Biography

Gregor Johann Mendel- an outstanding Czech naturalist. He was born in the Austrian Empire into a simple peasant family. At baptism he received the name Johann.

The boy was fond of studying nature since childhood, when he was still working, first as an assistant gardener, and then as a gardener. After studying for some time at the Olmutz Institute, in philosophical classes, in $1843$ he took the monastic vows and took the name Gregor. Then, from $1844 to $1848, Gregor Mendel studied at the Brunn Theological Institute and became a priest. During his studies, he independently studied many sciences, studied natural history at the University of Vienna.

It was in Vienna that Gregor Mendel became interested in research on hybridization processes and the statistical relationships of hybrids. Mendel paid special attention to questions of changes in the qualitative characteristics of plants. He chose peas as the object of experiments, which could be grown in the monastery garden. It was the observations of the results of these studies that formed the basis of the famous "Mendel's laws".

Encouraged by the first successes, Mendel transferred his experiments to a plant of the aster family (he crossed varieties of hawkweed) and crossed varieties of bees. The results of the experiments did not coincide with the results of experiments with peas. At that time, they did not yet know that the mechanism of inheritance of traits in these plants and animals differs from the mechanism of inheritance in peas.

Remark 1

Gregor Mendel was disillusioned with biological science. After his appointment as abbot of the monastery, he no longer engaged in science. But his merit is that he was the first to identify and describe the statistical patterns of inheritance of traits in hybrids. Let's get acquainted with them in more detail.

Mendel's first law

To facilitate the accounting of the results of the experiment, Gregor Mendel chose plants with clearly different characteristics. It was the color and shape of the seeds.

To begin with, he obtained the seeds of "pure lines" of plants. These seeds, upon further sowing and as a result of self-pollination, did not show splitting of traits.

When crossing different varieties of peas - with purple flowers and with white flowers, in the first generation of hybrids, Mendel received all plants with purple flowers. Similar results were obtained when the scientist took pea plants with yellow and green seeds or seeds of a smooth and wrinkled shape.

Based on the results of these experiments, Gregor Mendel deduced law of uniformity of hybrids of the first generation , which we know as Mendel's first law. Today it sounds like this:

“When crossing two homozygous organisms. which belong to pure lines and differ from each other in one pair of alternative manifestations of a certain trait, the entire first generation of hybrids (F1) will be completely uniform and will carry the manifestation of the trait of only one of the parents.

This law is also called the law of dominance of signs . It means that the dominant trait appears in the phenotype, suppressing the recessive one.

Mendel's second law

Conducting further experiments with hybrids of the first generation, Mendel found that with further crossing of hybrids of the first generation among themselves, hybrids of the second generation are characterized by splitting of characters with stable constancy. Today, this law is formulated as follows:

Definition 1

“After crossing two heterozygous descendants of the first generation with each other, splitting is observed in the second generation in a certain numerical ratio: according to the phenotype $3:1$, according to the genotype $1:2:1$.”

He got the name splitting law . It means that the recessive trait in hybrids of the first generation does not disappear, but is only suppressed and then manifests itself in the second hybrid generation.

Mendel's third law

In the first experiments, Gregor Mendel took into account only one pair of alternative signs. He became interested in the question, what if we take into account several signs. The signs began to combine with each other and at first caused confusion among the scientist. But upon closer examination, Mendel managed to deduce the pattern of splitting. It turned out that the hybrids of the first generation are uniform, and in the second generation, the phenotypic traits are split in the proportion of $9:3:3:1$, regardless of the other trait. This law was called law of independent succession . Today its wording looks like this:

Definition 2

“When two individuals are crossed that differ from each other in several pairs (two or more) of alternative traits, the genes and their corresponding traits are inherited from each other independently and can be combined in all possible combinations (similar to monohybrid crossing).”

Patterns discovered by Mendel anticipated the beginning new science- genetics.

Topic: Basic laws of trait inheritance. Laws of Mendel.

Lesson Objectives:

Educational:

To form ideas about monohybrid crossing, the first and second laws of G. Mendel.

To consolidate knowledge of the terms and symbols used in genetics.

To contribute to the formation of students' skills to find causal relationships between the genotype and phenotype, to continue the formation of a biological picture of the world.

Developing: To develop in students the ability to highlight the main thing, compare, contrast.

Educational:

To promote the development of interest in genetics as a science.

Cultivate a tolerant attitude towards people of different races.

Methods: Explanatory-motivating, partially exploratory, method of self-organization of cognitive work.

Lesson type: combined

Equipment:

Portrait of G. Mendel,

multimedia equipment,

Handout,

dynamic manual "Monohybrid crossing".

During the classes

1. Organizational moment. Examination. D / s.

2.Updating knowledge.

3. goal setting 1 min.

Teacher. Today we begin the study of the science of genetics, get acquainted with new concepts, terms, symbols; learn how to solve genetic problems.(slide 1 - topic and goals)

Before we start studying the topic, we will recall the definitions that we already know.
What is heredity?

Heredity is the property of all living organisms to transmit their characteristics and properties from generation to generation.

What is variability?

Variability is the property of all living organisms to acquire in the process individual development new signs and properties.

What is a hybrid?

Organisms resulting from crossbreeding.

Genotype is the totality of genes, the totality of all hereditary properties of an individual.
Genetics - , studying the laws of heredity and variability of living organisms.

The patterns by which signs are passed down from generation to generation were first discovered by the great Czech scientist Gregor Mendel (1822-1884) (slide 2) portrait

2. Preparation for learning new material: 1 min.

In the old movie "The Circus", an actress, a fair-skinned woman, had a child - a dark-skinned baby. Why?

Let's turn to the teachings of the founder of genetics, Gregor Mendel (portrait, slide 2)

The student will acquaint with the biography of G. Mendel.

Student:

Johann Mendel was born in 1822 into a poor peasant family in a small village in the Austrian Empire (today it is the territory of the Czech Republic. Having taken the monastic rank, Johann Mendel received his middle name - Gregor. Gregor Mendel became a monk at the age of 25, after which he took a course mathematics and natural sciences at the University of Vienna. Later, from 1868, he was the abbot of the Augustinian monastery in the Czech city of Brno and at the same time taught natural history and physics at the school. For many years Mendel -an amateur conducted experiments in the monastery garden, he begged for a small fenced-in plot for a garden and in 1865 published the work “Experiments on Plant Hybrids”, in which he outlined the basic laws of heredity.

He devoted many years of his life to the study of genetics.

3. Learning new material.

Genetics has its own terminology and symbolism.

Let's turn to the memos that are on your desk. Lay them out neatly.

Now let's get acquainted with the symbols that depict the crossing of hybrids (badges on cards, on each desk):

Slay.3

P - parents (from the Latin "parent" - parents)

♀ - "Mirror of Venus" - female,

♂ - "Shield and spear of Mars" - male

X - crossing.

F - from lat hybrid offspring, if the index is 1.2, etc., the numbers correspond to the serial number of generations (F1).

AA-dominant-homozygote

Aa-dominant-heterozygous

aa - recessive - homozygous

Mendel used peas for his research.(slide 4)

4. In his work, Mendel used the so-called hybridological method. The essence of this method lies in the crossing (hybridization) of organisms that differ from each other by any traits, and in the subsequent analysis of the nature of the inheritance of these traits in offspring. The hybridological method still underlies the research of all geneticists.

When conducting experiments, Mendel adhered to several rules.

First of all, working with garden peas, he used for crossing plants that belonged to different varieties. So, for example, one variety of peas was always yellow, while another was always green.(magnets on the board) Since peas are self-pollinating plants, natural conditions these varieties do not mix. Such varieties are called pure lines.

Secondly, to get more material for the analysis of the laws of heredity, Mendel not with one, but with several parent pairs of peas.

Thirdly, Mendel deliberately simplified the task by observing the inheritance of not all the traits of peas at once, but only one pair of them. For his experiments, he initially chose the color of pea seeds - peas. In cases where parental organisms differ in only one trait, for example, only in the color of the seeds or only in the shape of the seeds), the crossing is calledmonohybrid. Slide 5.

Fourth, having a mathematical education, Mendel applied quantitative methods to data processing: he not only noticed what the color of pea seeds was in offspring, but also accurately calculated how many such seeds appeared

It should be added that Mendel very successfully chose peas for experiments.

Why do you think this plant ? (Working with the textbook p. 101). Answer of the students.slide 4.

Peas are easy to grow, in the conditions of the Czech Republic they multiply several times a year, pea varieties differ from each other in a number of well-marked features, and, finally, in nature, peas are self-pollinating, but in the experiment this self-pollination is easy to prevent, and the experimenter can pollinate the plant with pollen from another plants, i.e., crosswise.

If we use terms that appeared many years after the work of Mendel, then we can say that Pea plants of one variety contain two genes only for yellow color, and the genes of plants of another variety contain two genes only for green color.

Genes responsible for the development of one trait (for example, seed color) are called allelic genes. slide 6.

If an organism contains two identical allelic genes (for example, both genes for green: seeds or, conversely, both genes for yellow seeds), then such organismscalled homozygous. If the allelic genes are different (that is, one of them determines the yellow and the other the green color of the seeds), then such organismscalled heterozygous. Slide 7.

Pure lines are formed by homozygous plants; therefore, during self-pollination, they always reproduce one variant of the manifestation of the trait. In Mendel's experiments, this was one of two possible colors for pea seeds - either always yellow or always green.

(Let's not forget that in those years when Mendel set up his experiments, about genes, chromosomes, and meiosis knew nothing!)

(Uniformity of hybrids of the first generation.) (Slide 8). Artificially crossing pea plants withyellow peas with plants havinggreen peas (i.e., carrying out monohybrid crossing), Mendel made sure that all seeds of hybrid descendants will beyellow colors. (I put magnets on the board).

Pupils work with cards on their desks.

He observed the same phenomenon in the experiment when crossing plants with smooth and wrinkled seeds - all hybrid plants had smooth seeds.

The sign that appears in hybrids (yellowness of seeds or smoothness of seeds) Mendel calleddominant , and the suppressed trait (i.e., the green color of the seeds or the wrinkling of the seeds) isrecessive .

It is customary to designate a dominant trait with a capital letter (A, B, C), and a recessive trait with a small letter (a, b, c).slide 9.

Based on these datax Mendel formulated the rule of uniformity of hybrids of the first generation : when crossing two homozygous organisms that differ from each other in one trait, all hybrids of the first generation will have the trait of one of the parents, and the generation for this trait will be uniform.
From the seeds obtained in the first generation, Mendel grew pea plants and crossed them again. In plants of the second generation, most of the peas were yellow, but there were also green peas. In total, from several crossed pairs of plants, Mendel received 6022 yellow and 2001 green peas. It is easy to count that 3/4 of the hybrid seeds were yellow and ¼ green. The phenomenon in which crossing leads to the formation of offspring partly with dominant, partly with recessive traits, is called splitting.

Experiments with other traits confirmed these results, and Mendel formulatedsplitting rule slide 10, 11: when two descendants (hybrids) of the first generation are crossed with each other in the second generation, splitting is observed and individuals with recessive traits appear again; these individuals make up one-fourth of the total number of second-generation descendants.

Law of purity of gametes. To explain the facts that formed the basis of the rule of uniformity of hybrids of the first generation and the rule of splitting, G. Mendel suggested that there are two “elements of heredity” (genes) in each somatic cell. In the cells of the hybrid of the first generation, although they have only yellow peas, both "elements" (both yellow and green) must be present, otherwise the second generation hybrids cannot have green peas. Communication between generations is provided through germ cells - gametes. This means that each gamete receives only one “element of heredity” (gene) out of two possible ones - “yellow” or green”. This hypothesis of Mendel that during the formation of gametes, only one of the two allelic genes enters each of them is calledgamete purity law . slide 12.

From the experiments of G. Mendel on monohybrid crossing, in addition to the law of purity of gametes, it also follows that genes are transmitted from generation to generation without changing. Otherwise, it is impossible to explain the fact that in the first generation after crossing homozygotes with yellow and green peas, all seeds were yellow, and in the second generation green peas appeared again. Therefore, the gene for “green peas” did not disappear and did not turn into a gene for “yellow peas”, but simply did not appear in the first generation, suppressed by the dominant gene for yellowness.

How to explain the patterns of genetics from the standpoint of modern science?

Cytological bases of patterns of inheritance in monohybrid crossing.

Let's depict a monohybrid cross in the form of a diagram . Symbol ♀ - "mirror of Venus" - denotes a female, symbol ♂ male, x - crossing, P - parental generation, F1 - first generation of offspring, F2 - second generation of offspring, A - gene responsible for dominant yellow color, a - gene , responsible for the recessive green color of pea seeds (Fig. 50).

The figure shows that in each gamete of the parent individuals there will be one gene (remember meiosis): in one case A, in the other - a. Thus, in the first generation, all somatic cells will be heterozygous - Aa. In turn, hybrids of the first generation can form A or a gametes with equal probability.

Random combinations of these gametes during the sexual process can give the following options: AA, Aa, aA, aa. The first three plants containing the A gene will, according to the dominance rule, have yellow peas, and the fourth - the recessive homozygote aa - will have green peas.slide 13.

Problem solution: gray dominant rabbits were crossed with white recessive ones.(Working with rabbit magnets).

- What are the rabbits like?

- Why?

Now let's try to explain the birth of a dark-skinned baby in a fair-skinned woman.

We studied crossing on one trait: the color is yellow and green in peas, and the color of the coat and rabbits, that is, according to one pair of traits, G. Mendel called this crossing monohybrid.

4. Control of acquired knowledge. 4 min.

On the tables a crossword puzzle (4 min.) Remember the definitions. Write the correct answer directly in the crossword puzzle. I wish you success.

1. The totality of all signs of an organism.

2. Dominance, in which the dominant gene does not always completely suppress the manifestation of the recessive gene.

3. Crossing, in which one pair of alternative traits is traced.

4. Distribution of dominant and recessive traits among offspring in the same numerical ratio.

5. Sex cells.

Peer-to-peer responses on the slide.

Debriefing: Now share your answers with each other. We will conduct a mutual check, the correct answers and evaluation criteria on the slide(slide 14)

Changed back.

Raise your hands who has 6 correct answers, who has 4 correct answers. Well done.

5. Consolidation of the acquired knowledge. 4 min.

Front work. The solution of the problem:

Task 1.slide 15.

The smooth shape of the seeds in peas dominates over the wrinkled. Homozygous plants were crossed.

How many plants in the first generation will be heterozygous?

How many seeds in the second generation will be homozygous for the dominant trait?

How many seeds will be heterozygous in the second generation?

How many wrinkled seeds will be in the second generation?

6. News of genetic science. (one of the students speaks)

Project: "Human Genome"

The international project was started in 1988. Several thousand people from more than 20 countries work in the project. Since 1989, Russia has also been participating in it. All chromosomes are divided between the participating countries, and Russia got 3, 13, 19 chromosomes. The main goal of the project is to determine the localization of all genes in the DNA molecule. By 1998, about half of the human genetic information had been deciphered.

Today it is established that predisposition to alcoholism and / or drug addiction can also have a genetic basis.

Today, on the basis of genes, a person can be recognized by trace amounts of blood, skin flakes, and so on.

Currently, the problem of the dependence of a person's abilities and talents on his genes is being intensively studied.

The main task of future research is to identify differences between people on genetic level. This will make it possible to create genetic portraits of people and more effectively treat diseases, assess the abilities and capabilities of each person, and assess the degree of adaptation of a particular person to a particular environmental situation.

Are there any among you who want to become a genetic scientist?

7. Reflection

So, have we reached the goal of the lesson? Proveproblem solving.

Task 4

The brown-eyed gene in humans dominates the blue-eyed gene. A blue-eyed, homozygous man married a brown-eyed woman whose father has brown eyes and whose mother has blue eyes. Determine the genotypes of each of the mentioned individuals, write down how the trait is inherited. We write the task condition:

gene trait

And brown

and blue

Write down the genotypes together. What will be the offspring? Those. half of the children of these parents will be with brown eyes, half with blue.

8. Summing up.

1. I worked at the lesson
2. With my work in the lesson, I
3. The lesson seemed to me
4. For the lesson I
5. My mood
6. The material of the lesson was

9. D/Z p.104 answer the questions in workbook. Learn terms and concepts.

1

5

4

2

Genetics- the science that studies heredity and variability of organisms.
Heredity- the ability of organisms to transmit their characteristics from generation to generation (features of structure, functions, development).
Variability- the ability of organisms to acquire new traits. Heredity and variability are two opposite but interrelated properties of an organism.

Heredity

Basic concepts
Gene and alleles. unit hereditary information is the gene.
Gene(from the point of view of genetics) - a section of the chromosome that determines the development of one or more traits in an organism.
alleles- different states of the same gene, located in a certain locus (region) of homologous chromosomes and determining the development of some kind of trait. Homologous chromosomes are found only in cells containing a diploid set of chromosomes. They are not found in germ cells (gametes) of eukaryotes and prokaryotes.

Sign (hair dryer)- some quality or property by which one organism can be distinguished from another.
domination- the phenomenon of the predominance of the trait of one of the parents in the hybrid.
dominant trait- a trait that appears in the first generation of hybrids.
recessive trait- a trait that outwardly disappears in the first generation of hybrids.

Dominant and recessive traits in humans

signs
dominant	recessive
Dwarfism	normal growth
Polydactyly (multi-fingeredness)	Norm
curly hair	Straight hair
Not red hair	Red hair
early baldness	Norm
Long eyelashes	short eyelashes
big eyes	Small eyes
Brown eyes	Blue or gray eyes
Myopia	Norm
Twilight vision (night blindness)	Norm
Freckles on the face	No freckles
Normal blood clotting	Weak blood clotting (hemophilia)
color vision	Lack of color vision (color blindness)

dominant allele - an allele that determines a dominant trait. Denoted by Latin capital letter: A, B, C, ... .
recessive allele - an allele that determines a recessive trait. It is indicated by a Latin lowercase letter: a, b, c, ....
The dominant allele ensures the development of the trait both in the homo- and heterozygous state, the recessive allele appears only in the homozygous state.
Homozygous and heterozygous. Organisms (zygotes) can be homozygous or heterozygous.
Homozygous organisms have two identical alleles in their genotype - both dominant or both recessive (AA or aa).
Heterozygous organisms have one of the alleles in the dominant form, and the other in the recessive form (Aa).
Homozygous individuals do not cleave in the next generation, while heterozygous individuals give cleavage.
Different allelic forms of genes arise as a result of mutations. A gene can mutate repeatedly, producing many alleles.
Multiple allelism - the phenomenon of the existence of more than two alternative allelic forms of a gene that have different manifestations in the phenotype. Two or more states of a gene result from mutations. A series of mutations causes the appearance of a series of alleles (A, a1, a2, ..., an, etc.), which are in different dominant-recessive relationships to each other.
Genotype is the totality of all the genes of an organism.
Phenotype - the totality of all the characteristics of an organism. These include morphological (external) signs (eye color, flower color), biochemical (shape of a structural protein or enzyme molecule), histological (cell shape and size), anatomical, etc. On the other hand, signs can be divided into qualitative ( eye color) and quantitative (body weight). The phenotype depends on the genotype and environmental conditions. It develops as a result of the interaction of the genotype and environmental conditions. The latter have less effect on qualitative features and mostly quantitative.
Crossing (hybridization). One of the main methods of genetics is crossing, or hybridization.
hybridological method - crossing (hybridization) of organisms that differ from each other in one or more characteristics.
hybrids - descendants from crosses of organisms that differ from each other in one or more characteristics.
Depending on the number of signs by which the parents differ, they distinguish different types crossing.
monohybrid cross A cross in which the parents differ in only one trait.
Dihybrid cross A cross in which the parents differ in two ways.
Polyhybrid cross - crossbreeding, in which parents differ in several ways.
To record the results of crosses, the following generally accepted notation is used:
P - parents (from lat. parental- parent);
F - offspring (from lat. filial- offspring): F 1 - hybrids of the first generation - direct descendants of parents P; F 2 - second generation hybrids - descendants from crossing F 1 hybrids among themselves, etc.
♂ - male (shield and spear - a sign of Mars);
♀ - female (a mirror with a handle - a sign of Venus);
X - cross icon;
: - splitting of hybrids, separates the digital ratios of different (by phenotype or genotype) classes of descendants.
The hybridological method was developed by the Austrian naturalist G. Mendel (1865). He used self-pollinating garden pea plants. Mendel crossed pure lines (homozygous individuals) that differ from each other in one, two or more traits. He received hybrids of the first, second, etc. generations. Mendel processed the data obtained mathematically. The results obtained were formulated in the form of laws of heredity.

G. Mendel's laws

Mendel's first law. G. Mendel crossed pea plants with yellow seeds and pea plants with green seeds. Both were pure lines, that is, homozygotes.

Mendel's first law - the law of uniformity of hybrids of the first generation (the law of dominance): when crossing pure lines, all hybrids of the first generation show one trait (dominant).
Mendel's second law. After that, G. Mendel crossed hybrids of the first generation among themselves.

Mendel's second law - the law of feature splitting: hybrids of the first generation, when they are crossed, split in a certain numerical ratio: individuals with a recessive manifestation of a trait make up 1/4 of total number descendants.

Splitting is a phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some are recessive. In the case of monohybrid crossing, this ratio looks like this: 1AA:2Aa:1aa, that is, 3:1 (in case of complete dominance) or 1:2:1 (in case of incomplete dominance). In the case of dihybrid crossing - 9:3:3:1 or (3:1) 2 . With polyhybrid - (3:1) n.
incomplete dominance. The dominant gene does not always completely suppress the recessive gene. Such a phenomenon is called incomplete dominance . An example of incomplete dominance is the inheritance of the color of the flowers of the night beauty.

Cytological basis of uniformity of the first generation and splitting of characters in the second generation consist in the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis.
Hypothesis (law) of purity of gametes states: 1) during the formation of germ cells, only one allele from an allelic pair enters each gamete, that is, the gametes are genetically pure; 2) in a hybrid organism, genes do not hybridize (do not mix) and are in a pure allelic state.
Statistical nature of splitting phenomena. From the hypothesis of purity of gametes it follows that the law of splitting is the result of a random combination of gametes carrying different genes. With the random nature of the connection of gametes overall result turns out to be normal. It follows that in monohybrid crossing, the ratio of 3:1 (in the case of complete dominance) or 1:2:1 (in case of incomplete dominance) should be considered as a regularity based on statistical phenomena. This also applies to the case of polyhybrid crossing. The exact fulfillment of the numerical ratios during splitting is possible only when in large numbers studied hybrid individuals. Thus, the laws of genetics are statistical in nature.
offspring analysis. Analyzing cross allows you to determine whether an organism is homozygous or heterozygous for a dominant gene. To do this, an individual is crossed, the genotype of which should be determined, with an individual homozygous for the recessive gene. Often one of the parents is crossed with one of the offspring. Such a crossing is called returnable .
In the case of homozygosity of the dominant individual, splitting will not occur:

In the case of heterozygosity of the dominant individual, splitting will occur:

Mendel's third law. G. Mendel carried out a dihybrid crossing of pea plants with yellow and smooth seeds and pea plants with green and wrinkled seeds (both pure lines), and then crossed their descendants. As a result, he found that each pair of traits during splitting in the offspring behaves in the same way as during monohybrid crossing (it splits 3: 1), that is, regardless of the other pair of traits.

Mendel's third law- the law of independent combination (inheritance) of traits: splitting for each trait occurs independently of other traits.

Cytological basis of independent combination is the random nature of the divergence of homologous chromosomes of each pair to different poles of the cell during meiosis, regardless of other pairs of homologous chromosomes. This law is valid only when the genes responsible for the development of different traits are located on different chromosomes. Exceptions are cases of linked inheritance.

Linked inheritance. Clutch failure

The development of genetics has shown that not all traits are inherited in accordance with Mendel's laws. Thus, the law of independent inheritance of genes is valid only for genes located on different chromosomes.
The patterns of linked inheritance of genes were studied by T. Morgan and his students in the early 1920s. 20th century The object of their research was the Drosophila fruit fly (its life span is short, and several tens of generations can be obtained in a year, its karyotype consists of only four pairs of chromosomes).
Morgan's Law: genes located on the same chromosome are predominantly inherited together.
Linked genes are genes that are on the same chromosome.
clutch group All genes on one chromosome.
In a certain percentage of cases, the clutch may be broken. The reason for the violation of linkage is crossing over (crossing of chromosomes) - the exchange of sections of chromosomes in prophase I of meiotic division. Crossover leads to genetic recombination. The farther apart the genes are, the more often crossing over occurs between them. This phenomenon is based on the construction genetic maps- determination of the sequence of genes in the chromosome and the approximate distance between them.

Sex Genetics

autosomes Chromosomes are the same for both sexes.
Sex chromosomes (heterochromosomes) Chromosomes that distinguish males and females from each other.
A human cell contains 46 chromosomes, or 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. Sex chromosomes are referred to as X- and Y-chromosomes. Women have two X chromosomes, while men have one X and one Y chromosome.
There are 5 types of chromosomal sex determination.

Types of chromosomal sex determination

Type	Examples
♀XX, ♂XY	Characteristic for mammals (including humans), worms, crustaceans, most insects (including fruit flies), most amphibians, some fish
♀ XY, ♂ XX	Characteristic for birds, reptiles, some amphibians and fish, some insects (lepidoptera)
♀ XX, ♂ X0	Occurs in some insects (Orthoptera); 0 means no chromosomes
♀ Х0, ♂ XX	Found in some insects (Hydroptera)
haplo-diploid type (♀ 2n, ♂ n)	It occurs, for example, in bees and ants: males develop from unfertilized haploid eggs (parthenogenesis), females develop from fertilized diploid ones.

sex-linked inheritance - inheritance of traits whose genes are located on the X and Y chromosomes. The sex chromosomes may contain genes that are not related to the development of sexual characteristics.
When XY is combined, most of the genes located on the X chromosome do not have an allele pair on the Y chromosome. Also, genes located on the Y chromosome do not have alleles on the X chromosome. Such organisms are called hemizygous . In this case, a recessive gene appears, which is present in the genotype in singular. So the X chromosome may contain a gene that causes hemophilia (reduced blood clotting). Then all male individuals who received this chromosome will suffer from this disease, since the Y chromosome does not contain a dominant allele.

blood genetics

According to the AB0 system, people have 4 blood groups. The blood group is determined by gene I. In humans, the blood group is provided by three genes IA, IB, I0. The first two are co-dominant with respect to each other, and both are dominant with respect to the third. As a result, a person has 6 blood groups according to genetics, and 4 according to physiology.

I group	0	I 0 I 0	homozygous
II group	BUT	I A I A	homozygous
		I A I 0	heterozygous
III group	AT	I B I B	homozygous
		I B I 0	heterozygous
IV group	AB	I A I B	heterozygous

In different peoples, the ratio of blood groups in the population is different.

Distribution of blood groups according to the AB0 system among different peoples,%

In addition, blood different people may differ in Rh factor. Blood can be Rh positive (Rh+) or Rh negative (Rh-). This ratio varies among different peoples.

The distribution of the Rh factor in different peoples,%

Nationality	Rh positive	Rh negative
australian aborigines	100	0
American Indians	90–98	2–10
Arabs	72	28
Basques	64	36
Chinese	98–100	0–2
Mexicans	100	0
Norse	85	15
Russians	86	14
Eskimos	99–100	0–1
Japanese	99–100	0–1

The Rh factor of the blood determines the R gene. R + gives information about the production of a protein (Rh-positive protein), but the R gene does not. The first gene dominates the second. If Rh + blood is transfused to a person with Rh - blood, then specific agglutinins are formed in him, and repeated administration of such blood will cause agglutination. When an Rh woman develops a fetus that has inherited a positive Rh from the father, an Rh conflict may occur. The first pregnancy, as a rule, ends safely, and the second - with a disease of the child or stillbirth.

Gene Interaction

A genotype is not just a mechanical set of genes. This is a historically established system of genes interacting with each other. More precisely, it is not the genes themselves (sections of DNA molecules) that interact, but the products formed on their basis (RNA and proteins).
Both allelic and non-allelic genes can interact.
Interaction of allelic genes: complete dominance, incomplete dominance, co-dominance.
Complete dominance - the phenomenon when a dominant gene completely suppresses the work of a recessive gene, as a result of which a dominant trait develops.
incomplete dominance - a phenomenon when a dominant gene does not completely suppress the work of a recessive gene, as a result of which an intermediate trait develops.
Codominance (independent manifestation) - a phenomenon when both alleles participate in the formation of a trait in a heterozygous organism. In humans, a series of multiple alleles represents the gene that determines the blood group. In this case, the genes that determine blood types A and B are codominant with respect to each other, and both are dominant with respect to the gene that determines blood type 0.
Interaction of non-allelic genes: cooperation, complementarity, epistasis and polymerism.
Cooperation - a phenomenon when, with the mutual action of two dominant non-allelic genes, each of which has its own phenotypic manifestation, a new trait is formed.
complementarity - the phenomenon when a trait develops only with the mutual action of two dominant non-allelic genes, each of which individually does not cause the development of a trait.
epistasis - the phenomenon when one gene (both dominant and recessive) suppresses the action of another (non-allelic) gene (both dominant and recessive). The suppressor gene (suppressor) can be dominant (dominant epistasis) or recessive (recessive epistasis).
Polymerism - the phenomenon when several non-allelic dominant genes are responsible for a similar effect on the development of the same trait. The more such genes present in the genotype, the more pronounced the trait. The phenomenon of polymerism is observed in the inheritance of quantitative traits (skin color, body weight, milk yield of cows).
In contrast to polymers, there is such a phenomenon as pleiotropy - multiple gene action, when one gene is responsible for the development of several traits.

Chromosomal theory of heredity

The main provisions of the chromosome theory of heredity:

• Chromosomes play the leading role in heredity;
• genes are located on the chromosome in a certain linear sequence;
• each gene is located in a certain place (locus) of the chromosome; allelic genes occupy the same loci in homologous chromosomes;
• genes of homologous chromosomes form a linkage group; their number is equal to the haploid set of chromosomes;
• between homologous chromosomes, the exchange of allelic genes (crossing over) is possible;
• the frequency of crossing over between genes is proportional to the distance between them.

Nonchromosomal inheritance

According to the chromosome theory of heredity, the DNA of chromosomes plays a leading role in heredity. However, DNA is also found in mitochondria, chloroplasts, and in the cytoplasm. Non-chromosomal DNA is called plasmids . Cells do not have special mechanisms for the uniform distribution of plasmids during division, so one daughter cell can receive one genetic information, and the second - a completely different one. The inheritance of genes contained in plasmids does not follow the Mendelian laws of inheritance, and their role in the formation of the genotype is still poorly understood.

In the previous article, we got acquainted with the fundamental concepts and methods of genetics. The time has come to apply them in the study of a new section - Mendelian genetics, based on the laws discovered by Gregor Mendel.

Mendel followed some principles in his research that led his work to success:

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Let's introduce some new terms that will be useful to us. Crossing can be:

• Monohybrid - if the crossed individuals differ in only one trait under study (seed color)
• Dihybrid - if the crossed individuals differ in two different characteristics (color and shape of seeds)

There are some designations in the scheme for solving the genetic problem: ♀ - female organism, ♂ - male organism, P - parental organisms, F 1 - first generation hybrids, F 2 - second generation hybrids. It probably makes sense to save the picture below to your gadget if you are just starting to study genetics;)

I hasten to inform you that marriages between people (as opposed to forced crossing of peas) occur only out of love and mutual consent! Therefore, in tasks where we are talking about humans, do not put the cross sign "×" between parental individuals. In this case, put the sign "→" - "Cupid's arrow" to delight the examiner :)

Mendel's first law - the law of uniformity

Genetic tasks often begin with it (as a first cross). This law states that when crossing homozygous individuals that differ in one or more pairs of alternative traits, all hybrids of the first generation will be uniform in these traits.

This law is based on a variant of interaction between genes - complete dominance. With this option, one gene - dominant, completely suppresses another gene - recessive. In the experiment we have just studied, Mendel crossed pure lines of peas with yellow (AA) and green (aa) seeds, resulting in all the offspring having yellow seeds (Aa) - it was uniform.

Often the genotype of an individual is not known and is a mystery. How to be a geneticist in this case? Sometimes the easiest way is to use analyzing cross - crossing a hybrid individual (for which the genotype is not known) with a homozygous for a recessive trait.

Analyzing the resulting offspring, we can draw a conclusion about the genotype of the hybrid individual.

In the considered case, if the genotype of the studied individual contains two dominant genes (AA), then the recessive trait cannot appear in the offspring, since all the offspring will be uniform (Aa). If the studied individual contains a recessive gene (Aa), then half of the offspring will have it (aa). As a result, the genotype of the hybrid individual becomes known.

In addition to complete dominance, there is incomplete dominance, which is characteristic of some genes. A well-known example of incomplete dominance is the inheritance of the color of the petals of the night beauty plant. In this case, the genes do not completely suppress each other - an intermediate sign appears.

Please note that the F 1 offspring also turned out to be uniform (only one option is possible - Aa), but phenotypically, in a heterozygote, the trait will appear as an intermediate state (AA - red, aa - white, Aa - pink). It can be compared to an artist's palette: imagine how red and white colors- it turns out pink.

Mendel's second law - the law of splitting

"When crossing heterozygous hybrids (Aa) of the first generation F 1 in the second generation F 2, splitting is observed according to this trait: according to the genotype 1: 2: 1, according to the phenotype 3: 1"

Crossing hybrids of the first generation (Aa) with each other, Mendel found that in the offspring of individuals with a dominant trait (AA, Aa - the yellow color of the seeds) is about 3 times more than individuals with a recessive (aa).

I sincerely wish that you learn to determine the splitting by genotype and phenotype yourself. This is not difficult to do: when it comes to the genotype, pay attention only to genes (letters), that is, if you have individuals AA, Aa, Aa, aa in front of you, you should take genotypes in turn and add up the number of identical genotypes. It is as a result of such actions that the ratio of the genotype is 1:2:1.

If you are faced with the task of calculating the ratio by phenotype, then do not look at the genes at all - this will only confuse you! Only the manifestation of the sign should be taken into account. The offspring produced 3 plants with yellow seeds and 1 with green, hence a phenotypic split of 3:1.

Mendel's third law - the law of independent inheritance

It is about dihybrid crossing, that is, we examine not one, but two traits in individuals (for example, seed color and seed shape). Each gene has two alleles, so don't be surprised by the AaBb genotypes :) It is important to note that this law is about genes that are located on different chromosomes.

Remember Mendel's third law as follows: "When crossing individuals that differ from each other in two (or more) pairs of alternative traits, genes and their corresponding traits are inherited independently of each other, combining with each other in all possible combinations.

The combinations of genes are reflected in the formation of gametes. In accordance with the rule outlined above, the AaBb diheterozygote forms 4 types of gametes: AB, ab, Ab, aB. I repeat - this is only if the genes are on different chromosomes. If they are in one, as with linked inheritance, then everything proceeds differently, but this is the subject of the next article.

Each individual AaBb forms 4 types of gametes, there are 16 possible hybrids of the second generation. With such an abundance of gametes and a large number of descendants, it is more reasonable to use the Punnett lattice, in which male gametes are located along one side of the square, and female gametes along the other. This helps to more visually represent the genotypes resulting from crossing.

As a result of crossing diheterozygotes among 16 descendants, 4 possible phenotypes are obtained:

• Yellow smooth - 9
• Yellow wrinkled - 3
• Green smooth - 3
• Green wrinkled - 1

It is obvious that splitting by phenotype among hybrids of the second generation is: 9:3:3:1.

The dominant gene is responsible for the development of normal eyeballs in humans. The recessive gene results in an almost complete absence of the eyeballs (anophthalmia). Heterozygotes have a small eyeball (microphthalmia). What structure of the eyeballs will be typical for the offspring if both parents suffer from microphthalmia?

Note that gene dominance is incomplete: a person with the Aa genotype will have intermediate value sign - microphthalmia. Since dominance is incomplete, genotypic and phenotypic segregation coincide, which is typical of incomplete dominance.

In this task, only ¼ of the offspring (25%) will have normal eyeballs. ½ of the offspring (50%) will have a small eyeball - microphthalmia, and the remaining ¼ (25%) will be blind with an almost complete absence of eyeballs (anophthalmia).

Don't forget that genetics is essentially a theory of probability. It is obvious that in life in such a family 4 healthy children with normal eyeballs can be born in a row, or vice versa - 4 blind children. It can be anything, but you and I must learn to talk about the "highest probability", according to which, with a probability of 50%, a child with microphthalmia will be born in this family.

Polydactyly and the absence of small molars are transmitted as autosomal dominant traits. The genes responsible for the development of these traits are located on different pairs of homologous chromosomes. What is the probability of having children without anomalies in a family where both parents suffer from both diseases and are heterozygous for these pairs of genes.

I want to immediately bring you to the idea of ​​Mendel's third law (the law of independent inheritance), which is hidden in the phrase "Genes ... are located in different pairs of homologous chromosomes." You will see later on how valuable this information is. Also note that this problem is about autosomal genes (located outside the sex chromosomes). Autosomal dominant inheritance means that the disease manifests itself if the gene is in the dominant state: AA, Aa - sick.

In this case, we will build a Punnett lattice, which will make the genotypes of the offspring more visual. You see that there is literally not a single living place on the offspring: almost all 16 possible offspring are sick with either one or the other disease, except for one, aabb. The probability of having such a child is very small 1/16 = 6.25%.

A blue-eyed myopic woman from a marriage with a brown-eyed man with normal vision had a brown-eyed myopic girl and a blue-eyed boy with normal vision. The myopia gene (A) is dominant in relation to the normal vision gene (a), and the brown-eyed gene (D) dominates the blue-eyed gene (d). What is the probability of a normal brown-eyed child being born in this family?

The first step in problem solving is very important. We took into account the descriptions of the genotypes of the parents, and, nevertheless, white spots remained. We do not know if a woman is heterozygous (Aa) or homozygous (aa) for the myopia gene. The situation is the same with a man, we cannot say for sure whether he is homozygous (DD) or heterozygous (Dd) for the brown-eyed gene.

The resolution of our doubts lies in the genotype of the offspring we are told about: "blue-eyed boy with normal vision" with the genotype aadd. A child always receives one chromosome from the mother, and the other from the father. It turns out that such a genotype could not have been formed if there had not been an a gene from the mother, and a d gene from the father. Therefore, the father and mother are heterozygous.

Now we can say for sure that the probability of a normal brown-eyed child being born in this family is ¼ or 25%, its genotype is Ddaa.

I did not forget that in the course of studying genetics you need to be taught to see various options inheritance on family tree(pedigree) =) From the previous article, we learned about how the autosomal recessive type of inheritance looks and is characterized, now let's talk about the autosomal dominant type that we encountered in the tasks above.

Autosomal dominant inheritance can be recognized by the following features:

• The disease manifests itself in every generation of the family (vertical transmission)
• Healthy children of sick parents have healthy children
• Boys and girls get sick equally often
• The ratio of sick and healthy 1:1

©Bellevich Yury Sergeevich

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The subject and history of the development of genetics

Genetics (from the Greek. genesis - origin) - the science of heredity and variability of organisms. The term "genetics" was proposed in 1906 by W. Batson. Heredity is the property of living beings to ensure material and functional continuity between generations, as well as to determine the specific nature of individual development in certain environmental conditions. Heredity is the reproduction of life (N. P. Dubinin). Variability is the occurrence of differences between organisms in a number of characteristics and properties.

Heredity, variability and selection are the basis of evolution. Thanks to them, a huge variety of living beings arose on the Earth. Mutations supply primary material for evolution. As a result of selection, positive traits and properties are preserved, which, due to heredity, are transmitted from generation to generation. Knowledge of the laws of heredity and variability contributes to the more rapid creation of new breeds of animals, plant varieties and strains of microorganisms.

S. M. Gershenzon identifies four main theoretical problems studied by genetics:

1) storage of genetic information (where and how genetic information is encoded);

2) the transfer of genetic information from cell to cell, from generation to generation;

3) implementation of genetic information in the process of ontogeny;

4) changes in genetic information in the process of mutations. The rapid development of genetics is due to the fact that it is open

Laws of inheritance. General terminology. Monohybrid crossing.

Laws of succession

The diploid chromosome set consists of pairs of homologous chromosomes. One chromosome from each pair is inherited from the mother's body, the other from the father's. As a result, each gene on a homologous chromosome has a corresponding gene located in the same location on the other homologous chromosome. Such paired genes are called alleles, or alleles. Alleles can be absolutely identical, but variations in their structure are also possible. When many alleles are known, which are alternative variants of a gene localized in a certain region of the chromosome, they speak of multiple allelism. In any case, only two alleles can be present in a normal diploid organism, since there are only pairs of homologous chromosomes.

Mendel's first law
Consider a situation in which organisms that differ in one pair of traits are crossed (monohybrid crossing). Let the color of the eyes be such a trait. In one parent, these are alleles A, respectively, its genotype for such alleles is AA. With this genotype, the eye color is brown. The other parent has allele a on both chromosomes (genotype aa), the color of the groove is blue. During the formation of germ cells, homologous chromosomes diverge into different cells. Since both alleles are the same in parents, they form only one type of germ cells (gametes). In one parent, the gametes contain only the A allele, in the other, only the a allele. Such organisms are said to be homozygous for a given pair of genes.

In the first generation (F1), the offspring will have the same Aa genotype and the same phenotype - brown eyes. The phenomenon in which only one trait from an alternative pair appears in the phenotype is called dominance, and the gene that controls this trait is dominant. Allele a does not appear in the phenotype, being present in the genotype in a "hidden" form. Such alleles are called recessive. In this case, the rule of uniformity of hybrids of the first generation is fulfilled: all hybrids have the same genotype and phenotype.

Mendel's second law.
Mendel's second law, or the law of independent distribution of genes. It is established through the analysis of inheritance in dihybrid and polyhybrid crosses, when the crossed individuals differ in two or more pairs of alleles. The independent distribution of genes occurs because during the formation of sweat cells (gametes), homologous chromosomes from one pair diverge independently of other pairs. Therefore, Mendel's second law, unlike the first, is valid only in cases where the analyzed pairs of genes are located on different chromosomes.

The law of independent combination, or Mendel's third law. Mendel's study of the inheritance of one pair of alleles made it possible to establish a number of important genetic patterns: the phenomenon of dominance, the invariance of recessive alleles in hybrids, the splitting of the offspring of hybrids in a ratio of 3: 1, and also to suggest that gametes are genetically pure, i.e. contain only one gene from allele pairs. However, organisms differ in many genes. It is possible to establish patterns of inheritance of two pairs of alternative traits or more by dihybrid or polyhybrid crossing.

monohybrid cross

Phenotype and genotype. Monohybrid called crossing, in which the parental forms differ from each other in one pair of contrasting, alternative characters.

sign- any feature of an organism, i.e., any individual quality or property of it, by which two individuals can be distinguished. In plants, these are the shape of the corolla (for example, symmetrical-asymmetrical) or its color (purple-white), the rate of plant maturation (early-late-ripening), disease resistance or susceptibility, etc.

The totality of all signs of an organism, starting with external and ending with the structural features and functioning of cells, tissues and organs, is called phenotype. This term can also be used in relation to one of the alternative signs.

The signs and properties of the organism are manifested under the control of hereditary factors, i.e. genes. The totality of all the genes in an organism is called genotype.

Examples of monohybrid crossing carried out by G. Mendel are crossings of peas with such clearly visible alternative traits as purple and white flowers, yellow and green color of unripe fruits (beans), smooth and wrinkled surface of seeds, their yellow and green color, etc.

Uniformity of hybrids of the first generation (Mendel's first law). When crossing peas with purple and white flowers, Mendel discovered that in all hybrid plants of the first generation (F1) the flowers are purple. At the same time, the white color of the flower did not appear (Fig. 3.1).

Mendel also established that all hybrids F1 turned out to be uniform (homogeneous) for each of the seven characteristics he studied. Consequently, in hybrids of the first generation, out of a pair of parental alternative traits, only one appears, and the trait of the other parent, as it were, disappears. The phenomenon of dominance in hybrids F1 signs of one of the parents Mendel called dominance and the corresponding sign is dominant. Features that do not appear in hybrids F1 he named recessive.

Since all hybrids of the first generation are uniform, this phenomenon was called by K. Correns the first laws of Mendel, or the law of uniformity of hybrids of the first generation, as well as dominance rule.

W laws of inheritance. Polyhybrid crossing.

Mendel's laws are the principles of the transmission of hereditary traits from parent organisms to their offspring, arising from the experiments of Gregor Mendel. These principles formed the basis for classical genetics and were subsequently explained as a consequence of the molecular mechanisms of heredity.