Meiotic - occurs during the prophase of the first division of meiosis, during the formation of germ cells.
Mitotic - during the division of somatic cells, mainly embryonic ones. Leads to a mosaic pattern in the manifestation of symptoms.
2. Depending on the molecular homology of the chromosome regions entering crossing over.
Normal (equal) – there is an exchange of different sections of chromosomes.
Unequal - there is a gap in non-identical regions of the chromosomes.
3. Depending on the number of formed chiasmata and chromosome breaks with subsequent recombination of genes.
Single
Multiple
Crossover value:
Leads to an increase in combinative variability
Leads to an increase in mutations.
23. Based on an analysis of the results of numerous experiments with Drosophila, Thomas Morgan formulated the chromosomal theory of heredity, the essence of which is as follows:
Material carriers of heredity - genes are located in chromosomes and are located in them linearly at a certain distance from each other.
Genes located on the same chromosome belong to the same linkage group. The number of linkage groups corresponds to the haploid number of chromosomes.
Traits whose genes are located on the same chromosome are inherited linked.
In the offspring of heterozygous parents, new combinations of genes located in the bottom pair of chromosomes can arise as a result of crossing over during the process of meiosis.
The frequency of crossing over, determined by the percentage of crossover individuals, depends on the distance between genes.
Based on the linear arrangement of genes on a chromosome and the frequency of crossing over as an indicator of the distance between genes, chromosome maps can be constructed.
24. Genetic map - a diagram of the location of structural genes and regulatory elements in a chromosome.
Initially, the relative position of genes on chromosomes was determined by the frequency of crossing over between them. The corresponding genetic distance was measured in centimorgans (or centimorganides, cM): 1 cM corresponds to a crossing over frequency of 1%. With this method of genetic mapping, the physical distance between genes often differed from their genetic distance, since crossing over does not occur with equal probability in different parts of the chromosomes. With modern methods of genetic mapping, the distance between genes is measured in thousands of nucleotide pairs (kb) and corresponds to the physical distance.
When creating a genetic map, the sequence of location of genetic markers is established (for this purpose, various DNA polymorphisms were used, i.e., inherited variations in the DNA structure) along the length of all chromosomes with a certain density, i.e. at a fairly close distance from each other.
The genetic map of marker sequences should facilitate the mapping of all human genes, especially the genes of hereditary diseases, which is one of the main goals of this program. In a short time, several thousand genes were genetically mapped.
The method for compiling genetic maps, developed in Drosophila, was transferred to plants (corn, snapdragons) and animals (mice).
Compiling genetic maps is a very labor-intensive procedure. The gene structures of chromosomes can be easily deciphered in those organisms that reproduce quickly. The latter circumstance is the main reason that the most detailed maps exist for Drosophila, a number of bacteria and bacteriophages, and the least detailed for plants.
25. Modification (phenotypic) variability - changes in the body associated with changes in phenotype due to environmental influences and, in most cases, of an adaptive nature. The genotype does not change. In general, the modern concept of “adaptive modifications” corresponds to the concept of “definite variability”, which was introduced into science by Charles Darwin.
The limit of manifestation of modification variability of an organism with an unchanged genotype is reaction norm . The reaction rate is determined by the genotype and varies among different individuals of a given species. In fact, the reaction norm is a spectrum of possible gene expression levels, from which the expression level most suitable for given environmental conditions is selected. The reaction norm has limits or boundaries for each biological species (lower and upper) - for example, increased feeding will lead to an increase in the weight of the animal, but it will be within the reaction norm characteristic of a given species or breed. The reaction rate is genetically determined and inherited. For different traits, the reaction norm limits vary greatly. For example, wide limits of the reaction norm are the value of milk yield, cereal productivity and many other quantitative characteristics), narrow limits are the color intensity of most animals and many other qualitative characteristics.
However, some quantitative traits are characterized by a narrow reaction rate (milk fat content, the number of toes in guinea pigs), while some qualitative traits are characterized by a wide reaction rate (for example, seasonal color changes in many animal species of northern latitudes). In addition, the boundary between quantitative and qualitative characteristics is sometimes very arbitrary.
Expressiveness– degree of phenotypic manifestation of the allele. For example, alleles of blood groups AB0 in humans have constant expressivity (they are always 100% expressed), and alleles that determine eye color have variable expressivity. A recessive mutation that reduces the number of eye facets in Drosophila reduces the number of facets in different ways in different individuals, up to their complete absence.
Penetrance– the probability of the phenotypic manifestation of a trait in the presence of the corresponding gene. For example, the penetrance of congenital hip dislocation in humans is 25%, i.e. Only 1/4 of recessive homozygotes suffer from the disease. Medical-genetic significance of penetrance: a healthy person, whose one of the parents suffers from a disease with incomplete penetrance, may have an undetected mutant gene and pass it on to his children.
26. Mutational variability
Mutational variability is the occurrence of changes in the hereditary material, in the DNA molecules themselves. Not only the composition of DNA can change, but also its quantity (the number of chromosomes). The mutagenic process is influenced by various factors of the external and internal environment.
Assuming that more than one gene can be located on one chromosome, the question should be raised about whether genes in a homologous pair of chromosomes can change places, that is, genes on the paternal chromosome move to the maternal one and back.
If such a process did not occur, then genes would be combined only through the random segregation of homologous chromosomes in meiosis. Consequently, the possibility of exchanging hereditary information between parental organisms would be limited only by Mendelian patterns of inheritance.
Research by T. Morgan and his school has shown that genes are regularly exchanged in a homologous pair of chromosomes. The process of exchange of genes, or homologous regions of homologous chromosomes, is called crossing over, or chromosome crossing. The presence of such a mechanism for the exchange of genes between interbreeding organisms, i.e., the process of gene recombination, expands the possibilities of combinative variability in evolution.
When crossing two organisms that differ in two linked genes AB/AB x ab/ab, a heterozygous form AB/ab arises.
In the case of complete linkage, the diheterozygote will produce only two types of gametes: AB and ab. When analyzing crossing, two classes of zygotes appear, AB/ab and ab/ab, in a 1:1 ratio. Individuals of both classes reproduce the characteristics of their parents. This picture resembles monohybrid rather than dihybrid segregation during test crosses.
But along with the phenomenon of complete coupling, there naturally exists the phenomenon of incomplete coupling. In the case of incomplete linkage when crossing heterozygous individuals of the genotype AB/ab with the recessive form ab/ab, not two, but four classes of phenotypes and genotypes appear in the offspring: AB/ab, ab/ab, Ab/ab, aB/ab. These classes, in terms of their qualitative composition, resemble splitting during analytical crossbreeding of a dihybrid, when free combination of genes is carried out. However, the numerical ratio of classes with incomplete linkage is different from free combination, which gives a ratio of 1: 1: 1: 1. With incomplete linkage, two new classes of zygotes arise with a different combination of genes than the parents, namely Ab/ab and aB/ab, which are always less than 50%.
The formation of new classes of zygotes in cleavage indicates that during the process of gametogenesis in forms heterozygous for two genes, not only gametes AB and ab are formed, but also Ab and aB. Consequently, the genes introduced into the F 1 hybrid by one chromosome somehow diverge during the formation of its gametes. How could gametes with such a new combination of genes appear? Obviously, they could only arise if there was an exchange of sections between homologous chromosomes, i.e. crossing over. Crossing over provides new combinations of genes located on homologous chromosomes. The phenomenon of crossing over, like adhesion, turned out to be common to all animals, plants and microorganisms.
Crossing over can only be detected if the genes are in a heterozygous state, i.e. AB/ab.
In the homozygous state of the AB/AB and ab/ab genes, chromosome crossover cannot be detected, since the exchange of identical sections does not produce new combinations of genes in gametes and offspring. The crossover of chromosomes can be judged on the basis of genetic analysis of the frequency of emerging recombinants, i.e., zygotes with a new combination of genes, and cytological studies of the behavior of chromosomes in meiosis.
Crossing occurs in prophase I of meiosis and is therefore called meiotic crossover. But sometimes crossover also occurs during mitosis in somatic cells, then it is called mitotic, or somatic.
Meiotic crossover occurs after homologous chromosomes pair up in the zygotene stage of prophase I, forming bivalents. In prophase I, each chromosome is represented by two sister chromatids, and crossover occurs not between chromosomes, but between chromatids. The expression “chromosome crossing” is a general concept, meaning that crossing over occurs between chromatids.
Suggestions about the connection between the phenomenon of heredity and chromosomes were first made at the end of the 19th century. This idea was developed in particular detail in his theory of “germ plasm” by A. Weisman (see the first lecture). Later, the American cytologist W. Setton drew attention to the correspondence of the nature of inheritance of characters in one of the grasshopper species to the behavior of chromosomes during the process of meiosis. He concluded that the hereditary factors that determine these traits are localized in chromosomes and that the law of independent combination of traits established by Mendel is limited. He believed that only those traits whose hereditary factors lie on different chromosomes can be combined independently. Since the number of traits far exceeds the number of pairs of chromosomes, many traits are controlled by genes on one chromosome, which must be inherited together.
The first case of joint inheritance was described in 1906 by English geneticists W. Batson and R. Punnett in sweet pea (Lathyrus odoratus L.). They crossed two races of sweet peas, differing in two traits. One race was characterized by the purple color of the flowers and the elongated shape of the pollen, the other by the red color and rounded shape. It turned out that purple color completely dominates over red, and the elongated shape of pollen over the round one. Each pair of characters individually gave a split of 3: 1. F 1 hybrids from crossing plants of these two races inherited the dominant characters of one of the parents, i.e. had purple flowers and elongated pollen. However, in F2, the ratio of the expected four phenotypes did not fit into the formula 9: 3: 3: 1, characteristic of independent inheritance. The main difference was that the combinations of traits that characterized the parents occurred more often than they should, while new combinations appeared in quantities less than expected. Parental phenotypes also prevailed in the generation from the analyzing cross. It seemed that the hereditary factors present in the parents tended to remain together during the process of inheritance. And, conversely, factors contributed by different parents seem to resist entering into one gamete. Scientists called this phenomenon “attraction” and “repulsion” of factors. When using parents with other combinations of these traits, Betson and Punnett obtained the same results.
Crossing over in grasshopper chromosomes
For several years, this case of unusual inheritance in the sweet pea was considered a deviation from Mendel's III law. An explanation was given by T. Morgan and his colleagues, who discovered many cases of similar inheritance of traits in Drosophila. According to their conclusions, the preferential transmission of original combinations of traits to offspring is due to the fact that the genes that determine them are located on the same chromosome, i.e. physically connected. This phenomenon was named by Morgan gene linkage. He also gave an explanation for incomplete linkage, suggesting that it is the result crossing over- crossing of homologous chromosomes, which exchange homologous regions during conjugation in meiotic prophase. Morgan came to this conclusion under the influence of data from the Dutch cytologist F. Janssens (1909), who studied meiosis and drew attention to the characteristic interweaving of chromosomes in prophase I, reminiscent of the Greek letter c. He called them chiasmas.
Morgan made a cross on Drosophila, which became genetic evidence of the presence of gene exchange. As parental forms, he used two lines of Drosophila, differing in two pairs of characters. Flies of the same line had a gray body (wild type trait) and reduced wings (recessive mutation vestigal, vg), and flies of the other line have a black body (recessive mutation black, b) and normal wings. All F 1 hybrids inherited the dominant characteristics of the wild type - a gray body and normal wings. Next, Morgan deviated from the usual crossing scheme and, instead of F 2, received a generation from crossing F1 hybrids with homozygous recessive individuals, i.e. carried out an analytical cross. In this way, he tried to accurately determine which types of gametes and in what quantities form F 1 hybrids. Two types of test crosses were carried out: in the first of them, hybrid females were crossed with homozygous recessive males ( bbvgvg), in the second, homozygous recessive females were crossed with hybrid males.
The results of the two test crosses were different. As can be seen from the diagram, F a direct crossing consists of four phenotypic classes. This suggests that the hybrid female produces four types of gametes, the fusion of which with the single gamete of the homozygous recessive leads to the manifestation of four different combinations of characters in F a. Morgan called two classes that repeat the phenotype of the parent individuals non-crossover, since they originated from the fusion of gametes formed without the participation of crossing over and gene exchange. In terms of quantity, these classes are more numerous (83%) than the other two classes - crossover (17%), characterized by new combinations of characteristics. Their appearance indicated that in meiosis, during the formation of part of the female’s gametes, the process of crossing over occurs and genes are exchanged. This type of inheritance is called incomplete linkage.
Different results were obtained in backcrossing, where the genotype of the hybrid male was analyzed. In F a, only two classes of individuals were represented in equal numbers, repeating the phenotype of the parental forms. This indicated that the hybrid male, unlike the hybrid female, formed two types of gametes with the original combination of genes with equal frequency. Such a situation could only occur if there was no crossing over and, therefore, no exchange of genes during the formation of gametes in the male. This type of inheritance was called complete linkage by Morgan. Later it was found that crossing over during the formation of gametes in males, as a rule, is absent.
Chromosome crossing occurs in prophase I of meiosis and is therefore called meiotic. It occurs after homologous chromosomes pair up at the zygotene stage, forming bivalents. In prophase I, each chromosome is represented by two sister chromatids, and crossover occurs not between chromosomes, but between the chromatids of homologs. Crossing over can only be detected if the genes are in a heterozygous state ( BbVv). In the homozygous state of genes, crossing over cannot be genetically detected, since the exchange of identical genes does not produce new combinations at the phenotypic level.
Scheme of inheritance of body color and wing shape in Drosophila
in the presence of gene linkage
T. Morgan's colleague A. Sturtevant suggested that the frequency of crossing over depends on the distance between genes, and complete linkage is found in genes located very close to each other. On this basis, he proposed using this indicator to determine the distance between genes. The frequency of crossing over is determined based on the results of the analyzing cross. The percentage of crossing over is calculated as the ratio of the number of crossover individuals Fa (i.e. individuals with new combinations of parental characteristics) to the total number of individuals of this offspring (in %). 1% crossing over is taken as a unit of distance between genes, which was later named centi-morganid (or simply morganid) in honor of T. Morgan. The frequency of crossover reflects the strength of linkage of genes: the lower the frequency of crossing over, the greater the strength of linkage and vice versa.
The study of the phenomenon of gene linkage allowed Morgan to formulate the main genetic theory - chromosomal theory of heredity. Its main provisions are as follows:
- Each type of living organism is characterized by a specific set of chromosomes - a karyotype. The specificity of the karyotype is determined by the number and morphology of chromosomes.
- Chromosomes are the material carriers of heredity and each of them plays a specific role in the development of an individual.
- Genes are arranged in a linear order on a chromosome. A gene is a section of a chromosome responsible for the development of a trait.
- Genes on one chromosome form a single linkage group and tend to be inherited together. The number of linkage groups is equal to the haploid set of chromosomes, since homologous chromosomes represent the same linkage group.
- Gene linkage can be complete (100% joint inheritance) or incomplete. Incomplete linkage of genes is the result of crossing over and exchange of sections of homologous chromosomes.
- The frequency of crossing over depends on the distance between genes on the chromosome: the further the genes are from each other, the more often a crossover occurs between them.
A crossover that occurs in one part of a chromosome is called single cross. Since the chromosome is a linear structure of considerable length, several crossovers can occur in it simultaneously: double, triple and multiple.
If crossing over occurs simultaneously in two adjacent regions of the chromosome, then the frequency of double crossovers turns out to be lower than that which can be calculated based on the frequencies of single crossovers. A particularly noticeable decrease is observed when genes are very close together. In this case, crossing over in one area mechanically prevents crossing over in another area. This phenomenon is called interference. As the distance between genes increases, the amount of interference decreases. The interference effect is measured by the ratio of the actual frequency of double crossovers to their theoretically expected frequency, in the case of their complete independence from each other. This ratio is called co-incident. The actual frequency of double crossovers is established experimentally during hybridological analysis based on the frequency of the phenotypic class of double crossovers. The theoretical frequency, according to the law of probability, is equal to the product of the frequencies of two single crosses. For example, if there are three genes on a chromosome A, b And With and crossing over between A And b occurs with a frequency of 15%, and between b And With- with a frequency of 9%, then in the absence of interference the frequency of double crossing over would be equal to 0.15 x 0.09 = 1.35%. With an actual frequency of 0.9%, the magnitude of co-incident is expressed as a ratio and equals:
| 0,009 | = 0,69 = 69% |
| 0,0135 |
Thus, in this case, only 69% of double crosses were realized due to interference.
Among the 8 phenotypic classes formed in Fa in the presence of three pairs of linked characters, two classes of double crossovers are the smallest, taking into account the phenomenon of interference and in accordance with the law of probability.
The existence of multiple crosses leads to an increase in the variability of hybrid offspring, since thanks to them the number of gene combinations and, accordingly, the number of gamete types in hybrids increases.
On determining the frequencies of single, double, triple, etc. intersections is the basis for the construction of genetic maps. A genetic map is a diagram that shows the order of genes on a chromosome. The basis for calculating the distance between genes is the percentage of single crossing over between them. Corrections are added to it for the value of double and more complex crosses, which clarify the calculation. If we have three genes, then the order of their relative positions in the chromosome is determined based on the phenotype of the double crossover class. In double crossing over, the middle gene is exchanged. Therefore, the trait in which double crossovers differ from their parents is determined by this gene. For example, if a homozygous gray long-winged female Drosophila with red eyes (all wild-type traits are dominant) was crossed with a homozygous dark (recessive mutation black) male with reduced wings (recessive mutation) and bright eyes (recessive mutation cinnabar), and in Fa the fewest paired classes (i.e. double crossovers) were gray flies with bright eyes and long wings and black flies with red eyes and reduced wings, then, therefore, the gene controlling eye color is average. A map segment with these three genes would look like this:
On the genetic map of any chromosome, the distance count starts from the zero point - the locus of the first gene - and not the distance between two neighboring genes is noted, but the distance in the morganids of each subsequent gene from the zero point.
Genetic maps have been compiled only for genetically well-studied objects, both prokaryotic and eukaryotic, such as, for example, phage l, E. coli, Drosophila, mouse, corn, and humans. They are the fruit of the enormous and systematic work of many researchers. The presence of such maps makes it possible to predict the nature of inheritance of the studied traits, and during breeding work, to carry out a conscious selection of pairs for crossing.
Genetic evidence of the presence of crossing over, obtained in the experiments of T. Morgan and his colleagues, received direct confirmation at the cytological level in the 30s. in the works of K. Stern on Drosophila and B. McClintock and G. Creighton on corn. They managed to construct a heteromorphic pair of chromosomes (pair of X chromosomes in Drosophila and IV pair of autosomes in maize), in which the homologues had different shapes. The exchange of sections between them led to the formation of different cytological types of this pair of chromosomes, which could be identified cytologically (under a microscope). Thanks to genetic marking, each cytological type of bivalent corresponded to a certain phenotypic class of offspring.
In the 30s T. Paynter discovered giant or polytene chromosomes in the salivary glands of Drosophila. Due to their large size and clear structural organization, they have become the main object of cytogenetic research. Each chromosome is characterized by a specific pattern of dark stripes (disks) and light spaces (between disks), corresponding to the heterochromatic and euchromatic regions of the chromosome. The constancy of this internal structure of giant chromosomes made it possible to check how much the order of genes established on the basis of determining the frequency of crossing over reflects the actual location of genes on the chromosome. For this purpose, a comparison is made of the structure of a normal chromosome and a chromosome carrying a chromosomal mutation, for example, loss or duplication of a chromosome section. Such a comparison fully confirms the correspondence of the order of genes on genetic maps to their location on chromosomes. A graphical representation of a giant chromosome indicating the localization of genes in certain parts of it is called a cytological map.
The phenomenon of crossing over has been found not only in germ cells, but also in somatic cells. Typically, homologous chromosomes do not conjugate in prophase of mitosis and are located separately from each other. However, as early as 1916, researchers were sometimes able to observe patterns of synapsis of homologous chromosomes in mitotic prophase with the formation of crossover figures (chiasmata). This phenomenon is called somatic, or mitotic, crossing over. At the phenotypic level, it is judged by a mosaic change in characteristics in certain areas of the body. Thus, in female wild-type Drosophila heterozygous for the recessive mutations yellow (yellow body) and singed (singed bristles), as a result of somatic crossover, spots with recessive characteristics may appear. In this case, depending on where the crossover occurs: between the above genes or beyond them, either a spot with both mutant traits or with one of them is formed.
A: on the left - half of the chest is normal (+), on the right - mutant without bristles (aC); B and C - mosaic halves of the breast, consisting of sections of wild-type (white) and mutant tissue (black).
Typically, crossing over involves the exchange of homologous regions of chromosomes of equal size. But occasionally, asymmetrical breaks in chromatids and the exchange of unequal sections are possible, i.e. unequal crossing over. As a result of such an exchange, both alleles of a gene can end up on one chromosome (duplication), and a deficiency occurs in the other homologue. A similar change was found in the Drosophila X chromosome in a region containing a dominant mutation Bar (B), which determines the development of strip-shaped eyes with a reduced number of facets (in homozygotes 70 instead of 700). Duplication of this gene as a result of unequal crossing over leads to a further reduction in the number of facets (up to 25). Cytologically, unequal crossing over is easily detected by changes in the pattern of giant chromosomes.
Chromosome crossing, as a complex physiological process, is strongly influenced by external and internal factors. The structure of the chromosome, primarily the presence of large blocks of heterochromatin, has a great influence on the frequency of crossing over. It has been established that in Drosophila crossing over rarely occurs near the centromere and at the ends of chromosomes, which is due to the presence of pericentromeric and telomeric heterochromatin. Tight spiralization of heterochromatic regions of the chromosome reduces the distance between genes and prevents their exchange. The frequency of crossing over is affected by various chromosomal rearrangements and gene mutations. If there are several inversions in a chromosome, they can become “blockers” of the crossover. In corn, genes have been discovered that disrupt the conjugation process and thereby prevent crossing over.
In most of the animals and plants studied, meiotic crossover occurs in both sexes. But there are certain species of animals in which crossing over occurs only in the homogametic sex, and is absent in the heterogametic sex. Moreover, crossing over does not occur not only in sex chromosomes, but also in autosomes. A similar situation is observed in male Drosophila and female silkworms with the XY karyotype. However, in many species of mammals, birds, fish and insects, heterogamety of sex does not affect the process of crossing over.
The crossing over process is influenced by the functional state of the body. It has been established that the frequency of crossover depends on age, as does the level of anomalies in meiosis. With age, there is a decrease in the activity of enzymatic systems, including those that regulate the process of exchange of chromosome sections.
The frequency of crossover can be increased or decreased by the influence of various environmental factors on the body, such as high and low temperatures, ionizing radiation, dehydration, changes in the concentration of calcium, magnesium ions, etc. in the environment, the action of chemical agents, etc. In particular, it was found that in Drosophila the frequency of crossing over increases with increasing temperature.
In conclusion, the process of crossing over is very important from an evolutionary point of view. It is the mechanism by which genetic recombination occurs and new favorable genotypes are created. Combinative variability, along with mutational variability, is the basis for the creation of new forms.
rice. 1How the “usual” resolution of crossovers occurs is clear from the figure. It’s not very clear from the drawing how resolution occurs with “jumping” (vertical lines). In order to understand this, we need to move from flat DNA to three-dimensional.
rice. 2 The left picture is similar to the diagrams we drew above. In the middle picture the same structure is drawn as it looks in real life. By turning the lower part of the middle picture along the arrow, we get the right picture. If we cut with a knife between the numbers 1, we will get a “left path”, there will be no crossing over. And if we cut 2 between the numbers, we get the “right path”, crossing over. (But if “knife cutting” 1 and 2 are equal, then why does the first occur much more often than the second? - “Cutting” depends not on how the DNA molecule has turned in space, but on which proteins work at the crossover site.)
Same thing with terms
"Left end" is called invasive, the process of its integration into homologous DNA - invasion. Once the invasive end has joined with homologous DNA, the result is heteroduplex(a section of DNA containing chains from different molecules). The loop displaced by the invasive end is called D-loop. The crossover between DNA strands is called Holiday structure– in figure No. 2 she is depicted three times, in three different poses. Few? - Here you have it in the form of a cartoon.
Resolution of the Holiday structure can occur via recombination or conversion pathways. Recombination pathway(vertical lines in Fig. 1, cutting through numbers 2 in Fig. 2, right scissors in Fig. 3) leads to recombination, chromosomes change their parts. Conversion path(horizontal lines in Fig. 1, cutting through the numbers 1 in Fig. 2) leads to conversion.
Conversion
Maternal and paternal DNA are not exactly the same (otherwise why would we cross over).
Accordingly, in a heteroduplex, the paternal and maternal strands are not completely complementary.
Reparation enzymes correct non-complementary pairs of nucleotides, and whose letter they correct - father's or mother's - is random.
For example, if mother’s DNA was A=T, and father’s DNA was G≡C, then the heteroduplex turns out to be A=C - repair enzymes correct it either to A=T or to G≡C.
Accordingly, if the mother was AA, and the father was aa, then the heteroduplex will be Aa - repair enzymes correct it either to AA or to aa, strange splits are obtained:
In fact, it was these informal splits that in 1964 forced Robin Holiday to come up with the crossing over model - which (with modifications, of course) has survived to this day. For my part, I congratulate you for almost making it to the end of the article. Let's check if you understood anything? Here's an unchewed drawing for you.
Crossing over(from English crossing–over– crossover) is an exchange of homologous sections of homologous chromosomes (chromatids).
The “break-reunion” crossing-over mechanism
According to the Janssens–Darlington theory, crossing over occurs in prophase of meiosis. Homologous chromosomes with chromatid haplotypes AB And ab form bivalents. In one of the chromatids in the first chromosome there is a break in the area A–B, then in the adjacent chromatid of the second chromosome there is a break in the area a–b. The cell seeks to correct the damage using repair-recombination enzymes and attach chromatid fragments. However, in this case, it is possible to join crosswise (crossing over), and recombinant haplotypes (chromatids) Ab And aB . In anaphase of the first division of meiosis, the divergence of two-chromatid chromosomes occurs, and in the second division, the divergence of chromatids (single-chromatid chromosomes) occurs. Chromatids that did not participate in crossing over retain their original combinations of alleles. Such chromatids (single-chromatid chromosomes) are called non-crossover; with their participation, non-crossover gametes, zygotes and individuals will develop. Recombinant chromatids that were formed during crossing over carry new combinations of alleles. Such chromatids (single-chromatid chromosomes) are called crossover, with their participation, crossover gametes, zygotes and individuals will develop.
Thus, due to crossing over, recombination– the emergence of new combinations (haplotypes) of hereditary inclinations in chromosomes.
Note. According to other theories, crossing over is associated with DNA replication: either in pachytene of meiosis or in interphase (see below). In particular, it is possible to change the matrix at a replication fork.
Interference is the suppression of crossing over in areas immediately adjacent to the point of the exchange that occurred. Consider an example described in one of Morgan's early works. He studied the frequency of crossing over between genes w (white- white eyes) at (yellow– corpus luteum) and m(miniature - small wings), localized on the X chromosome D. melanogaster. Distance between genes w And at the percentage of crossing over was 1.3, and between genes at And m– 32.6. If two acts of crossing over are observed by chance, then the expected frequency of double crossing over should be equal to the product of the crossover frequencies between genes at And w and genes w And m. In other words, the double crossover rate would be 0.43%. In fact, only one double crossing over per 2205 flies was detected in the experiment, i.e. 0.045%. Morgan's student G. Moeller proposed to determine the intensity of interference quantitatively by dividing the actually observed double crossing-over frequency by the theoretically expected (in the absence of interference) frequency. He called this indicator co-incident coefficient, i.e. coincidences. Möller showed that in the Drosophila X chromosome interference is especially strong at short distances; as the interval between genes increases, its intensity decreases and at a distance of about 40 morganids or more, the co-incidence coefficient reaches 1 (its maximum value).
Types of crossing over:
1.Double and multiple crossing over
2.Somatic (mitotic) crossing over
3. Unequal crossing over
The evolutionary significance of crossing over
As a result of crossing over, unfavorable alleles, initially linked to favorable ones, can move to another chromosome. Then new haplotypes arise that do not contain unfavorable alleles, and these unfavorable alleles are eliminated from the population.
Biological significance of crossing over
Due to linked inheritance, successful combinations of alleles are relatively stable. As a result, groups of genes are formed, each of which functions as a single supergene, controlling several traits. At the same time, during crossing over, recombinations occur - i.e. new combinations of alleles. Thus, crossing over increases the combinative variability of organisms.
This means that...
a) in the course of natural selection, “useful” alleles accumulate in some chromosomes (and carriers of such chromosomes gain an advantage in the struggle for existence), while undesirable alleles accumulate in other chromosomes (and carriers of such chromosomes drop out of the game - are eliminated from populations)
b) during artificial selection, alleles of economically valuable traits accumulate in some chromosomes (and the carriers of such chromosomes are retained by the breeder), while undesirable alleles accumulate in other chromosomes (and the carriers of such chromosomes are discarded).
