LINKAGE OF GENES is a phenomenon based on the localization of genes on one chromosome. With complete linkage of genes, only two types of gametes are formed (with the original combinations of linked genes), with incomplete linkage, new combinations of alleles of linked genes are formed. Incomplete linkage of genes is the result of crossing over between linked genes, so complete linkage of genes is possible in organisms in which cells do not normally cross over (for example, germ cells of Drosophila males). Thus, full linkage of genes is rather an exception to the rule of incomplete linkage of genes. In addition, full linkage of genes can be mimicked by the phenomenon of pleiotropy.

Crossing over (another name in biology is cross) is the process of exchanging sections of homologous chromosomes during conjugation in prophase I of meiosis.

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The basic concepts of genetics are heredity, inheritance, dominance, recessiveness, allelic genes, homo- and heterozygosity

Genetics is the science of the laws of heredity and variability of organisms and methods of managing them .. heredity is the property of organisms to provide material and .. inheritance the transfer of genetic information of genetic traits from one generation of organisms to another ..

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Features of the inheritance of sex-linked traits
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Population-statistical method. Its capabilities and significance
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Hereditary (genotypic) variability
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In 1906, W. Batson and R. Pennet, crossing sweet pea plants and analyzing the inheritance of pollen form and flower color, found that these traits do not give independent distribution in the offspring, hybrids always repeated the traits of parental forms. It became clear that not all traits are characterized by independent distribution in the offspring and free combination.

Each organism has a huge number of characteristics, and the number of chromosomes is small. Consequently, each chromosome carries not one gene, but a whole group of genes responsible for the development of different traits. He studied the inheritance of traits whose genes are localized on the same chromosome. T. Morgan. If Mendel conducted his experiments on peas, then for Morgan the main object was the Drosophila fruit fly.

Drosophila every two weeks at a temperature of 25 ° C gives numerous offspring. The male and female are outwardly well distinguishable - the male has a smaller and darker abdomen. They have only 8 chromosomes in the diploid set, they multiply quite easily in test tubes on an inexpensive nutrient medium.

By crossing a Drosophila fly with a gray body and normal wings with a fly with a dark body color and rudimentary wings, in the first generation Morgan obtained hybrids with a gray body and normal wings (the gene that determines the gray color of the abdomen dominates the dark color, and the gene that determines development of normal wings, - over the gene of underdeveloped ones). When conducting an analyzing crossing of an F 1 female with a male who had recessive traits, it was theoretically expected to obtain offspring with combinations of these traits in a ratio of 1:1:1:1. However, in the offspring, individuals with signs of parental forms clearly predominated (41.5% - gray long-winged and 41.5% - black with rudimentary wings), and only an insignificant part of the flies had a combination of signs other than that of the parents (8.5% - black long-winged and 8.5% - gray with rudimentary wings). Such results could only be obtained if the genes responsible for body color and wing shape are on the same chromosome.

1 - non-crossover gametes; 2 - crossover gametes.

If the genes for body color and wing shape are localized on the same chromosome, then with this crossing, two groups of individuals should have been obtained, repeating the signs of parental forms, since the maternal organism should form only two types of gametes - AB and ab, and the paternal one - one type - ab . Therefore, two groups of individuals with the genotype AABB and aabb should be formed in the offspring. However, individuals appear in the offspring (albeit in small numbers) with recombined traits, that is, those with the genotype Aabb and aaBb. In order to explain this, it is necessary to recall the mechanism of the formation of germ cells - meiosis. In the prophase of the first meiotic division, homologous chromosomes conjugate, and at this moment an exchange of sites can occur between them. As a result of crossing over, in some cells, chromosome sections are exchanged between genes A and B, gametes Ab and aB appear, and, as a result, four groups of phenotypes are formed in the offspring, as with free combination of genes. But, since crossing over occurs when a small part of gametes is formed, the numerical ratio of phenotypes does not correspond to the ratio of 1:1:1:1.

clutch group- genes located on the same chromosome and inherited together. The number of linkage groups corresponds to the haploid set of chromosomes.

Linked inheritance- inheritance of traits whose genes are localized on the same chromosome. The strength of linkage between genes depends on the distance between them: the farther the genes are located from each other, the higher the frequency of crossing over and vice versa. Full grip- a type of linked inheritance, in which the genes of the analyzed traits are located so close to each other that crossing over between them becomes impossible. Incomplete clutch- a type of linked inheritance, in which the genes of the analyzed traits are located at a certain distance from each other, which makes crossing over between them possible.

Independent Inheritance- inheritance of traits whose genes are localized in different pairs of homologous chromosomes.

Non-crossover gametes- gametes, during the formation of which crossing over did not occur.

Non-recombinants- hybrid individuals that have the same combination of traits as the parents.

Recombinants- hybrid individuals having a different combination of characters than their parents.

The distance between genes is measured in morganides— arbitrary units corresponding to the percentage of crossover gametes or the percentage of recombinants. For example, the distance between the genes for gray body color and long wings (also black body color and rudimentary wings) in Drosophila is 17%, or 17 morganides.

In diheterozygotes, dominant genes can be located either on the same chromosome ( cis-phase), or in different ( trans phase).

1 - Mechanism of cis-phase (non-crossover gametes); 2 - trans-phase mechanism (non-crossover gametes).

The result of T. Morgan's research was the creation of chromosome theory of heredity:

1. genes are located on chromosomes; different chromosomes contain an unequal number of genes; the set of genes for each of the nonhomologous chromosomes is unique;
2. each gene has a specific location (locus) on the chromosome; allelic genes are located in identical loci of homologous chromosomes;
3. genes are located on chromosomes in a certain linear sequence;
4. genes located on the same chromosome are inherited together, forming a linkage group; the number of linkage groups is equal to the haploid set of chromosomes and is constant for each type of organism;
5. gene linkage can be broken during the process of crossing over, which leads to the formation of recombinant chromosomes; the frequency of crossing over depends on the distance between genes: the greater the distance, the greater the value of crossing over;
6. Each species has its own set of chromosomes, the karyotype.

Go to lectures №17 Basic concepts of genetics. Laws of Mendel»

Linked inheritance - inheritance of traits whose genes are localized on the same chromosome. The strength of linkage between genes depends on the distance between them: the farther the genes are located from each other, the higher the frequency of crossing over and vice versa. Along with traits that are inherited independently, there must also be those that are inherited linked to each other, since they are determined by genes located on the same chromosome. Such genes form clutch group. The number of linkage groups in organisms of a certain species is equal to the number of chromosomes in the haploid set (for example, in Drosophila 1 pair = 4, in humans 1 pair = 23).

Full grip- a kind of linked inheritance, in which the genes of the analyzed traits are located so close to each other that crossing over between them becomes impossible.

Incomplete clutch- a type of linked inheritance, in which the genes of the analyzed traits are located at a certain distance from each other, which makes crossing over between them possible.

(Crossover gametes- gametes, in the process of formation of which there was a crossing-over. As a rule, crossover gametes make up a small part of the total number of gametes.

Crossing over- exchange of sections of homologous chromosomes in the process of cell division, mainly in the prophase of the first meiotic division, sometimes in mitosis. The experiments of T. Morgan, K. Bridges and A. Sturtevant showed that there is no absolutely complete linkage of genes, in which genes would always be transmitted together. The probability that two genes located on the same chromosome will not diverge during meiosis ranges from 1-0.5. In nature, incomplete linkage predominates due to the crossing of homologous chromosomes and recombination of genes. The cytological pattern of crossing over was first described by the Danish scientist F. Janssens.

Crossing over occurs only when the genes are in a heterozygous state (AB / av). If the genes are in a homozygous state (AB / AB or aB / aB), the exchange of identical regions does not give new combinations of genes in gametes and in a generation. The frequency (percentage) of crossover between genes depends on the distance between them: the further they are located from each other, the more often crossing over occurs. T. Morgan proposed to measure the distance between genes by crossing over as a percentage, according to the formula:

N1/N2 X 100 = % Crossover,

where N1 is the total number of individuals in F;

N2 is the total number of crossover individuals.

The segment of the chromosome, on which 1% of crossing over takes place, is equal to one morganide (a conditional measure of the distance between genes). The crossover frequency is used to determine mutual arrangement genes and the distance between them. To build a human genetic map, new technologies are used, in addition, cytogenetic maps of chromosomes have been built.

There are several types of crossing over: double, multiple (complex), irregular, uneven.

Crossing over leads to a new combination of genes, causing a change in the phenotype. In addition, along with mutations, it is an important factor evolution of organisms.

The result of T. Morgan's research was his creation of the chromosome theory of heredity:

· genes are located on chromosomes; different chromosomes contain an unequal number of genes; the set of genes for each of the nonhomologous chromosomes is unique;

· each gene has a specific location (locus) on the chromosome; allelic genes are located in identical loci of homologous chromosomes;

· genes are located on chromosomes in a certain linear sequence;

· genes located on the same chromosome are inherited together, forming a linkage group; the number of linkage groups is equal to the haploid set of chromosomes and is constant for each type of organism;

· gene linkage can be broken during the process of crossing over, which leads to the formation of recombinant chromosomes; the frequency of crossing over depends on the distance between genes: the greater the distance, the greater the value of crossing over;

· each species has a set of chromosomes characteristic only for it - a karyotype.

Inheritance of sex and sex-linked traits. Sex chromosomes and their role in sex determination. Sex inheritance. The sex of an individual is a complex trait, formed both by the action of genes and by the conditions of development. In humans, one of the 23 pairs of chromosomes is the sex chromosomes, denoted as X and Y. Women are homogametic sex, i.e. They have two X chromosomes, one from their mother and one from their father. Males are heterogametic, have one X-one Y-chromosome, and X is transmitted from the mother, and Y from the father. Note that the heterogametic sex is not always necessarily male; for example, in birds, these are females, while males are homogametic. There are other mechanisms of sex determination. So, in a number of insects, the Y chromosome is absent. In this case, one of the sexes develops in the presence of two X chromosomes, and the other - in the presence of one X chromosome. In some insects, sex is determined by the ratio of the number of autosomes and sex chromosomes. In a number of animals, the so-called. redefinition of sex, when, depending on environmental factors, the zygote develops either into a female or into a male. The development of sex in plants has as diverse genetic mechanisms as in animals.

Traits linked to the X chromosome. If the gene is located on the sex chromosome (it is called sex-linked), then its expression in descendants follows rules other than for autosomal genes. Consider genes located on the X chromosome. A daughter inherits two X chromosomes, one from her mother and one from her father. The son has only one X-chromosome - from the mother; from his father he receives a Y-chromosome. Therefore, the father passes on the genes on his X chromosome only to his daughter, while the son cannot receive them. Since the X chromosome is more "rich" in genes compared to the Y chromosome, in this sense, the daughter is genetically more similar to the father than the son; the son is more like his mother than his father.

One of the historically best known sex-linked traits in humans is hemophilia, resulting in severe bleeding from minor cuts and extensive bruising from bruises. It is caused by a recessive defective allele 0, which blocks the synthesis of a protein necessary for blood clotting. The gene for this protein is located on the X chromosome. A heterozygous woman +0 (+ means the normal active allele, dominant in relation to the hemophilia 0 allele) does not get hemophilia, and her daughters, too, if the father does not have this pathology. However, her son can get the 0 allele, and then he develops hemophilia. Recessive diseases caused by genes on the X chromosome affect women much less often than men, since they have the disease only when they are homozygous - the presence of a recessive allele in each of the two homologous X chromosomes; males get sick whenever their single X chromosome carries the defective allele.

Linkage to the Y chromosome.Information about the genes located on the Y chromosome is very scarce. It is assumed that it practically does not carry the genes that determine the synthesis of proteins necessary for the functioning of the cell. But it plays a key role in the development of the male phenotype. The absence of a Y chromosome in the presence of only one X chromosome leads to the so-called. Turner syndrome: the development of a female phenotype with poorly developed primary and secondary sexual characteristics and other abnormalities. There are men with an additional Y-chromosome (XYY); they are tall, aggressive and often have abnormal behavior. In the Y chromosome, several genes have been identified that are responsible for the regulation of the synthesis of specific enzymes and hormones, and violations in them lead to pathologies of sexual development. There are a number of morphological traits that are believed to be determined by genes on the Y chromosome; among them is the development of ear hair. Such signs are transmitted only through the male line: from father to son.

genetic sex determination, determined by the set of sex chromosomes, supports equal reproduction of females and males. Indeed, female eggs contain only the X chromosome, since women have the XX genotype on the sex chromosomes. The genotype of men is XY, and therefore the birth of a girl or a boy in each case is determined by whether the sperm carries the X or Y chromosome. Since, in the process of meiosis, the chromosomes have an equal chance of getting into the gamete, then half of the gametes produced by male individuals contain the X-chromosome, and half the Y-chromosome. Therefore, half of the offspring are expected to be of one sex and half of the other.

It should be emphasized that it is impossible to predict the birth of a boy or a girl in advance, since it is impossible to predict which male sex cell will participate in the fertilization of the egg: carrying the X- or Y-chromosome. Therefore, the presence of more or less boys in the family is a matter of chance.

Task number 4. A tall sweet pea plant with green wrinkled seeds is crossed with a plant that has dwarf growth and green round seeds. Segregation was obtained in the offspring: 3/4 of tall plants with green round seeds and 1/4 of tall plants with yellow round seeds. Determine the genotypes of the original plants and F 1 hybrids.

Given:

BUT- high growth
a- dwarfism
AT- green seeds
b- yellow seeds
FROM- round seeds
With- wrinkled seeds

Genotypes P and F 1 – ?

Solution

Based on the nature of the splitting of traits in hybrids, we draw conclusions about the nature of the genes and the traits they encode. For a pair of genes that determine the height of plants, a tall plant was homozygous, since there is no splitting among hybrids for this trait. For a pair of genes that determine the color of the seed, both plants were heterozygous, since 3: 1 splitting takes place among hybrids for this trait. The parent plant with round seeds was a dominant homozygous, since the seeds of all hybrids turned out to be round.

Answer

III. Homework

Solve the genetic problem.

Chickens have feathered legs BUT) dominate naked ( a), rose-shaped comb ( AT) – over simple ( b), white plumage ( FROM) – above the colored ( With). A hen with feathered legs, a pink crest and white plumage is crossed with the same rooster. Among the offspring was a chicken with bare legs, a simple comb and colored feathers. Determine the genotypes of the parents.

Lesson #12-13 Linked inheritance of genes. Full and incomplete clutch. Genetic maps

Equipment: tables by general biology illustrating the linked inheritance of genes and traits.

During the classes

I. Knowledge Test

Checking the solution of the problem at home.

Given:

BUT- feathered legs
a- bare feet
AT- rose comb
b- simple comb
FROM- white plumage
With- painted plumage

P genotypes?

Solution

When two parental individuals are crossed, each of which was a carrier of three dominant traits, a carrier of three recessive traits appears among the hybrids. This indicates the triheterozygosity of each of the parents.

(31 op., pink, white: 9 heads, ros., white: 3 heads, ave., white: 5 op., ave., white: 9 op., ros., dyed: 3 op., ave., dyed: 3 head, rose, dyed: 1 head, ave., dyed.)

Answer:

Independent work

Each student receives a card with the text of the genetic problem and solves it in a notebook.

Task number 1. Tomato plants of the Golden Beauty variety have yellow fruits and high growth, the Karlik variety is undersized with red fruits. How can you get a homozygous dwarf variety with yellow fruits using these varieties?

Task number 2. Tomato fruits are red and yellow, smooth and fluffy. The red gene is dominant, the fluffy gene is recessive. Both pairs are on different chromosomes. What offspring can be expected from crossing heterozygous tomatoes with an individual homozygous for both recessive genes?

Task number 3. Tomato fruits are red and yellow, smooth and fluffy. The red gene is dominant, the fluffy gene is recessive. Both pairs are on different chromosomes. Of the tomatoes harvested on the collective farm, there were 36 tons of smooth red and 12 tons of red fluffy ones. How many yellow fluffy tomatoes will be in the collective farm crop if the source material was heterozygous for both traits?

Task number 4. In cattle, the polled gene dominates the horned gene, and the black gene dominates the red gene. Both pairs of genes are not linked, that is, they are located on different pairs of chromosomes. In the breeding sovkhoz, black polled cows were crossed with a black polled bull for a number of years. 896 heads of young animals were obtained, of which 535 were black polled calves and 161 were red polled. How many horned calves were there and how many of them are red?

Task number 5. In cattle, the polled gene dominates the horned gene, and the black gene dominates the red gene. Both pairs of genes are not linked, that is, they are located on different pairs of chromosomes. On the farm, 984 calves were obtained from 1000 horned red cows. Of these, 472 are red, 483 are polled, and 501 are horned. Determine the genotypes of the parents and the percentage of black calves.

Task number 6. The human has a gene brown eyes dominates the gene blue eyes, and the ability to use predominantly the 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?

Task number 7. In humans, the gene for brown eyes dominates over the gene for blue eyes, and the ability to use predominantly the right hand dominates over left-handedness. Both pairs of genes are located on different chromosomes. 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. Determine the probability of the birth in this family of blue-eyed children, owning mainly the left hand.

Task number 8. By crossing phlox plants with white funnel-shaped flowers with a plant having creamy saucer-shaped flowers, 96 plants with white saucer-shaped flowers were obtained. Determine the genotypes of the original plants if it is known that each of the traits is inherited by a monogenic type and the traits are inherited independently. What are the dominant traits?

Task number 9. By crossing two white-flowered phlox plants with saucer-shaped flowers in F 1, 49 plants with white saucer-shaped flowers, 24 with white funnel-shaped flowers, 17 with cream-colored saucer-shaped flowers and 5 with cream-colored funnel-shaped flowers were obtained. Is it possible to determine, based on the results of this crossing, how these traits are inherited? Determine the genotypes of the original plants. What splitting should occur if the original plants are crossed with a plant with cream funnel-shaped flowers from F 1 ?

Task number 10. During self-pollination of two tomato plants with red two-nested fruits, one of them gave only plants with red two-nested fruits, and 24 plants with red two-nested fruits and 10 plants with red multi-nested fruits were obtained from the second. Is it possible to determine the genotypes of the original plants? What crosses need to be made to test your hypothesis?

II. Learning new material

Linked inheritance of genes

G. Mendel traced the inheritance of seven pairs of traits in peas. Many researchers, repeating Mendel's experiments, confirmed the laws he discovered. It was recognized that these laws were of a universal nature. However, in 1906, the English geneticists W. Batson and R. Pennet, crossing sweet pea plants and analyzing the inheritance of pollen shape and flower color, found that these traits do not give independent distribution in the offspring. Descendants always repeated the features of parental forms. Gradually, the facts of exceptions to Mendel's third law accumulated. It became clear that not all genes are characterized by independent distribution in the offspring and free combination.

Any organism has a variety of morphological, physiological, biochemical and other features and properties, and each feature or property is controlled by one or more genes located in the chromosomes.

However, if the number of genes in an organism is enormous and can number in the tens of thousands, then the number of chromosomes is relatively small and, as a rule, is measured in a few tens. Therefore, hundreds and thousands of allelic genes forming linkage groups are localized in each pair of chromosomes.

A complete correspondence between the number of linkage groups and the number of pairs of chromosomes has been established. For example, maize has a set of chromosomes 2n = 20 and 10 linkage groups, and Drosophila has 2n = 8 and 4 linkage groups, that is, the number of linkage groups is equal to the haploid set of chromosomes.

Thomas Morgan law

Genes located on the same chromosome are transmitted together, and the way they are inherited differs from the inheritance of genes located on different pairs of homologous chromosomes.

So, for example, with an independent distribution of chromosomes, a dihybrid AaBb forms four types of gametes ( AB, aB, Ab, ab), and under the condition of complete linkage, the same dihybrid will give only two types of gametes ( AB and ab), since these genes are located on the same chromosome.

The development of the problem of linked inheritance of genes belongs to the school of T. Morgan (1866–1945). If Mendel conducted his experiments on peas, then for Morgan the main object was the Drosophila fruit fly. A fly every two weeks at a temperature of 25 ° C gives numerous offspring. The male and female are clearly distinguishable - the male's abdomen is smaller and darker. In addition, they differ in numerous characteristics and can multiply in test tubes on a cheap nutrient medium.

Studying patterns of inheritance genes located on the same chromosome, Morgan came to the conclusion that they are inherited linked. This is T. Morgan's law.

Full and partial clutch

To determine the type of inheritance of two pairs of genes (linked or independent), it is necessary to carry out an analyzing cross and, based on its results, draw a conclusion about the nature of gene inheritance. Consider three possible options test cross results.

1) Independent Inheritance.

If, as a result of analyzing crosses, four classes of phenotypes are formed among hybrids, then the genes are inherited independently.

2) Full linkage of genes.

With full linkage of genes BUT and AT according to the results of analyzing crosses, they find -
Xia two phenotypic classes of hybrids that completely copy the parents.

3) Incomplete linkage of genes.

In case of incomplete linkage of genes BUT and AT when analyzing crosses, four phenotypes appear, two of which have a new combination of genes: Ab‖ab; aB‖ab. The appearance of such forms indicates that the dihybrid with gametes AB│ and ab│ forms crossover gametes Ab│ and aB│. The appearance of such gametes is possible only as a result of the exchange of sections of homologous chromosomes, that is, in the process of crossing over. The number of crossover gametes is much less than non-crossover ones.

The crossover frequency is proportional to the distance between the genes. The closer the genes are located on the chromosome, the closer the linkage between them and the less often they are separated when they cross. And vice versa, the farther the genes are separated from each other, the weaker the linkage between them and the more often they cross over. Therefore, the distance between genes in chromosomes can be judged by the frequency of crossover.

Genetic maps

Genetic mapping is usually understood as determining the position of a gene in relation to other genes.

Consider the procedure for compiling genetic maps.

1. Establishment of the linkage group (that is, the determination of the chromosome in which the given gene is localized). To do this, it is necessary to have at least one marker gene in each linkage group.

2. Finding the location of the studied gene in the chromosome. To do this, the mutant form is crossed with the normal form and the result of crossing over is taken into account.

3. Determination of the distance between linked genes, which makes it possible to compile genetic maps of chromosomes, which indicate the order of the genes in the chromosomes and their relative distances from each other. The higher the crossover frequency, the further apart the genes are. If it is established that between linked genes BUT and AT crossover frequency is 10%, and between genes AT and FROM– 20%, it is obvious that the distance Sun 2 times more than AB. The distance between genes is expressed in units corresponding to 1% crossing over. These units are called morganides.

Thus, based on data on the frequency of crossing over, genetic maps are compiled.

III. Consolidation of knowledge

Solution of the genetic problem

A female Drosophila, heterozygous for the recessive genes for dark body color and miniature wings, was crossed with a male with a dark body and miniature wings. From this crossing was obtained:

- 244 flies with a dark body and miniature wings;
– 20 flies with gray body color and miniature wings;
– 15 flies with dark body color and normal wings;
– 216 flies with gray body color and normal wings.

Based on the given data, determine whether these two pairs of genes are linked or not. How are genes linked?

Given:

BUT- gray body
a- dark body
AT– normal wings
b- miniature wings

The nature of inheritance of genes BUT and AT -?

Solution

The results of splitting among hybrids (two phenotypic classes are dominant and repeat phenotypically and genotypically parental forms, and the other two classes of phenotypes are represented by a small number of individuals) indicate incomplete linkage of genes BUT and AT.

Answer: genes BUT and AT are inherited in a linked manner; bonding is incomplete.

IV. Homework

Study the paragraph of the textbook (linked inheritance of genes, T. Morgan's law, complete and incomplete linkage, genetic maps).

Solve the genetic problem.

Determine the frequency of crossing over between genes if, when crossing gray long-winged flies with black short-winged flies in F 1, all flies were gray long-winged, and in the analyzing crossing of F 1 females with a black short-winged male, the following was obtained:

– 722 gray long-winged flies;
– 139 gray short-winged flies;
– 161 black long-winged fly;
– 778 black short-winged flies.

Lesson #14-15 Chromosomal theory of heredity. Non-chromosomal (cytoplasmic) inheritance

Equipment: tables on general biology, illustrating the linked inheritance of genes and traits; portraits of scientists who have made a special contribution to the creation of the chromosome theory of heredity.

During the classes

I. Knowledge Test

1. Oral knowledge test on:

Linked inheritance of genes. T. Morgan's law.
Complete and incomplete linkage of genes.
Genetic maps and the order of their compilation.

2. Checking the solution of the problem at home.

Given:

BUT- gray body
a- dark body
AT- long wings
b- short wings

Crossover frequency - ?

Solution

We draw up schemes for the first and second crossings.

N (hybrids) = 722 + 778 + 161 + 139 = 1800 individuals.

N (recombinants) = 161 + 139 = 300 individuals.

(300 × 100) : 1800 = 16.6%

Answer: 16,6%.

II. Problem solving in the classroom

Task number 1. Assuming that the genes BUT and AT are linked and the cross between them is 10%, then which gametes and in what quantity will be formed by a diheterozygote AB‖ab?

Solution

Crossover between genes BUT and AT is 10%, which means that crossover gametes are formed in the amount of 10% (5% of each type). The share of non-crossover gametes remains 90% (45% of each type).

Answer: AB (45%); ab (45%); Ab (5%); aB (5%).

Task number 2. The smooth form of corn seeds dominates over the wrinkled, the colored seeds dominate over the uncolored ones. Both traits are linked. When crossing corn with smooth colored seeds with a plant with wrinkled uncolored seeds, offspring were obtained:

painted smooth - 4152 individuals;
colored wrinkled - 149;
unpainted smooth - 152;
unpainted wrinkled - 4163.

Determine the distance between genes.

Given:

BUT- smooth seeds
a- wrinkled seeds
AT- colored seeds
b- uncolored seeds

Distance AB – ?

Solution

We draw up a crossover scheme.

2) Calculate the total number of hybrids:

N (hybrids) = 4152 + 4163 + 152 + 149 = 8616 individuals.

3) We count the number of recombinant individuals:

N (recombinants) = 152 + 149 = 301 individuals.

4) Determine the crossover frequency:

(301 × 100) : 8616 = 3.5%

5) One percent of crossing over equals 1 morganide, so the distance between genes BUT and AT equals 3.5 morganides.

Answer: 3.5 morganides.

III. Learning new material

Chromosomal theory of heredity

Thanks to the research of A. Weisman, T. Boveri, T. Morgan and other prominent geneticists and cytologists, by the 40s. 20th century The chromosome theory of heredity was formulated. The modern chromosome theory of heredity includes the following postulates:

1) the signs of organisms are formed under the action of genes located on chromosomes;
2) there are chromosomes in every cell, and their number is constant for each species;
3) gametes contain a haploid set of chromosomes;
4) in the zygote and somatic cells - a paired, diploid set of chromosomes. Half of the chromosomes of the zygote of maternal origin, the other - paternal;
5) chromosomes retain structural and genetic individuality in life cycle organisms;
6) in the chromosome, the genes are located linearly and within the same chromosome form a linkage group. The number of linkage groups is equal to the haploid number of chromosomes;
7) the frequency of crossing over occurring in meiosis is proportional to the distance between genes.

Thus, the chromosome theory of heredity is an outstanding achievement of biological science. It was the result of combining knowledge gained in two biological disciplines: genetics and cytology.

Cytoplasmic inheritance

Along with the facts confirming the chromosome theory of heredity, in the process of the formation of genetics as a science, facts about heredity began to accumulate that do not obey established patterns: inheritance only through the maternal line, deviations from Mendelian numerical relations, etc.

These cases could only be explained by the localization of the genes that determine this trait in the cytoplasm, that is, the cytoplasm also plays a role in the formation of some traits. This phenomenon is called cytoplasmic, or extrachromosomal, heredity. For example, the chloroplasts of higher plants during sexual reproduction are transmitted through the maternal line, male gametes do not contain them, therefore, the zygote contains the chloroplasts that were in the egg. Chloroplasts contain their own circular DNA, which ensures the synthesis of certain proteins and RNA responsible for a number of traits.

At the night beauty and snapdragon known phenomena of variegation associated with mutations in the DNA of some chloroplasts. These mutations can cause chloroplasts to lose their green color. When a cell divides, the distribution of chloroplasts to daughter cells occurs randomly; daughter cells may contain colored, colorless, or both chloroplasts. If the egg contained colorless chloroplasts, then an uncolored plant will develop from the zygote, having used up the supply of nutrients, it dies. If the egg contains only green chloroplasts, a normal green plant will develop. If both green and colorless chloroplasts get into the egg, then the plant will be variegated. In the same way, traits associated with mutations that have occurred in mitochondria are inherited.

IV. Homework

Study the paragraph of the textbook and the notes made in the class (provisions of the chromosome theory of heredity, cytoplasmic inheritance).

Information block.

Linkage of genes.

At the beginning of the 20th century (1902-1907), the American scientist W. Setton and the German embryologist T. Boveri discovered parallelism in the inheritance of traits and the behavior of the chromosomes of the cell nucleus during gametogenesis and during fertilization. This confirmed the localization of hereditary information in chromosomes. It has been established that the number of genes significantly exceeds the number of chromosomes. So a person has 46 chromosomes, and genes from 70,000 to 100,000. Therefore, a large number of genes are localized in each chromosome. Genes on the same chromosome are inherited together (linked). An experimental study of this phenomenon was carried out by the American geneticist T. Morgan and his colleagues: A. Sturtevant, A. Bridges and G. Möller in 1910-1916. These studies confirmed the chromosomal localization of genes and formed the basis of the chromosomal theory of heredity.

The main provisions of the chromosome theory of heredity.

1. Each gene occupies a certain place in the chromosome - a locus.

2. Genes in the chromosome are arranged linearly in a certain sequence.

3. Different chromosomes contain an unequal number of genes. The set of genes on each non-homologous chromosome is unique.

4. The genes of one chromosome form a "linkage group" and are inherited together, i.e. linked.

5. The number of linkage groups is equal to the number of chromosomes in the haploid set (there are four in Drosophila, 10 in corn, 20 in mice, and 23 in humans).

6. An exchange of allelic genes can occur between homologous chromosomes, i.e. crossing over.

7. Crossover frequency is directly proportional to the distance between genes in a linkage group.

8. A special unit, the morganide (M), is taken as a unit of distance between genes in a linkage group. 1M=1% crossover.

Distinguish between complete and incomplete linkage of genes.

Full grip.

In experiments on Drosophila, it was found that the development

traits that are inherited are linked controlled by the genes of one

chromosomes. Genes for body color (b - gray and B - black) and wing length

(v - normal and V - short, rudimentary wings) are localized in one pair of homologous chromosomes.

Crossing of gray flies with normal wings and gray flies with rudimentary wings gives in the first generation of gray hybrids with normal

When conducting analyzing crosses, males were selected from p, because it is known that male Drosophila have achiasmatic spermatogenesis (i.e., crossing over does not occur and the completeness of gene coupling is not disturbed in any way). As a result of such crossing, individuals of two phenotypes similar to the original parental forms, and in equal amounts: splitting according to the phenotype 1:1.

Considered together, the results of both crossings convince us that the development of the analyzed traits is controlled by different genes, and linked inheritance is explained by the localization of genes on the same chromosome. Completeness of coupling in this case is not violated in any way. This linkage of genes is complete.

To study incomplete linkage from P, females (genotype B||b) were selected; in C females, crossing over occurs during gametogenesis. Therefore, a diheterozygous individual forms additional, i.e. crossover varieties of gametes. The probability of their formation is determined by the probability of crossing over, i.e. depends on the distance between the genes in the linkage group.

Not recombinant individuals; in Recombinant individuals; at

they have the same formation of their genotypes

combinations that the original ones involved crossover

parent forms of the gamete.

AT this example splitting by phenotype in the offspring obtained the following: gray flies with long wings - 41.5%; blacks with short wings - 41.5%; gray short-winged - 8.5%; black long-winged - 8.5%. Thus, the probability of the appearance of recombinant individuals in the offspring is 17%. Therefore, the distance between genes B and V in the linkage group is 17 morganiids.

The predominance of gray long-winged and black short-winged flies in the offspring indicates that genes B and V; b and v are indeed linked. On the other hand, the appearance of recombinant individuals indicates that in a certain number of cases there is a break in the linkage between genes B and V and genes B and V. This is the result of crossing over.

An example of complete linkage of genes in humans is the inheritance of the Rh factor. It is due to three pairs of C, D, K, closely linked to each other, so the inheritance of Rh - belonging occurs according to the type of monohybrid crossing. Another example of close linkage of genes in humans is the inheritance of cataracts and polydactyly. The genes for hemophilia and color blindness are located on the X chromosome at a distance of 9.8 morganids (M), i.e. undergo crossing-over, so they are inherited as incompletely linked. The autosomal genes of the Rh factor and the forms of erythrocytes, located at a distance of 3 M from each other, are also an example of incomplete linkage.

In 1909, F. Janssens, while studying meiosis in amphibians, discovered chiasmata (crossings) of chromosomes, which are cytological evidence of crossing over. Since that time, many attempts have been made to explain the mechanism of this phenomenon. There are several theories of crossover. The most common are two hypotheses.