In 1906, W. Batson and R. Punnett, crossing sweet pea plants and analyzing the inheritance of pollen shape and flower color, discovered that these characteristics do not give independent distribution in the offspring; hybrids always repeated the characteristics of the parent 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, but 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 one chromosome. T. Morgan. If Mendel conducted his experiments on peas, then for Morgan the main object was the fruit fly Drosophila.
Drosophila produces numerous offspring every two weeks at a temperature of 25 °C. The male and female are clearly distinguishable in appearance - the male has a smaller and darker abdomen. They have only 8 chromosomes in a diploid set and reproduce 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 having 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, - above the gene of underdeveloped wings). When carrying out an analytical 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 characteristics of the parental forms clearly predominated (41.5% - gray long-winged and 41.5% - black with rudimentary wings), and only a small part of the flies had a combination of characters different from those 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 located on the same chromosome.
1 - non-crossover gametes; 2 - crossover gametes.
If the genes for body color and wing shape are localized on one chromosome, then this crossing should have resulted in two groups of individuals repeating the characteristics of the parental forms, since the maternal organism should form gametes of only two types - AB and ab, and the paternal one - one type - ab . Consequently, 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 characters, that is, having genotypes Aabb and aaBb. In order to explain this, it is necessary to recall the mechanism of formation of germ cells - meiosis. In the prophase of the first meiotic division, homologous chromosomes are conjugated, and at this moment an exchange of regions can occur between them. As a result of crossing over, in some cells, sections of chromosomes 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 the free combination of genes. But, since crossing over occurs during the formation of a small part of gametes, the numerical ratio of phenotypes does not correspond to the ratio 1:1:1:1.
Clutch group- genes localized on the same chromosome and inherited together. The number of linkage groups corresponds to the haploid set of chromosomes.
Chained 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 further 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 characteristics as their parents.
Recombinants- hybrid individuals that have a different combination of characteristics than their parents.
The distance between genes is measured in Morganids— conventional 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 morganids.
In diheterozygotes, dominant genes can be located either on one chromosome ( cis phase), or in different ( trans phase).
1 - Cis-phase mechanism (non-crossover gametes); 2 - trans-phase mechanism (non-crossover gametes).
The result of T. Morgan's research was the creation of chromosomal theory of heredity:
- genes are located on chromosomes; different chromosomes contain different numbers of genes; the set of genes of each of the non-homologous 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 specific linear sequence;
- genes localized 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 disrupted during 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 magnitude of crossing over;
- Each species has a unique set of chromosomes - a karyotype.
Go to lectures No. 17“Basic concepts of genetics. Mendel's laws"
Chromosomal level of organization of hereditary material. Chromosomes as gene linkage groups.
It follows from the principles of genetic analysis that independent combination of traits can only be carried out under the condition that the genes that determine these traits are located in different pairs of chromosomes. Consequently, in each organism, the number of pairs of characters for which independent inheritance is observed is limited by the number of pairs of chromosomes. On the other hand, it is obvious that the number of characteristics and properties of an organism controlled by genes is extremely large, and the number of pairs of chromosomes in each species is relatively small and constant. It remains to be assumed that each chromosome contains not one gene, but many. If this is so, then it should be recognized that Mendel’s third rule concerns only the distribution of chromosomes, and not genes, i.e. its action is limited. Analysis of the manifestation of the third rule showed that in some cases new combinations of genes were completely absent in hybrids, i.e. complete linkage was observed between the genes of the original forms and a 1:1 split was observed in the phenotype. In other cases, a combination of traits was observed with less frequency than expected from independent inheritance.
In 1906, W. Betson described a violation of the Mendelian law of independent inheritance of two characters. Questions arose: why are not all traits inherited and how are they inherited, how are genes located on chromosomes, what are the patterns of inheritance of genes located on the same chromosome? The chromosomal theory of heredity, created by T. Morgan, in 1911, was able to answer these questions.
T. Morgan, having studied all the deviations, proposed to call the joint inheritance of genes, limiting their free combination, linkage of genes or linked inheritance.
Patterns of complete and incomplete coupling. Clutch groups in humans.
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 identical sections of homologous chromosomes with the genes they contain is called chromosome crossing or crossing over. Crossing over occurs in meiosis. It provides new combinations of genes located on homologous chromosomes. The phenomenon of crossing over, like gene linkage, is characteristic of animals, plants, and microorganisms. The exceptions are male fruit flies and female silkworms. Crossing over ensures the recombination of genes and thereby significantly increases the role of combinative variability in evolution. The presence of crossing over can be judged by taking into account the frequency of occurrence of organisms with a new combination of characteristics. The phenomenon of crossing over was discovered by Morgan in Drosophila.
Recording the genotype of a diheterozygote with independent inheritance:
A IN
Recording the genotype of a diheterozygote with linked inheritance:
Gametes with chromosomes that have undergone crossing over are called crossover, and those that have not undergone are called non-crossover.
AB, AB AB, AB
Non-crossover gametes. Crossover gametes.
Accordingly, organisms that arise from a combination of crossover gametes are called crossovers or recombinants, and those arising from a combination of non-crossover gametes - non-crossovers or non-recombinants .
The phenomenon of crossing over, as well as the linkage of genes, can also be considered in the classic experiment of T. Morgan when crossing Drosophila.
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Sign |
P♀ B.V. x♂ bv |
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gray body color |
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black body color |
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normal wings |
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vestigial wings |
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Analysis cross 1. Complete linkage of genes. 2. Incomplete linkage of genes. |
1. Full grip P♀ bv x♂ B.V. F 2 bv bv splitting – 1:1 |
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2. Incomplete traction (crossing over) P:♀ B.V. x♂ bv G: BV bv Bv bV bv non-crossover crossover F 2 B.V. bv Bv bV non-crossovers – 83% crossovers – 17% |
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To measure the distance between genes by test crossing, you can use the formula:
Where:
X– distance between genes in % crossing over or in morganids;
A– number of individuals of the 1st crossover group;
V– number of individuals of the 2nd crossover group;
n– total number of hybrids in the experiment;
100% – coefficient for conversion to percentage.
Based on a study of linked inheritance, Morgan formulated a thesis that was included in genetics under the name Morgan's rule : genes localized on the same chromosome are inherited linked, and the strength of linkage depends on the distance between them.
Linked genes are arranged in a linear order and the frequency of crossing over between them is directly proportional to the distance between them. However, this thesis is typical only for genes that are close to each other. In the case of relatively distant genes, some deviation from this dependence is observed.
Morgan proposed expressing the distance between genes as the percentage of crossing over between them. The distance between genes is also expressed in morganids or centimorganids. Morganidae is the genetic distance between genes where crossing over occurs with a frequency of 1%.
The frequency of crossing over between two genes can indicate the relative distance between them. So, if between genes A And IN crossing over is 3%, and between genes IN And WITH– 8% crossing over, then between A And WITH crossing over should occur at a frequency of either 3+8=11% or 8-3=5%, depending on the order in which these genes are located on the chromosome.
A ─ ─ ─ B ─ ─ ─ ─ ─ ─ ─ ─ C B ─ ─ ─ A ─ ─ ─ ─ ─ ─ ─ ─ C
Task 1. Cataracts and polydactyly are inherited as dominant autosomal traits. The woman inherited cataracts from her father and polydactyly from her mother. The genes are linked, the distance between them is 3M. What are the genotypes and phenotypes of the children from the marriage of this woman and a man normal for these characteristics? What is the probability of having healthy children?
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cataract |
P♀ aB x ♂ aw |
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polydactyly |
X = AB = 3 Morgue. |
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P♀ aB x ♂ aw |
Answer: the probability of having a healthy child is 1.5%, having one characteristic is 48.5%, having both characteristics is 1.5% |
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G: (аВ) (Ав) (ав) |
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F1 aB Av aw AB aw aw aw aw 48,5% 48,5% 1,5% 1,5% |
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Genetic map chromosomes is a diagram showing the order of genes at their relative distance from each other. The distance between linked genes is judged by the frequency of crossing over between them. Genetic maps of all chromosomes have been compiled for the most genetically studied organisms: Drosophila, chickens, mice, corn, tomatoes, Neurospora. Genetic maps of all 23 chromosomes have also been compiled for humans.
After establishing the linear discreteness of chromosomes, the need arose to compile cytological maps for the purpose of comparison with genetic maps compiled on the basis of taking into account recombinations.
Cytological card is a map of a chromosome that determines the location and relative distance between genes on the chromosome itself. They are constructed based on the analysis of chromosomal rearrangements, differential coloring of polytene chromosomes, radioactive labels, etc.
To date, genetic and cytological maps have been constructed and compared for a number of plants and animals. The reality of this comparison confirms the correctness of the principle of the linear arrangement of genes on a chromosome.
In humans, some cases of linked inheritance can be named.
The genes that control the inheritance of ABO blood groups and nail and patella defect syndrome are inherited linked.
The genes for the Rh factor and the oval shape of red blood cells are linked.
The third autosome contains the genes for the Lutheran blood group and the secretion of antigens A and B with saliva.
The genes for polydactyly and cataracts are inherited linked.
The X chromosome contains the genes for hemophilia and color blindness, as well as the genes for color blindness and Duchenne muscular dystrophy.
Autosome 6 contains subloci A, B, C, D/DR of the HLA system, which control the synthesis of histocompatibility antigens.
Inheritance of X-linked and holandric traits.
Traits controlled by genes located on the sex chromosomes are called adhered to the floor. More than 60 sex-linked diseases have been described in humans, most of which are inherited recessively. Genes on sex chromosomes can be divided into 3 groups:
Genes partially linked to sex. They are located in paired segments X And Y chromosomes . Partially sex-linked diseases include: hemorrhagic diathesis, convulsive disorders, retinitis pigmentosa, xeroderma pigmentosa, and general color blindness.
Genes are completely sex-linked. They are located in the area X chromosome , for which there is no homologous region in Y chromosome (heterological). These genes control diseases: optic atrophy, Duchenne muscular dystrophy, color blindness, hemophilia, and the ability to smell hydrocyanic acid.
Genes located in the region Y chromosomes , for which there is no homologous locus in X chromosome are called holandric . They control symptoms: syndactyly, hypertrichosis of the auricle.
The color blindness gene occurs in 7% of men and 0.5% of women, but 13% of women are carriers of this gene.
Sex-linked inheritance was described by T. Morgan using the example of inheritance of the eye color trait in Drosophila.
Several patterns of inheritance of sex-linked traits have been noted:
passed cross to cross (from father to daughter, from mother to son);
the results of direct and back crossings do not coincide;
in the heterogametic sex, the trait manifests itself in any state (dominant or recessive).
Basic provisions of the chromosomal theory of heredity.
The main provisions of the chromosomal theory of heredity can be formulated as follows:
Genes are located on chromosomes. Each gene on a chromosome occupies a specific locus. Genes on chromosomes are arranged linearly.
Each chromosome represents a group of linked genes. The number of linkage groups in each species is equal to the number of pairs of chromosomes.
Allelic genes are exchanged between homologous chromosomes—crossing over.
The distance between genes on a chromosome is proportional to the percentage of crossing over between them. Knowing the distance between genes, you can calculate the percentage of genotypes in the offspring.
The law of independent distribution of traits (Mendel's third law) is violated if the genes that determine different traits are located on the same chromosome. Such genes are usually inherited together, i.e. chained inheritance. The phenomenon of linked inheritance was studied by Thomas Morgan and his associates and is therefore called Morgan's law.
T. Morgan's law can be formulated as follows: genes located on the same chromosome form a linkage group and are often inherited together, while the frequency of joint inheritance depends on the distance between genes (the closer, the more often).
The reason why linked inheritance is disrupted is crossing over, which occurs in meiosis during the conjugation of chromosomes. In this case, homologous chromosomes exchange their sections, and thus previously linked genes can end up on different homologous chromosomes, which determines the independent distribution of traits.
For example, gene A is linked to gene B (AB), and the homologous chromosome contains recessive alleles of the corresponding genes (ab). If, during the process of crossing over, homologous chromosomes almost never exchange sections so that one gene moves to another chromosome, while the other remains in the same one, then such an organism forms gametes of only two types: AB (50%) and ab (50%). If an exchange of the corresponding sections occurs, then a certain percentage of the gametes will contain the Ab and aB genes. Usually their percentage is less than with independent distribution of genes (when A and B are on different chromosomes). If, with an independent distribution of all types of gametes (AB, ab, Ab, aB), there will be 25% each, then in the case of linked inheritance of gametes Ab and aB there will be less. The fewer there are, the closer the genes are located to each other on the chromosome.
Sex-linked inheritance is especially distinguished when the gene under study is located on the sex (usually X) chromosome. In this case, the inheritance of one trait is studied, and the second is gender. If an inherited trait is sex-linked, then it is inherited differently during reciprocal crosses (when the trait is first possessed by the female parent, then by the male).
If the mother has the aa genotype, and the father exhibits a dominant trait (there is definitely one gene A), then in the case of sex linkage, all daughters will have a dominant trait (in any case, they will receive his only X chromosome from the father, and all sons will have a recessive one) (from the father one gets the Y chromosome, which does not contain the corresponding gene, and from the mother, in any case, gene a.) If the trait were not sex-linked, then among both sexes of children there could be owners of a dominant trait.
When the genes under study are linked in an autosome, such linkage is called autosomal. Linkage is called complete if the parental combinations of alleles are not disrupted from generation to generation. This happens very rarely. Usually, incomplete linked inheritance is observed, which violates both Mendel's third law and Morgan's law (in its abbreviated form: genes located on the same chromosome are inherited together).
Genes on a chromosome are arranged linearly. The distance between them is measured in centimorgans (cm). 1 cm corresponds to the presence of 1% of crossover gametes. By conducting various crosses and statistically analyzing the descendants, scientists identify linked genes, as well as the distance between them. Based on the data obtained, genetic maps are constructed, which reflect the localization of genes on chromosomes.
The phenomenon of linked inheritance and its cytological basis
Note 1
The law of independent combination of genes is based on the principles that genes that determine certain traits and characteristics are localized on homologous chromosomes, and genes encoding different traits are located on different chromosomes. But the number of traits far exceeds the number of chromosomes in living organisms. The logical conclusion follows from this that each organism has a number of genes that can be independently combined in meiosis, but are limited by the number of pairs of chromosomes. As a result, there is far more than one gene on each chromosome.
Chromosomes are inherited as a single unit. They retain their integrity during conjugation and segregation in meiosis. Therefore, genes contained on the same chromosome are usually inherited together.
Genes that are localized on the same chromosome and can be inherited together constitute a linkage group. And the joint inheritance of genes is accordingly called gene linkage.
In organisms of a certain species, the number of linkage groups is equal to the number of chromosomes in the haploid set.
Chromosomal theory of heredity
The phenomenon of linked inheritance of traits was first described in 1906 by W. Betson and R. Punnett in experiments conducted with sweet peas. But they could not explain the results of the experiments and came to the conclusion that the rule of independent combination of characteristics established by G. Mendel was limited.
Experimental studies of the phenomenon of linked inheritance were successfully carried out by the outstanding American naturalist and geneticist Thomas Hunt Morgan. He and his assistants and collaborators A. Stervant, G. Miller and K. Bridges conducted thorough research. The results of these studies made it possible to propose and substantiate chromosomal theory of heredity .
Experiments by T. H. Morgan
To conduct research, T.H. Morgan chose the Drosophila fly as an object. Since then, this fly has become a classic subject for various genetic experiments. They are easy to keep and reproduce quickly. And the small number of chromosomes makes observation easier.
Example 1
The following experiment was carried out. Scientists crossed Drosophila males that were homozygous for dominant traits of body color and wing shape (namely, a gray body and normal wings) with females that were homozygous for recessive traits (a black body and underdeveloped wings). The genotypes of the studied individuals were designated accordingly EEVV And hervv . All first-generation hybrids were characterized by a gray body and normal wings. They were heterozygous. Their genotype could be written as EeVv . Then an analytical cross was carried out. To do this, first-generation hybrids were crossed with homozygotes for recessive traits. Theoretically, it could be assumed that a splitting of features would occur and the proportion of the results obtained would look like this: $1: 1: 1: 1$. In other words, each option will cost approximately $25$%. In fact, $41.5$% of individuals had a gray body and normal wings, $41.5$% had a black body and underdeveloped wings, $8.5$% had a gray body and underdeveloped wings, $8.5$% had a black body and normal wings. The experimental results allowed Morgan to formulate two important assumptions.
- The genes that determine body color and wing shape are localized on one chromosome and are subsequently inherited linked.
- During the process of meiosis and the formation of gametes, the homologous chromosomes of some individuals exchanged sections and formed a new linkage group.
Crossing over phenomenon
Definition 1
The phenomenon of chromosome crossing during meiosis and the subsequent exchange of chromosome sections is called crossing over .
It increases combinative variability, promoting the emergence of new combinations of alleles. The following patterns of crossing over were established:
- The strength of linkage between two genes that are located on the same chromosome is inversely proportional to the distance between them.
- The frequency of crossing over, which occurs between two linked genes, is a relatively constant value for each specific pair of genes.
The main conclusion of Morgan's hypothesis was that genes are located on the chromosome along its entire length, one after another in a linear order.
Biology lesson for 10th grade
"Linked inheritance of genes."
Lesson objectives.
To form in students an understanding of linked inheritance, linkage groups, and genetic mapping.
Teach schoolchildren to explain the causes of linked inheritance of genes, as well as the disruption of linkage between them, which occurs in the prophase of the first meiotic division.
Convince high school students that genetic mapping makes it possible to establish the true location (localization) of individual genes on a chromosome, and then to influence the material basis of heredity.
Equipment. Table with mutations in Drosophila, formation of germ cells and crossing over, instruction cards, test text, Cyril and Methodius disk for grade 10, projector, computer, screen.
Lesson type. Studying new material and initially consolidating knowledge and methods of activity.
Methods used in the lesson: reproductive, partially search.
During the classes.
Teacher activities
Student activities
Updating knowledge.
Today we continue to study the section “Patterns of Heredity and Variation”, we will again solve genetic problems.
Now we open the notebooks, write down the number, but leave room for the topic of the lesson, you will tell me it yourself later. We have already solved problems about humans, dogs, cats, but the object in the first problem today will be the fruit fly Drosophila - the favorite object of all geneticists. And why she is so loved, ____________ will tell us
She prepared a short message. Your task is to listen carefully to the message and write it down in your notebook - Straight to points 1, 2..
One student makes a message. The guys listen and write down the merits in a notebook.
Advantages of the Drosophila fly as an object of genetics.
-easily bred in captivity,
-very prolific
-has great variability,
-easy to learn,
-small number of chromosomes.
Creating a problem situation, formulating the topic and purpose of the lesson.
Now we know a lot about the darling of geneticists, and we can safely begin the task as real researchers.
Task.
In the Drosophila fly, gray body coloring dominates over dark ones, and long wings dominate over rudimentary wings. By crossing a gray fly with long wings with a dark fly with rudimentary wings in F 1, all the flies with a gray body and long wings were obtained. Next, F 1 was crossed with a recessive homozygote. What offspring should be expected from this cross?
One student decides at the blackboard, the rest at their desks
solve the problem.
Theoretically, everything is correct, according to Mendel’s law, we should get 4 genotypes, 4 phenotypes of 25% each, but in fact, as a result of such crossing, we received 42% of flies with gray long wings and 42% of flies with dark rudimentary wings. I write the result on the board. In what case do you think such a result could occur? Please note that there are more flies with the same characteristics as the parents.
How can these genes be inherited?
Where are they located?
So we come to the topic of today's lesson - What inheritance will we study today? I write the topic on the board.
And 8% are recombined traits. I write on the board. Studying this topic, we must answer why in our problem we got such a ratio that does not correspond to Mendel’s laws. And as a result, 8% appeared with recombined characteristics. This is our goal for today's lesson.
Any organism has many more genes than chromosomes. Humans have 23 pairs of chromosomes, 22 pairs of autosomes and 1 pair of sex chromosomes, and about a million genes. This means that several thousand genes are located on one chromosome.
You will learn from today’s lesson how genes located on the same chromosome are inherited and what laws apply in this case.
Children make guesses.
These gene pairs are inherited
Together. In other words, they are linked.
These genes are localized (or located) on the same chromosome.
Linked inheritance of genes.
Write down the topic in your notebook.
Learning new material.
Teacher activities
Information on the computer screen
Student activities
Slide lesson No. 31 "Linked inheritance of genes" Look at the lesson plan guys - what questions we will study today.
Slide 2 (Non-allelic genes).
Research by geneticists has shown that Mendel's laws are valid if the genes responsible for different traits are located in different pairs of homologous chromosomes (Bottom figure)
And if different genes are located in one pair of homologous chromosomes (Top picture), then the laws are different.
Slide 3 (Linked genes).
Read the information carefully and complete the task A instruction card. __ minutes to work.
So let's see what have you learned?
Which genes are called linked?
Give examples of linked traits.
Slide 4 (Morgan's experiments).
The patterns of inheritance when genes are located on the same chromosome were established by the American biologist Thomas Morgan.
I have prepared a message about the life of this scientist. Your task is to listen carefully and briefly write down Thomas Morgan's contributions to biology.
If Mendel's favorite object was the pea, then Morgan's favorite was the fruit fly. Thomas, conducting an experiment according to the conditions of our problem, obtained a result that we already know, contrary to Mendel’s laws. Let's hear what conclusions he came to.
Open slide 5 (Law of linked inheritance).
Look at how Morgan's law is formulated and write it down in your notebook.
Genes localized on one chromosome form clutch group and are inherited together. Review the diagram, how many linkage groups exist in different organisms.
This law explains why 84% (42%+42%) had a genotype and phenotype similar to their parents. But how new combinations appeared is still an open question.
Open slide 6.
Now read the textbook on page 219, paragraph 2 and complete the task on the instruction card. To work___minutes.
Let's check what you've learned along the way to your goal.
How can genes be linked?
Under what condition is complete adhesion observed?
Under what condition is incomplete adhesion observed?
When is gene linkage broken?
It turns out that when males are heterozygous. And females are homozygous, the linkage is complete, but if it’s the other way around, then the linkage is not complete.
Slide 8 (Genetic maps).
This pattern: the higher the crossover frequency, the farther the genes are located, they are used from each other to compile genetic maps.
What are genetic maps, why are they compiled and which ones have already been compiled. _____________, who prepared a message on this topic, will tell us. Your task is to listen carefully to the message, write down the definition and meaning of genetic cards.
So, what is a genetic map we see on the slide.
Where can genetic maps be used?
Slide 9 (Conclusions).
Read the lesson plan.
Listen and look at the drawings.
Study the information on the slide. Perform independent work.
Answer questions
Genes located on the same chromosome are called linked and are usually inherited together without independent distribution.
Tomato
Red color - dark stems and leaves
White flowers - light stems and leaves
Animals
Long neck - long limbs,
Short neck - short limbs
Human
Dark eyes – dark hair
Light eyes - light hair.
Listen to the message, write down the scientist’s contribution in a notebook.
Watch a video clip of Morgan's experiments.
Answer questions.
Law of linked inheritance: genes located on the same chromosome are inherited together (linked).
They look at the diagram.
Complete the task.
Answer questions.
As a result of crossing over
Complete and incomplete.
During the process of meiosis during conjugation.
Listen, record
This is a diagram of the relative location of genes that are in the same linkage group. It shows the sequence of genes on a chromosome and the distance between them.
The creation of genetic maps is necessary for the development of genetics, selection, genetic engineering, as well as evolutionary research.
Consolidation and testing of mastery of the material.
Before the final test, carefully review your notes, read the conclusions from the lesson, and begin the test work.
Generalization
Why did we get such a split in our problem?
Do Morgan's experiments 2 deny Mendel's law?
Lesson grades:
Homework. Paragraph 57 and notes in the notebook.
They write a test paper.
1 option
1. Genes located on the same chromosome during meiosis end up in...
A) different chromosomes,
B) one chromosome,
B) spiralization of chromosomes.
2. What genetic pattern is illustrated by the following fact: roses with red flowers have dark stems and leaves, and roses with white flowers have light stems and leaves?
A) linked inheritance of genes,
B) complete dominance,
3. What does Morgan's law reflect?
A) the law of uniformity,
B) the law of linked inheritance of traits, if the genes are on the same chromosome,
C) the law of independent segregation of characters if genes are located in different pairs of homologous chromosomes.
4. How many pairs of chromosomes are responsible for the inheritance of body color and wing shape in Drosophila?
A) four, B) two,
5. Determine in the figure which genes have the highest probability of crossover
a in c d A) ab and AB
B) sun and sun
B) cd and CD
A B C D
6. When ... ... genes, homologous chromosomes exchange their sections. This provides the possibility of the emergence of new combinations of genes and traits.
Final testing for the lesson
Topic: “Linked inheritance of genes”
Option 2
1. The law of chained inheritance states that:
A) genes of one somatic cell are inherited together,
B) genes located on the same chromosome are inherited together
C) genes located on different chromosomes are inherited together
2. Determine in the figure which genes have the highest probability of crossover
a in c d A) ab and AB
B) sun and sun
B) cd and CD
A B C D
3. Typically, people with dark hair have brown eyes, and people with blond hair have blue or gray eyes. What is the reason for this?
A) complete dominance
B) linked inheritance of genes,
B) intermediate inheritance.
4. Genes located on the same chromosome are inherited together. This is the law:
A) G. Mendel,
B) N. Vavilova,
B) T. Morgana,
5. Genes located on the same pair of chromosomes are called:
A) recessive
B) allelic
B) linked
6. All genes on one chromosome form one... ...they end up in one gamete during meiosis.
Application
Message "The Life and Work of Thomas Morgan"
Full name Thomas Hunt Morgan was born in 1866 in Lexington (Kentucky). At the age of 20 he graduated from the university of his native state, and 5 years later from the university in Baltimore. He immediately became a professor at the beginning of college, then at Columbia University, and from 1928 until the end of his life he headed a laboratory at the California Institute of Technology.
Colleagues at Columbia University were surprised when he, already widely known as an embryologist, decided to take up the fashionable but unsettled science of genetics. Morgan usually worked with rabbits, but he was not given money to run a large vivarium. The choice of the subject of experiments - the tiny fruit fly Drosophila - was his greatest success. Discoveries were not slow to appear. In general, Morgan confirmed Mendel's conclusions, but significantly supplemented them. Traits were discovered that were inherited together. And the number of such groups is equal to the number of chromosomes. Further, Morgan and his students showed that genes on chromosomes are arranged linearly, like beads on a string. And finally, the cohesion rule turned out to be not absolute. Thus, the cytological mechanism of Mendel's laws was clarified. Morgan's discoveries led to the final proof and completion of the chromosomal theory of heredity. Morgan showed a way to calculate the distance between genes to draw genetic maps.
Each such discovery can rightfully be called the greatest. But not only they brought Morgan worldwide fame. From his laboratory came the genetics of the first half of the 20th century, which is now called classical. Morgan gave genetics an object of research, a set of methods, and trained numerous students, many of whom also became world famous. This is his greatest merit. For his work on the study of heredity, Thomas Morgan was awarded the Nobel Prize in 1933. For a number of years he was president of the US National Academy of Sciences, and in 1932 he became an honorary member of the USSR Academy of Sciences. Morgan died in 1945.
Message "Genetic maps"
A genetic map is a diagram of the relative location of genes in the same linkage group. It shows the sequence of genes on a chromosome and the distance between them.
The distance between genes located on the same chromosome is determined as the percentage of gametes during the formation of which gene recombination occurred. This distance is expressed in morganids, in honor of T. Morgan. 1%=1 morganide. Based on the results of analyzing crosses, the distances between genes in the chromosomes of Drosophila, mice, silkworms, and yeast in barley, peas, cotton, corn, wheat, and tomato plants were determined (the slide shows the genetic map of tomato chromosome 2). Each pair of chromosomes has a unique pattern - a cytological map, and the genetic map represents its schematic reflection (drawing). Human chromosomes are also mapped. Based on new technologies, in 1989 the international program “Human Genome” was adopted, within the framework of which scientists from different countries, including Russia, work to determine the complete sequence of all nucleotide links of the human genome and determine their location. Currently, about 10 thousand genes have been mapped. On television and in the press you can sometimes hear assumptions about the loss of the Y chromosome and the extinction of the male sex after 5 million years, but scientists have deciphered the code of this chromosome and claim that it will live for at least another 50-60 million years. The Y chromosome contains only 78 genes, which is much less than other chromosomes. In order to draw up the map, it was necessary to study 300 thousand 269 inhabitants of different countries.
Maps were constructed based on the data obtained. We can confidently expect in the near future the emergence of molecular genetic maps that will carry not only comprehensive information about the location of genes on the chromosome, but also complete information about their nucleotide sequences.
There are also serious prospects for using genetic maps in practice. Human genetic maps can be very useful in the development of medicine and healthcare. Already at present, knowledge about the localization of a gene on a specific chromosome is used in the diagnosis of a number of severe hereditary human diseases. In the future, not only will the use of this approach expand dramatically, but there will also be opportunities for gene therapy, i.e. correction of gene structure or function.
Animal and plant breeding is another important area in which genetic maps are already being used. In microbiology, the use of genetic maps is also very important. The microbiological industry, not only of the near future, but also of today, is already unthinkable without detailed knowledge of genetic maps. The creation of groups of microorganisms capable of synthesizing proteins, hormones and other complex organic compounds necessary for pharmacology and agriculture is possible only on the basis of genetic engineering methods, i.e. is based on knowledge of the genetic maps of the corresponding microorganisms.
In the future, the number of species of plants, animals and microorganisms for which detailed genetic maps will be built. Will increase significantly.
A message about the Drosophila fly
Drosophila, a genus of dipteran insects in the fruit fly family. Length up to 3.5 mm. About 1000 species, widely distributed, more numerous in the tropics and subtropics. The common fruit fly is a classic object of genetics because...
-easily bred in captivity (in laboratories it lives well in test tubes on mashed bananas or semolina porridge with raisins seeded with yeast cells),
-very prolific (every 10-15 days at optimal temperature gives rise to a new generation and in each generation up to a thousand descendants),
-has great variability textbook figure 108
-easy to learn (Flies that have been euthanized for a while with ether can be clearly seen under a magnifying glass, changed individuals can be selected, and then crossed; males and females are easily distinguishable: the male has a smaller and darker abdomen),
-small number of chromosomes (in the diploid set 8) figure 110 of the textbook.
The study of the inheritance of traits in Drosophila served as the experimental basis for the chromosomal theory of heredity.
Task.
INSTRUCTION CARD
For the lesson on “Chained Inheritance”
Task.
In the Drosophila fly, gray body coloring dominates over dark ones, and long wings dominate over rudimentary wings. By crossing a gray fly with long wings with a dark fly with rudimentary wings in F 1, all the flies with a gray body and long wings were obtained. Next, F 1 was crossed with a recessive homozygote. What offspring should be expected from this cross?
Task A.
Write the definition in your notebook linked genes – This…
Write down examples of linked traits in different organisms in your notebook.
Task B.
Answer the question orally - why did individuals with recombined traits appear?
Write down the wording of Morgan's law or the law of linked inheritance in your notebook?
Task B.
Why, after all, among the second generation hybrids do a small number of individuals appear with recombination of parental characteristics?
Write down a diagram in your notebook, answering the question of what the linkage of genes might be.
Gene linkage
complete
incomplete
If there is no crossing of chromosomes and gene exchange.
If crossing over occurs and homologous chromosomes exchange sections.
In what process can crossing over (exchange of genes between chromosomes) occur?
When is crossover more likely?