Question 1. What is hybridization?
The crossing of two organisms is called hybridization.
Question 2. Which cross is called monohybrid?
Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative (mutually exclusive) characteristics.
Question 3. What phenomenon is called dominance?
G. Mendel called the predominance of the trait of one of the parents in a hybrid dominance.
Question 4. Which trait is called dominant and which is called recessive?
A trait that appears in a first-generation hybrid and suppresses the development of another trait was called dominant (from the Latin dominus - master), and the opposite, i.e. suppressed, was called recessive (from the Latin recess - retreat, removal).
Question 5. Tell us about Mendel’s experiments on monohybrid crossing of pea plants.
When Mendel crossed purple-flowered and white-flowered peas, he discovered that all of the first generation (F1) hybrid plants had purple flowers. At the same time, the white color of the flower did not appear.
Mendel also established that all F1 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. Mendel called the phenomenon of predominance of the traits of one of the parents in F1 hybrids dominance, and the corresponding trait - dominant. He called traits that do not appear in F1 hybrids recessive.
Question 6. Which organism is called homozygous; heterozygous?
If the genotype of an organism (zygote) contains two identical allelic genes, absolutely identical in nucleotide sequence, such an organism is called homozygous for this gene. An organism can be homozygous for dominant (AA or BB) or recessive (aa or bb) genes. If the allelic genes differ from each other (one of them is dominant and the other is recessive (Aa, Bb), such an organism is called heterozygous.
Question 7. Formulate Mendel’s first law. Why is this law called the law of dominance?
This law can be formulated as follows: 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.
Since all first-generation hybrids are uniform, this phenomenon was called Mendel's first laws by K. Correns, or the law of uniformity of first-generation hybrids, as well as the rule of dominance.
Question 8. Using additional sources of information, give examples of incomplete dominance of traits in humans.
In humans, incomplete dominance manifests itself when the hair structure is inherited. The gene for curly hair does not fully dominate over the gene for straight hair, and heterozygotes exhibit an intermediate manifestation of the trait - wavy hair.
Another example is sickle cell anemia, which is based on a gene mutation that leads to the replacement of one of the 287 amino acids in the hemoglobin protein - valine - with glutamic acid. As a result, the structure of hemoglobin changes and red blood cells take on the shape of a sickle, which leads to oxygen deficiency. Homozygous organisms die at an early age, while heterozygotes are viable, but suffer from shortness of breath during exercise.
Question 9. Which night beauty plants should be crossed with each other so that the offspring will be half plants with pink flowers and half with white flowers?
Briefly describing the main stages of the “exposure” of the experiments of Gregor Johann Mendel. The name of this scientist is present in all school biology textbooks, as well as illustrations of his experiments on pea breeding. Mendel is rightfully considered the discoverer of the laws of heredity, which became the first step towards modern genetics.
Mendel's pattern of inheritance of traits
Textbook "General Biology"A large-scale experiment conducted by an Augustinian monk interested in natural sciences lasted from 1856 to 1863. Over these few years, Mendel selected 22 varieties of peas, which clearly differed from each other in certain characteristics. After this, the researcher began experiments on the so-called monohybrid crossing: Mendel crossed varieties that differed from each other only in the color of the seeds (some were yellow, others were green).
It turned out that
during the first crossing, the green seeds “disappear” - this rule is called the “law of uniformity of first-generation hybrids.” But in the second generation, green seeds appear again, and in a ratio of 3:1.
(Mendel received 6,022 yellow seeds and 2,001 green.) The researcher called the “winning” trait dominant and the “losing” trait recessive, and the pattern that emerged became known as the “law of segregation.”
This rule means that 75% of second-generation hybrids will have external dominant traits, and 25% will have recessive traits. As for the genotype, the ratio will be as follows: 25% of the plants will inherit a dominant trait from both father and mother, the genes of 50% will carry both traits (the dominant one will appear - yellow peas), and the remaining 25% will be completely recessive.
Mendel's third law - the law of independent combination - was derived by the researcher during the crossing of plants that differed from each other in several characteristics. In the case of peas, this was the color of the peas (yellow and green) and their surface (smooth or wrinkled). The dominant traits were yellow color and smooth surface, recessive traits were green color and wrinkled surface. Gregor Mendel found out that these characteristics will be combined independently of each other. At the same time, it is easy to calculate that according to the phenotype - external characteristics - the offspring will be divided into four groups: 9 yellow smooth, 3 yellow wrinkled, 3 green smooth and 1 green wrinkled pea.
If we take into account the results of splitting for each pair of characters separately, it turns out that the ratio of the number of yellow seeds to the number of green ones and the ratio of smooth seeds to wrinkled ones for each pair is equal to 3:1.
In 1866, the results of Gregor Mendel’s work were published in the next volume of the Proceedings of the Society of Natural Scientists entitled “Experiments on Plant Hybrids,” but his work did not arouse interest among his contemporaries. In 1936, the theoretical geneticist and statistician at the University of Cambridge, Ronald Fisher, declared that Mendel's results were "too good to be true." However, it was not he who began accusing the researcher of falsifying the facts - apparently, Walter Weldon, a biologist from the University of Oxford, was the first to do so. In October 1900, a few months after renewed interest in Mendel's work, the scientist wrote in a personal letter to his colleague, mathematician Karl Pearson, that he had come across the research of "a certain Mendel" who was cross-breeding peas. Over the next year, Weldon studied the monk's work and became increasingly convinced that the proportions obtained by Mendel would not be so “pure” if using actually existing in nature - and not artificially bred - varieties of peas.
In addition, the biologist was also confused by the fact that Mendel operated with binary categories: yellow - green, smooth - wrinkled. According to Weldon, such a clear division of characteristics is very far from reality: so, to what category did the researcher classify seeds of yellow-green, indeterminate color?
Most likely, they were classified so as to fit into the proposed model, argued the biologist, to whom the figures cited by Mendel - 5474 peas with a dominant trait out of 7324 grown seeds (that is, 74.7%, whereas theoretically there should have been 75%) - seemed too "good". “He is either a liar or a magician,” Weldon wrote in a letter to Pearson in 1901.
Illustration from Weldon's 1902 article. The images clearly demonstrate that not all seeds can be classified as “yellow,” “green,” “smooth,” or “wrinkled.”
Science. W. F. R. Weldon, 1902.However, some of those who found Mendel's results incredibly good still decided to speak out in his defense - Ronald Fisher was one of these scientists. He stated that the theoretical model of the inheritance of traits should have been born immediately after the experiments began - and only a truly outstanding mind could develop it. Experiments, according to Fisher, became a carefully prepared illustration of the theory later, and it was not the scientist himself who could “falsify” the results of pea breeding, but the gardeners caring for the plants, who were familiar with the theoretical calculations of the researcher.
By the middle of the twentieth century, the debate around the question of Mendel's adherence to scientific ethics had somewhat subsided - this was due to the fact that genetics at that time was under the strong influence of political factors, in particular, the dominance of "Lysenkoism" in Soviet Union.
Under these conditions, Western scientists preferred not to voice doubts about the reliability of Mendel’s experiments, and the topic was forgotten, however, apparently only for a while.
The authors of the Science article once again argue that the figures he gives are too good to be true, classifying traits into only two categories is not justified, and also agree that the monk could consider yellow peas as green if this fit better with the theory. Nevertheless, this does not detract from the scientist’s merits: the laws he formulated really work, and their discovery became the first stage in the development of modern genetics.
Having worked through these topics, you should be able to:
- Give definitions: gene, dominant trait; recessive trait; allele; homologous chromosomes; monohybrid crossing, crossing over, homozygous and heterozygous organism, independent distribution, complete and incomplete dominance, genotype, phenotype.
- Using the Punnett grid, illustrate crossbreeding for one or two traits and indicate what numerical ratios of genotypes and phenotypes should be expected in the offspring from these crosses.
- Explain the rules of inheritance, segregation, and independent distribution of characters, the discovery of which was Mendel's contribution to genetics.
- Explain how mutations can affect the protein encoded by a particular gene.
- Indicate the possible genotypes of people with blood groups A; IN; AB; ABOUT.
- Give examples of polygenic traits.
- Indicate the chromosomal mechanism of sex determination and types of inheritance of sex-linked genes in mammals, and use this information when solving problems.
- Explain the difference between sex-linked traits and sex-dependent traits; give examples.
- Explain how human genetic diseases such as hemophilia, color blindness, and sickle cell anemia are inherited.
- Name the features of methods of selection of plants and animals.
- Indicate the main directions of biotechnology.
- Be able to solve simple genetic problems using this algorithm:
Algorithm for solving problems
- Determine the dominant and recessive traits based on the results of crossing the first generation (F1) and the second (F2) (according to the conditions of the problem). Enter the letter designations: A - dominant and - recessive.
- Write down the genotype of an individual with a recessive trait or an individual with a known genotype and gametes based on the conditions of the problem.
- Record the genotype of the F1 hybrids.
- Draw up a scheme for the second crossing. Record the gametes of F1 hybrids in a Punnett grid horizontally and vertically.
- Record the genotypes of the offspring in the gamete intersection cells. Determine the ratios of phenotypes in F1.
Task design scheme.
Letter designations:
a) dominant trait _______________
b) recessive trait _______________
Gametes
F1(first generation genotype)
| gametes | ||
| ? | ? |
Punnett grid
| F2 |
|
Phenotype ratio in F2: _____________________________
Answer:_________________________
Examples of solving monohybrid crossing problems.
Task.“There are two children in the Ivanov family: a brown-eyed daughter and a blue-eyed son. The mother of these children is blue-eyed, but her parents had brown eyes. How is eye color inherited in humans? What are the genotypes of all family members? Eye color is a monogenic autosomal trait.”
The eye color trait is controlled by one gene (by condition). The mother of these children is blue-eyed, and her parents had brown eyes. This is only possible if both parents were heterozygous, therefore, brown eyes dominate over blue ones. Thus, grandparents, father and daughter had the genotype (Aa), and mother and son had the genotype aa.
Task."A rooster with a rose-shaped comb was crossed with two hens, also having a rose-shaped comb. The first gave 14 chickens, all with a rose-shaped comb, and the second gave 9 chickens, of which 7 with a rose-shaped and 2 with a leaf-shaped comb. The shape of the comb is a monogenic autosomal trait. What are genotypes of all three parents?
Before determining the genotypes of the parents, it is necessary to find out the nature of inheritance of the comb shape in chickens. When a rooster was crossed with a second hen, 2 chicks with leaf combs were produced. This is possible if the parents are heterozygous; therefore, it can be assumed that the rose-shaped comb in chickens is dominant over the leaf-shaped one. Thus, the genotypes of the rooster and the second hen are Aa.
When crossing the same rooster with the first hen, no splitting was observed, therefore, the first hen was homozygous - AA.
Task.“In a family of brown-eyed, right-handed parents, fraternal twins were born, one of whom is brown-eyed, left-handed, and the other blue-eyed, right-handed. What is the probability of the next child being born similar to his parents?”
The birth of a blue-eyed child to brown-eyed parents indicates the recessiveness of blue eye color, respectively, the birth of a left-handed child to right-handed parents indicates the recessivity of better control of the left hand compared to the right. Let's introduce allele designations: A - brown eyes, a - blue eyes, B - right-handed, c - left-handed. Let's determine the genotypes of parents and children:
| R | AaBv x AaBv |
| F, | A_bb, aaB_ |
A_вв is a phenotypic radical, which shows that this child is left-handed with brown eyes. The genotype of this child may be Aavv, AAvv.
Further solution of this problem is carried out in the traditional way, by constructing a Punnett lattice.
| AB | Av | aB | Av | |
| AB | AABB | AAVv | AaBB | AaVv |
| Av | AAVv | AAbb | AaVv | Aaww |
| aB | AaBB | AaVv | aaBB | AaVv |
| aw | AaVv | Aaww | aaVv | Aaww |
9 variants of descendants that interest us are underlined. There are 16 possible options, so the probability of a child being born similar to their parents is 9/16.
Ivanova T.V., Kalinova G.S., Myagkova A.N. "General Biology". Moscow, "Enlightenment", 2000
- Topic 10. "Monohybrid and dihybrid crossing." §23-24 pp. 63-67
- Topic 11. "Genetics of sex." §28-29 pp. 71-85
- Topic 12. "Mutational and modification variability." §30-31 pp. 85-90
- Topic 13. "Selection." §32-34 pp. 90-97
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 a pair of 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:
1) 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);
2) crossing F 1 hybrids with each other and obtaining second generation hybrids (F 2);
3) mathematical analysis of the 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 initial homozygous parental forms were taken: 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 located in different pairs of homologous pea chromosomes.
During meiosis, homologous chromosomes of different pairs are randomly combined in gametes. If the paternal chromosome of the first pair gets into the gamete, then with equal probability both the paternal and maternal chromosomes of the second pair can get into this gamete. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. 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:
1. 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.
2. 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.
3. 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 for white petals A. Organism Aa has pink petals. Thus, the night beauty does not have a gene for pink color, pink color arises from the combination (combination) of 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 it can be confidently stated that the emergence of two identical people is almost impossible (with the exception of identical twins arising from one 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 – double or multiple repetition of 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 living conditions and tolerate low temperatures and drought more easily. That is why they are widespread in the northern and high mountain regions.
Mendel's laws- a set of basic provisions concerning the mechanisms of transmission of hereditary characteristics from parent organisms to their descendants; these principles underlie classical genetics. Typically, three laws are described in Russian-language textbooks, although the “first law” was not discovered by Mendel, and the “hypothesis of the purity of gametes,” of all the laws discovered by him, has the most general meaning and most deserves the name “law.”
Story
It should be noted that Mendel himself did not formulate his conclusions as “laws” and did not assign any numbers to them. Moreover, many of the facts “discovered” by him were well known for a long time, as Mendel himself points out in his work.
By the middle of the 19th century, the phenomenon of dominance was discovered (O. Sarge, C. Naudin, etc.). Often all hybrids of the first generation are similar to each other (uniformity of hybrids) and according to this trait they are all identical to one of the parents (its trait is dominant). They also showed that recessive (not manifested in first-generation hybrids) traits do not disappear; When hybrids are crossed with each other in the second generation, some of the hybrids have recessive characteristics (“return to parental forms”). It was also shown (J. Goss et al.) that among the second generation hybrids with a dominant trait there are different ones - those that give and those that do not give segregation during self-pollination. However, none of these researchers was able to provide a theoretical basis for their observations.
Mendel's main achievement was the creation of a theory of heredity that explained the patterns of inheritance he studied.
Mendel's methods and progress of work
- Mendel studied how individual traits are inherited.
- Mendel chose from all the characteristics only alternative ones - those that had two clearly different options in his varieties (the seeds are either smooth or wrinkled; there are no intermediate options). Such a conscious narrowing of the research problem made it possible to clearly establish the general patterns of inheritance.
- Mendel planned and carried out a large-scale experiment. He received 34 varieties of peas from seed-growing companies, from which he selected 22 “pure” varieties (which do not produce segregation according to the studied characteristics during self-pollination). Then he carried out artificial hybridization of the varieties, and crossed the resulting hybrids with each other. He studied the inheritance of seven traits, studying a total of about 20,000 second-generation hybrids. The experiment was facilitated by a successful choice of object: peas are normally self-pollinating, but artificial hybridization is easy to carry out.
- Mendel was one of the first in biology to use precise quantitative methods to analyze data. Based on his knowledge of probability theory, he realized the need to analyze a large number of crosses to eliminate the role of random deviations.
Law of Uniformity of First Generation Hybrids
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 called the manifestation of the trait of only one of the parents in hybrids as dominance.
When crossing organisms that differ in one pair of contrasting traits, for which alleles of one gene are responsible, the first generation of hybrids is uniform in phenotype and genotype. According to the phenotype, all hybrids of the first generation are characterized by a dominant trait; according to the genotype, all the first generation of hybrids are heterozygous
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. The offspring from tall and short plants were tall. So, first-generation hybrids are always uniform in this characteristic and acquire the characteristic of one of the parents. This sign (stronger, dominant), always suppressed the other ( recessive).
Codominance and incomplete dominance
Some opposing characters are not in the relation of complete dominance (when one always suppresses the other in heterozygous individuals), but in the relation incomplete dominance. For example, when pure snapdragon lines with purple and white flowers are crossed, the first generation individuals have pink flowers. When pure lines of black and white Andalusian chickens are crossed, gray chickens are born in the first generation. With incomplete dominance, heterozygotes have characteristics intermediate between those of recessive and dominant homozygotes.
With codominance, in contrast to incomplete dominance, heterozygotes exhibit characteristics simultaneously (mixed). A typical example of codominance is the inheritance of ABO blood groups in humans. All offspring of people with genotypes AA (second group) and BB (third group) will have the AB genotype (fourth group). Their phenotype is not intermediate between the phenotypes of their parents, since both agglutinogens (A and B) are present on the surface of erythrocytes.
The phenomena of co-dominance and incomplete dominance of characters is slightly modified by Mendel’s first law: “Hybrids of the first generation from crossing pure lines of individuals with opposite characters are always the same in this trait: they exhibit a dominant trait if the characters are in a relationship of dominance, or a mixed (intermediate) trait if they are in a relationship of co-dominance (incomplete dominance).”
Law of character splitting
Definition
Law of segregation, or Mendel's second law.
The crossing of organisms of two pure lines, differing in the manifestations of one studied trait, for which alleles of one gene are responsible, is called monohybrid crossing. Law of splitting: during monohybrid crossing in the second generation of hybrids, a split in phenotype is observed in a ratio of 3:1: about 3/4 of the second generation hybrids have a dominant trait, about 1/4 has a recessive trait.
The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation
Explanation
Law of gamete purity: each gamete contains only one allele from a pair of alleles of a given gene of the parent individual.
Normally, the gamete is always pure from the second gene of the allelic pair. This fact, which could not be firmly established in Mendel's time, is also called the gamete purity hypothesis. This hypothesis was later confirmed by cytological observations. Of all the laws of inheritance established by Mendel, this “Law” is the most general in nature (it is fulfilled under the widest range of conditions).
Definition
Law of independent inheritance(Mendel's third law) - each pair of characters is inherited independently of other pairs and gives a 3:1 split for each pair (as in a monohybrid cross). When plants differing in several characters, such as white and purple flowers and yellow or green peas, were crossed, the inheritance of each character followed the first two laws and in the offspring they were combined in such a way as if their inheritance occurred independently of each other. The first generation after crossing had a dominant phenotype for all traits. In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1, that is, 9/16 were with purple flowers and yellow peas, 3/16 were with white flowers and yellow peas, 3/16 were with purple flowers and green peas, 1 /16 with white flowers and green peas.
Explanation
Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes. During meiosis, homologous chromosomes of different pairs are randomly combined in gametes. If the paternal chromosome of the first pair gets into the gamete, then with equal probability both the paternal and maternal chromosomes of the second pair can get into this gamete. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. (It later turned out that of the seven pairs of characters studied by Mendel in the pea, which has a diploid number of chromosomes 2n = 14, the genes responsible for one of the pairs of characters were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since as linkage between these genes was not observed due to the large distance between them).
Basic provisions of Mendel's theory of heredity
In modern interpretation, these provisions are as follows:
- Discrete (separate, non-mixable) hereditary factors - genes are responsible for hereditary traits (the term “gene” was proposed in 1909 by V. Johannsen)
- Each diploid organism contains a pair of alleles of a given gene responsible for a given trait; one
one of them was received from the father, the other from the mother.
- Hereditary factors are transmitted to descendants through germ cells. When gametes are formed, each of them contains only one allele from each pair (the gametes are “pure” in the sense that they do not contain the second allele).
Conditions for the fulfillment of Mendel's laws
According to Mendel's laws, only monogenic traits are inherited. If more than one gene is responsible for a phenotypic trait (and the absolute majority of such traits), it has a more complex pattern of inheritance.
Conditions for fulfilling the law of segregation during monohybrid crossing
Splitting 3:1 by phenotype and 1:2:1 by genotype is performed approximately and only under the following conditions:
- A large number of crosses (large number of offspring) are studied.
- Gametes containing alleles A and a are formed in equal numbers (have equal viability).
- There is no selective fertilization: gametes containing any allele fuse with each other with equal probability.
- Zygotes (embryos) with different genotypes are equally viable.
