second generation (F 2): when crossing individuals of the first generation obtained after monohybrid crossing, the offspring show splitting of characters in a certain quantitative ratio, which for dominant recessive inheritance is 3:1, and for intermediate inheritance 1:2:1 (the number 2 means that hybrids are individuals with an intermediate trait).
Consider examples.
1. By crossing pea plants with smooth seeds (F 1 obtained after crossing plants with smooth and wrinkled seeds), we get the second generation (F 2), while 3/4 of the offspring have smooth seeds, and 1/4 - wrinkled.
This phenomenon can be explained as follows. Plants of the first generation are heterozygous (let us designate them Aa). They produce two types of gametes (let's call them A and a). These gametes are characteristic of both paternal and maternal organisms. When implementing the processes of fertilization, four combinations are possible (the gene received from the mother's organism is in the first place in them, we will single it out): AA, Aa, aA and aa. The AA combination corresponds to a homozygous dominant trait (smooth seeds); combinations of Aa and aA correspond to a heterozygote (smooth seeds), and the last combination of aa is homozygous for a recessive trait. Thus, in the second generation, three different genotypes arise for this trait, and only two phenotypes correspond to them.
2. Crossing violet plants with pink flowers (F 1), we get F 2, in which 1/4 of the offspring have white flowers, 1/4 of the offspring are red, and half of the offspring (2/4) are pink. The explanation of this phenomenon is the same as for example 1, but here we observe a difference - three genotypes for this trait (A 1 A 1, A 1 A 2 and A 2 A 1; A 2 A 2) correspond to three phenotypes (white, pink and red flowers).
3. Mendel's third law - the law of polyhybrid crossing or the law of independent splitting of traits.
This law manifests itself in the second generation with dihybrid and polyhybrid (tri-, tetra-, etc.) crossing: when crossing individuals of the first generation obtained by crossing in a dihybrid (polyhybrid) type, in the offspring (in the second generation) there is a splitting of characters (for dominant-recessive nature of inheritance) in quantitative terms, expressed by the formula (3 + 1) n, where n = 2, 3, 4, etc.
For cytological explanation, it is convenient to use the Punnett lattice. Let's analyze the information given in the figure. First, we crossed plants of the common pea species with yellow and smooth seeds (we will designate the gene for the yellow color of the seed as A, and for the smooth shape - B) with plants that had green wrinkled seeds (we denote the gene for the green color of the seed as a, the gene for the wrinkled shape - b). All resulting first-generation plants are heterozygous and have yellow smooth seeds (dominant recessive inheritance in which the genes for yellow color and smooth seed surface dominate over the genes for green color and wrinkled shape). Name the law in this case.
After crossing the plants of the first generation, they received F 2 - the second generation, in which we observe the law of independent splitting of signs: 1/16 of all plants have green wrinkled seeds, 3/16 are green and smooth; 3/16 are yellow and wrinkled, while the rest (9/16) are yellow and smooth. Therefore, during dihybrid crossing, four phenotypes appear in F 2 (according to these traits).
In a dihybrid cross, each plant forms four types of gametes, and for two parents these gametes can give 16 combinations. As a result, it turns out that 1/16 of the generation is homozygous for recessive and the same number for dominant traits, and all other individuals are heterozygous for at least one trait; absolutely heterozygous (on two grounds) only 4/16 of the generation.
The calculation shows that the four phenotypes in a dihybrid cross correspond to nine phenotypes (do this calculation yourself).
It should be noted that Mendel's third law is valid if the genes responsible for these traits are in different pairs of chromosomes; thus, the seed color gene is located in one pair of homologous chromosomes, and the gene that determines the shape of the seeds is located in the other.
Probably, there are cases when the genes responsible for certain traits are contained in one pair of chromosomes. For such variations, Mendel's laws (except the first) are not applicable. These cases are subject to Morgan's law.
Morgan's law - the law of linked inheritance of traits
A number of organisms have a small number of chromosomes; therefore, many genes that determine various groups of alternative traits are located in one homologous pair of chromosomes, i.e. are linked and are transmitted to offspring together. So, in the fruit fly Drosophila, the gene that determines the length of the wings and the gene responsible for body color are located on homologous chromosomes.
Dihybrid crossing, carried out according to these traits in the second generation, will not give independent splitting of traits, that is, it will not correspond to Mendel's third law. This phenomenon was discovered by T. Morgan and formulated it in the form of the law of linked inheritance:
During dihybrid crossing of organisms in which genes are located in one pair of homologous chromosomes, in the second generation, splitting of characters is observed not according to the third, but according to the second Mendel's law.
By crossing fruit flies with dark body color and normal wings (dominant traits) with flies with shortened wings and a gray body (recessive traits), a heterozygous generation (F 1) with dark bodies and normal wings was obtained.
When crossing individuals of the first generation, organisms were obtained in which 1/4 of the generation had shortened wings and a gray body, and 1/3 of the generation had normal wings and a dark body. This is due to the fact that the genes for body color and wing length are located in one pair of homologous chromosomes, i.e., they are linked. However, among the F 2 individuals, both insects with a dark body and shortened wings and individuals with a gray body and normal wings were observed. This is due to crossing over, in which chromosomes, as a result of conjugation and crossing, exchange sections of homologous chromosomes. But these phenomena are random and do not obey mathematical laws.
Law of homologous series of hereditary variability
In the process of studying the patterns of inheritance of mutational (hereditary) variability, N. I. Vavilov discovered a law known in science as the law of homologous series of hereditary variability, which was formulated as follows:
If species and genera are genotypically related to each other, by a unity of origin, then they form series of forms of organisms similar in their characteristics, i.e., homologous series.
So, wheat, rye, barley are phylogenetically close species - genera of the class of monocotyledonous angiosperms. They are cereals. In nature, spinous forms of cereals are common, since spinousness is a form of adaptation of cereal plants against being eaten by animals. For practical needs, man has developed awnless forms, which are more convenient for economic activity than spinous ones. In the process of developing awnless varieties of cereals, all these three species, belonging to different genera, went through the same stages of "artificial evolution", giving similar intermediate forms:
awned forms → low awned forms → awnless forms.
These forms are typical for wheat, and for rye, and for barley.
Homologous series are known not only for cereals, but also for other plants.
Analyzing cross
As shown above, in order to identify patterns of inheritance of traits, it is necessary to subject homozygous individuals to primary crossing. However, the phenotype for this trait is not always a sign of the homozygosity of a given organism, for example, peas with yellow seeds can be either homozygous (AA) for a dominant trait or heterozygous (Aa). Therefore, a method for detecting homozygosity is needed, which is analyzing cross.
For analysis crosses, organisms with a recessive alternative trait are used, and these organisms are crossed with organisms whose homozygosity is to be determined. If in the first generation there is no splitting of traits, then these organisms are homozygous for the dominant trait, otherwise (organisms with recessive traits will appear in this generation), the organisms under study are heterozygous.
Consider an example. When crossing guinea pigs with short hair (recessive trait) (aa is the designation of the parent organism that gives gametes a) with pigs with long hair (dominant trait), offspring with long hair were obtained in the first generation. Conclusion - long-haired pigs are homozygous (AA - the designation of the parent organism that produces gametes A), since the zygote of the first generation will correspond to the designation Aa. The case when long-haired pigs were heterozygous, characterize yourself. Also answer the question: is it possible to use homozygous organisms with dominant traits for analyzing crosses and why? Prove your answer using cytological representations.
Gene Interaction
When studying the patterns of inheritance of traits, it is necessary to take into account the nature of the impact of some genes on others. In the previous subsections, it was shown that allelic genes have a certain effect on each other, in which either a dominant-recessive nature of the interaction is observed, or when allelic genes interact with each other, a new trait arises, intermediate between the original traits - with the same effect of genes on each other .
In genetic studies, it was found that non-allelic genes can also interact with each other, and when they interact, new signs appear in the body, i.e. a new phenotype emerges. So, when crossing chickens with pink and walnut combs, they got the first offspring of chickens with pea-shaped combs. Crossing individuals with each other led to the splitting of traits not according to Mendel's second law (as it was supposed, because outwardly crossing was monohybrid), but according to the third law - the law of independent splitting of traits.
It was found that 1/16 of the offspring had a simple crest, 3/16 - rose-shaped, 3/16 - nut-shaped, and the rest (9/16) - pea-shaped. Consequently, the pink and nut-shaped forms of the crest are not determined by one gene, but are the result of the interaction of two non-allelic genes, since the nature of the splitting of characters corresponds to dihybrid crossing.
Multiple action of genes
Geneticists have found that one gene can either affect a single specific trait, or affect several traits, i.e. have multiple actions. Thus, the watershed has a flower color gene, while the red color gene affects the color of the leaves (the watershed with red flowers has purple leaves). In addition, this gene influences the stem length and seed mass - the stem in the columbine with red flowers is longer, and the seeds have a greater mass than the seeds in the columbine with a different flower color.
The Drosophila fly has a gene that determines eye color. If Drosophila contains a gene that causes the absence of pigment in the eye, then these flies have low fecundity, a shorter lifespan and a specific coloration of the internal organs.
