Determination of the genotypes of organisms based on the genotypes and phenotypes of parents and offspring. Survival of the fittest Problem solving algorithm
Having worked through these topics, you should be able to:
- Give definitions: gene, dominant trait; recessive trait; allele; homologous chromosomes; monohybrid crossing, crossing over, homozygous and heterozygous organism, independent distribution, complete and incomplete dominance, genotype, phenotype.
- Using the Punnett grid, illustrate crossbreeding for one or two traits and indicate what numerical ratios of genotypes and phenotypes should be expected in the offspring from these crosses.
- Explain the rules of inheritance, segregation, and independent distribution of characters, the discovery of which was Mendel's contribution to genetics.
- Explain how mutations can affect the protein encoded by a particular gene.
- Indicate the possible genotypes of people with blood groups A; IN; AB; ABOUT.
- Give examples of polygenic traits.
- Indicate the chromosomal mechanism of sex determination and types of inheritance of sex-linked genes in mammals, and use this information when solving problems.
- Explain the difference between sex-linked traits and sex-dependent traits; give examples.
- Explain how human genetic diseases such as hemophilia, color blindness, and sickle cell anemia are inherited.
- Name the features of methods of selection of plants and animals.
- Indicate the main directions of biotechnology.
- Be able to solve simple genetic problems using this algorithm:
Algorithm for solving problems
- Determine the dominant and recessive traits based on the results of crossing the first generation (F1) and the second (F2) (according to the conditions of the problem). Enter the letter designations: A - dominant and - recessive.
- Write down the genotype of an individual with a recessive trait or an individual with a known genotype and gametes based on the conditions of the problem.
- Record the genotype of the F1 hybrids.
- Draw up a scheme for the second crossing. Record the gametes of F1 hybrids in a Punnett grid horizontally and vertically.
- Record the genotypes of the offspring in the gamete intersection cells. Determine the ratios of phenotypes in F1.
Task design scheme.
Letter designations:
a) dominant trait _______________
b) recessive trait _______________
Gametes
F1(first generation genotype)
| gametes | ||
| ? | ? |
Punnett grid
| F2 |
|
Phenotype ratio in F2: _____________________________
Answer:_________________________
Examples of solving monohybrid crossing problems.
Task.“There are two children in the Ivanov family: a brown-eyed daughter and a blue-eyed son. The mother of these children is blue-eyed, but her parents had brown eyes. How is eye color inherited in humans? What are the genotypes of all family members? Eye color is a monogenic autosomal trait.”
The eye color trait is controlled by one gene (by condition). The mother of these children is blue-eyed, and her parents had brown eyes. This is only possible if both parents were heterozygous, therefore, brown eyes are dominant over blue ones. Thus, grandparents, father and daughter had the genotype (Aa), and mother and son had the genotype aa.
Task."A rooster with a rose-shaped comb was crossed with two hens, also having a rose-shaped comb. The first gave 14 chickens, all with a rose-shaped comb, and the second gave 9 chickens, of which 7 with a rose-shaped and 2 with a leaf-shaped comb. The shape of the comb is a monogenic autosomal trait. What are genotypes of all three parents?
Before determining the genotypes of the parents, it is necessary to find out the nature of inheritance of the comb shape in chickens. When a rooster was crossed with a second hen, 2 chicks with leaf combs were produced. This is possible if the parents are heterozygous; therefore, it can be assumed that the rose-shaped comb in chickens is dominant over the leaf-shaped one. Thus, the genotypes of the rooster and the second hen are Aa.
When crossing the same rooster with the first hen, no splitting was observed, therefore, the first hen was homozygous - AA.
Task.“In a family of brown-eyed, right-handed parents, fraternal twins were born, one of whom is brown-eyed, left-handed, and the other blue-eyed, right-handed. What is the probability of the next child being born similar to his parents?”
The birth of a blue-eyed child to brown-eyed parents indicates the recessiveness of blue eye color, respectively, the birth of a left-handed child to right-handed parents indicates the recessivity of better control of the left hand compared to the right. Let's introduce allele designations: A - brown eyes, a - blue eyes, B - right-handed, c - left-handed. Let's determine the genotypes of parents and children:
| R | AaBv x AaBv |
| F, | A_bb, aaB_ |
A_вв is a phenotypic radical, which shows that this child is left-handed with brown eyes. The genotype of this child may be Aavv, AAvv.
Further solution of this problem is carried out in the traditional way, by constructing a Punnett lattice.
| AB | Av | aB | Av | |
| AB | AABB | AAVv | AaBB | AaVv |
| Av | AAVv | AAbb | AaVv | Aaww |
| aB | AaBB | AaVv | aaBB | AaVv |
| aw | AaVv | Aaww | aaVv | Aaww |
9 variants of descendants that interest us are underlined. There are 16 possible options, so the probability of a child being born similar to their parents is 9/16.
Ivanova T.V., Kalinova G.S., Myagkova A.N. "General Biology". Moscow, "Enlightenment", 2000
- Topic 10. "Monohybrid and dihybrid crossing." §23-24 pp. 63-67
- Topic 11. "Genetics of sex." §28-29 pp. 71-85
- Topic 12. "Mutational and modification variability." §30-31 pp. 85-90
- Topic 13. "Selection." §32-34 pp. 90-97
Genotype and phenotype are concepts that teenagers become familiar with in the last grades of secondary school. But not everyone understands what these words mean. We can guess that this is some kind of classification of people's characteristics. What is the difference between these consonant names?
Human genotype
A genotype refers to all hereditary characteristics of a person, that is, a set of genes located on chromosomes. The genotype is formed depending on the inclinations and adaptation mechanisms of the individual. After all, every living organism is in certain conditions. Animals, birds, fish, protozoa and other types of living organisms adapt to the conditions where they live. Likewise, a person living in the southern part of the globe can easily tolerate high air temperatures or too low temperatures due to the color of his skin. Such adaptation mechanisms work not only in relation to the geographical location of the subject, but also other conditions; in a word, this is called the genotype.
What is a phenotype?
To know what genotype and phenotype are, you need to know the definition of these concepts. We have already dealt with the first concept, but what does the second mean? The phenotype includes all the properties and characteristics of an organism that it acquired during development. When a person is born, he already has his own set of genes that determine his adaptability to external conditions. But in the course of life, under the influence of internal and external factors, genes can mutate and change, so a qualitatively new structure of human characteristics appears - the phenotype.
The history of these concepts
What genotype and phenotype are can be understood by learning the history of the origin of these scientific terms. At the beginning of the twentieth century, the science of the structure of a living organism and biology was actively studied. We remember Charles Darwin's theory of evolution and the emergence of man. He was the first to put forward the Temporary hypothesis about the separation of cells in the body (gemmules), from which another individual could subsequently emerge, since these are germ cells. Thus, Darwin developed the theory of pangenesis.
41 years later, in 1909, the botanist Wilhelm Johansen, based on the concept of “genetics” already known in those years (introduced in 1906), introduced a new concept into the terminology of science - “gene”. The scientist replaced with it many words that were used by his colleagues, but which did not reflect the entire essence of the innate properties of a living organism. These are words such as “determinant”, “germ”, “hereditary factor”. During the same period, Johansen introduced the concept of “phenotype,” emphasizing the hereditary factor in the previous scientific term.
Human genotype and phenotype - what is the difference?
By highlighting two concepts about the properties and characteristics of a living organism, Johansen clearly defined the difference between them.
- Genes are passed on to offspring by an individual. An individual receives its phenotype during its life development.
- Genotype and phenotype also differ in that genes in a living being appear as a result of the combination of two sets of hereditary information. The phenotype appears on the basis of the genotype, undergoing various changes and mutations. These changes occur under the influence of external conditions of existence of a living organism.
- The genotype is determined through complex DNA analysis; an individual's phenotype can be seen by analyzing basic appearance criteria.
It should be noted that living organisms have different levels of adaptability and sensitivity to the conditions around them. This determines how much the phenotype will be changed during life.
Differences between people by genotype and phenotype
Although we belong to the same biological species, we are very different from each other. No two people are alike; each person’s genotype and phenotype will be individual. This is manifested if you place completely different people in conditions that are equally unusual for them, for example, send an Eskimo to the villages of South Africa, and ask a resident of Zimbabwe to live in the tundra. We will see that this experiment will not be successful, since these two people are accustomed to living in their own geographical latitudes. The first difference between people in terms of geno- and phenotypic characteristics is adaptation to climatic and geographical factors.
The next difference is dictated by the historical-evolutionary factor. It lies in the fact that as a result of population migrations, wars, the culture of certain nationalities, and their mixing, ethnic groups were formed that have their own religion, national characteristics and culture. Therefore, you can see clear differences between the style and way of life, for example, of a Slav and a Mongol.
Differences between people can also be based on social parameters. This takes into account the level of people’s culture, education, and social aspirations. It was not for nothing that there was such a thing as “blue blood,” which indicated that the genotype and phenotype of a nobleman and a commoner were significantly different.
The last criterion for differences between people is the economic factor. Depending on the provision of a person, family and society, needs arise, and, consequently, differences between individuals.
Allelic genes. So, we have established that heterozygous individuals have two genes in each cell - A And A, responsible for the development of the same trait. Genes that determine the alternative development of the same trait and are located in identical regions of homologous chromosomes are called allelic genes or alleles. Any diploid organism, be it a plant, animal or human, contains two alleles of any gene in each cell. The exception is sex cells - gametes. As a result of meiosis, the number of chromosomes in them decreases by 2 times, so each gamete has only one allelic gene. Alleles of one gene are located in one place on homologous chromosomes.
Schematically, a heterozygous individual is designated as follows:
Homozygous individuals with this designation look like this:
or , but they can also be written as AA And ahh.
Phenotype and genotype. Considering the results of self-pollination of F 2 hybrids, we discovered that plants grown from yellow seeds, being externally similar, or, as they say in such cases, having the same phenotype, have a different combination of genes, which is usually called a genotype. Thus, the phenomenon of dominance leads to the fact that with the same phenotype, individuals can have different genotypes. The concepts of “genotype” and “phenotype” are very important in genetics. The totality of all the genes of an organism constitutes its genotype. The totality of all the characteristics of an organism, from external to the features of the structure and functioning of cells and organs, constitutes the phenotype. The phenotype is formed under the influence of the genotype and environmental conditions.
Analyzing crossing. It is not always possible to determine its genotype based on the phenotype of an individual. In self-pollinating plants, the genotype can be determined in the next generation. For cross-breeding species, so-called test crossing is used. During analytical crossing, an individual whose genotype should be determined is crossed with individuals homozygous for the recessive gene, i.e., having the aa genotype. Let's look at analytical crossing using an example. Let individuals with genotypes AA And Ahh have the same phenotype. Then, when crossed with an individual that is recessive for a given trait and has the genotype ahh, the following results are obtained:
From these examples it is clear that individuals homozygous for the dominant gene do not produce cleavage in F1, but heterozygous individuals, when crossed with a homozygous individual, produce cleavage in F1.
Incomplete dominance. Heterozygous organisms do not always exactly correspond in phenotype to the parent who is homozygous for the dominant gene. Often heterozygous offspring have an intermediate phenotype, in such cases they speak of incomplete dominance (Fig. 36). For example, when crossing a night beauty plant with white flowers (aa) with a plant that has red flowers (AA), all F 1 hybrids have pink flowers (Aa). When hybrids with pink flower color are crossed with each other in F 2, splitting occurs in the ratio 1 (red): 2 (pink): 1 (white).
Rice. 36. Intermediate inheritance in the night beauty
The principle of gamete purity. Hybrids, as we know, combine different alleles introduced into the zygote by parental gametes. It is important to note that different alleles that end up in the same zygote and, therefore, in the organism that develops from it, do not affect each other. Therefore, the properties of the alleles remain constant regardless of which zygote they were in before. Each gamete always contains only one allele of a gene.
The cytological basis of the principle of gamete purity and the law of segregation is that homologous chromosomes and the allelic genes located in them are distributed in meiosis among different gametes, and then during fertilization they are reunited in the zygote. In the processes of divergence in gametes and association into zygotuallelic genes, they behave as independent, integral units.
- Would the definition be correct: a phenotype is a set of external characteristics of an organism?
- What is the purpose of testing crossbreeding?
- What do you think is the practical significance of knowledge about genotype and phenotype?
- Compare the types of inheritance of genetic traits during crossings with the behavior of chromosomes during meiosis and fertilization.
- When crossing gray and black mice, 30 offspring were obtained, of which 14 were black. It is known that gray color is dominant over black. What is the genotype of the parent generation mice? See the solution to the problem at the end of the textbook.
- A blue-eyed man, both of whose parents had brown eyes, married a brown-eyed woman whose father had brown eyes and whose mother had blue eyes. From this marriage a blue-eyed son was born. Determine the genotypes of all mentioned individuals.
I'm currently reading a book on population genetics (I've already given a couple of interesting examples from there) and recently read an excellent example of how the phrase "natural selection - survival of the fittest" is a simplification and even an incorrect statement in the light of modern evolutionary theory. This example also indicates how much this modern theory of evolution has absorbed the non-obvious nuances of such a simple and obvious, at first glance, principle as natural selection. At the same time, it has in no way become anti-Darwinian, on the contrary - Darwin's theory undoubtedly lies at its very basis, but at the same time we have become much better aware of the real consequences of this theory.
In short, if this topic is at least somewhat interesting to you, then I recommend following the cut - you won’t be disappointed.
So let's start with the classic story of sickle cell disease. For those who don’t know, in short: We all have two copies of the hemoglobin gene (one from mom, the other from dad). For most people in the world, both copies of the hemoglobin gene have a sequence that we will call the A allele (for such people we will call AA). These people are susceptible to malaria, which is widespread in central Africa. There is an alternative variant of the hemoglobin gene - the S allele. People who have two S alleles (SS) are resistant to malaria, but suffer from a very serious disease - hemolytic anemia, which is much worse than malaria. People who have both the A allele and the S allele (denoted AS) are resistant to malaria, but suffer slightly from anemia. This is a classic example from textbooks on the theory of evolution, which demonstrates balancing selection - the S allele (despite the fact that it causes serious disease) spreads through the population of people in areas with malaria, but only until the probability of having both a father and a mother with this allele remains low. If it becomes too common, many children get two copies of it and die from anemia.
Now let's move on to some lesser-known facts. In addition to alleles A and S, there is also allele C. People with the AC genotype are no different from people with the AA genotype, but people with the CC genotype (two copies of this allele) are resistant to malaria and do not suffer from anemia at all!
We summarize all of the above in the table of the survival factor (fitness) of people with different genotypes. If we take the fitness of AA people in areas without malaria as 1, then in areas with malaria the fitness of different genotypes will be:
From this table it is clear that the “strongest” of all people are people with the CC genotype. If natural selection were simply survival of the fittest, then we would expect the CC genotype to become the dominant genotype in malaria areas within a fairly short time. However, this does not happen! Moreover, calculations show that he basically cannot spread into a population in which the A and S alleles already exist.
I won’t present the mathematics here, but in general the logic of this conclusion boils down to the following: Let’s assume that in a population of people in which alleles A and S are already common, allele C appeared (as a result of mutation or migration from another area). Since its frequency is still very low; people with the CC genotype are very rare. They survive well in these conditions and produce offspring, spreading their genes (allele C), but there are very few of them and their children are very likely to have either the AC genotype or the SC genotype. The AC genotype has no advantages over the AA genotype and is worse than the AS genotype. The SC genotype is worse than AA and AS. That is, although people with the CC genotype themselves are very well adapted, their children are less well adapted than other people on average. As a result, the C allele (which gives its homozygotes resistance to malaria without the negative effects of anemia) disappears from the population through natural selection!
Even more interesting is the influence of social factors on this process. The above reasoning is valid for a population in which crossings occur completely randomly. We know, however, that in human populations there are often rules regarding marriages - incest is often prohibited, but in many cases (in small closed settlements) incest cannot be avoided, if not in the first generation, then in the second or third, people unwittingly marry relatives. To avoid inbreeding, it is often customary to take a bride from a remote settlement. In short, the rate of inbreeding in human populations may be either higher or lower than expected from random mate selection.
One of the properties of closely related crosses is an increase in the number of homozygotes (AA, SS, SS) in the offspring and a decrease in the number of heterozygotes (AS, AC, SC). Where does this lead? If inbreeding is frequent, then the probability of occurrence of SS and CC genotypes is higher than predicted in previous calculations. The SS genotype is very negative, and the CC genotype is very favorable. If the probability of their occurrence increases, then the balance quickly shifts in favor of the C allele. As soon as the probability of closely related crosses exceeds their random probability by 4%, the C allele acquires an advantage and spreads throughout the population, and the S allele disappears. Conversely, if inbreeding is actively prevented in a population, then the chances of the C allele to spread are significantly reduced and it very quickly disappears from the population. The positive effect of inbreeding!
That is, we see that in this situation, natural selection not only does NOT lead to the reproduction of the strongest (phenotypes or genotypes), but selection also largely depends on how exactly crossings occur in the population.
By the way, this example also illustrates the correctness of Dawkins’ idea of a selfish gene: if you look from the point of view of gametes (carrying individual alleles of the hemoglobin gene), then the above situation is in no way paradoxical, and is even quite obvious: a gamete carrying the C allele has less fitness than the gamete carrying the S allele. Therefore, it disappears. The fitness of the genotypes or phenotypes of organisms is not a decisive factor for natural selection.
Tasks: introduce students to the basic laws of heredity, teach them to solve genetic problems of various types.
Lesson No. 1–2. Subject and tasks of genetics. Hybridological method of studying heredity. Heredity and its material carriers. Allelic genes. Genotype. Phenotype. G. Mendel's first law
Equipment: portraits of outstanding Russian geneticists, tables on general biology.
During the classes
I. Learning new material
1. Subject and significance of genetics
Genetics (from Greek. genesis– origin) – the science of heredity and variability of living organisms and methods of managing them.
Since ancient times, people have noticed the similarity between parents and children, but they could not explain its reasons. An explanation for this similarity was given only in the 20th century.
The results of G. Mendel's experiments on pea genetics were published in 1865. However, the birth of genetics is usually attributed to 1900, when botanists G. de Vries, K. Correns and E. Cermak discovered Mendel's laws for the second time. The term “genetics” was proposed in 1906 by W. Batson.
In the history of the development of genetics, several stages can be distinguished.
1. Beginning of the 20th century. – development of Mendelism and the formation of genetics (G. de Vries, A. Weisman, O. Hertwig, etc.).
2. 1920–1940s XX century – formation of the chromosomal theory of heredity (T. Morgan, A.S. Serebrovsky, N.I. Vavilov, S.S. Chetverikov, etc.).
3. Since the 1950s. to the present - the development of genetics at the molecular level (D. Watson, F. Crick, etc.).
The importance of genetics is very great. Firstly, it is the scientific basis of selection. Secondly, it allows you to prevent, identify and more effectively treat hereditary diseases (more than 3 thousand human diseases are hereditary). Genetic engineering, which is essentially applied molecular genetics, allows you to change the genetic programs of organisms and “design” biological systems with properties beneficial to humans.
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2. Outstanding domestic geneticists
Russian scientists made a significant contribution to the development of genetics. Let's name them.
- Koltsov Nikolai Konstantinovich (1872–1940) - in 1928, put forward a hypothesis of the molecular structure of chromosomes and developed the first diagram of their structure. He founded the Institute of Experimental Biology and was its director.
- Timofeev-Resovsky Nikolai Timofeevich (1900–1981) is one of the founders of population and radiation genetics.
- Chetverikov Sergei Sergeevich (1880–1959) is one of the founders of evolutionary and population genetics. He was the first to connect the doctrine of heredity with the doctrine of evolution.
- Serebrovsky Alexander Sergeevich (1892–1948) – organizer and first head of the Department of Genetics at Moscow State University. He proposed a diagram of the linear structure of a gene and a method for determining gene sizes.
- Vavilov Nikolai Ivanovich (1887–1943) - created the scientific foundations of modern selection and the doctrine of the world centers of origin of cultivated plants. In 1920, he formulated the law on homological series in hereditary variability, which in significance can be compared with the periodic system of D.I. Mendeleev.
The history of genetics in our country is tragic. In the 1950s The discussion between scientists from a scientific plane was transferred to an ideological plane. Genetics was declared a pseudoscience and completely destroyed, and many scientists were repressed. The consequences of those tragic events have not yet been fully overcome.
3. Hybridological method of studying heredity
One of the main methods of genetics is the hybridological method. Hybridization(from lat. hibrida– cross) – crossing of individuals that differ in one or more characteristics. Daughter organisms obtained during hybridization are called hybrids.
If the original parent forms differ from each other in one pair of characters, then such a cross is called monohybrid(from Greek monos– one and lat. hibrida). If the original parent forms differ in two pairs of characters, then we are talking about dihybrid crossing (from Greek. di– a prefix denoting twice, double and lat. hibrida), and if there are three or more signs, then they talk about polyhybrid crossing (from Greek. policy- a lot of lat. hibrida).
4. Heredity and its material structures
Heredity– the ability of parents to pass on their characteristics and developmental characteristics to the next generation. This property, inherent in all organisms without exception, performs two main functions:
– ensuring the continuity of properties and characteristics between generations;
– ensuring accurate transmission of the type of development specific to each organism, the formation during ontogenesis of certain characteristics and properties, and a certain type of metabolism.
How is the continuity of generations carried out? In sexual reproduction - through germ cells, and in asexual reproduction - through somatic cells. But cells do not carry the signs and properties of future organisms, but only the makings that make it possible to develop these signs and properties. The carriers of these inclinations are genes.
Gene(from Greek genos– genus, origin) is a section of a DNA molecule (or a section of a chromosome) that determines the possibility of developing a separate elementary trait.
The location of a gene on a chromosome is called locus(from lat. locus- place). In all organisms of a given species, each gene is located in the same locus of a strictly defined chromosome. In the haploid set of chromosomes there is only one gene that determines the development of this trait. In a diploid set of chromosomes, two homologous chromosomes and, accordingly, two genes determine the development of one trait.
Genes located in the same loci of homologous chromosomes and determining the development of one particular trait are called allelic(from Greek allelon– mutually).
Letter designations are used for genes. Allelic genes are usually designated by letters of the same name. If two allelic genes are completely identical in structure, that is, they have the same nucleotide sequence, they are designated by the same letters (both in capitals or both in lowercase - ahh or AA).
Random and unpredictable gene transformations (mutations) lead to the fact that almost every gene is represented by several, two or more, varieties. These forms of genes cause changes in the original trait. A gene can mutate repeatedly, that is, a whole series of allelic genes (alleles) can arise. Thus, alleles are variations of the same gene. For example, the seed color gene in peas is represented by two alleles: a yellow allele and a green allele.
5. Genotype and phenotype
Genotype(from Greek genos And typos– imprint) – the totality of all the genes of a given cell or organism.
Phenotype(from Greek pheno- I manifest, discover and typos) – the totality of all the characteristics and properties of an individual. Phenotypic include not only external, visible signs, but also biochemical signs (shape of a protein molecule, concentration of glucose in the blood, etc.), histological (shape and size of cells, tissue structure, etc.), anatomical, etc.
The terms “genotype” and “phenotype” were introduced by V. Johansen at the beginning of the 20th century.
6. G. Mendel's first law
The most important patterns of inheritance were discovered by the Czech scientist G. Mendel in experiments on plants. Johann Mendel was born in 1822 into a peasant family. After graduating from high school, in 1843 he was tonsured a monk at the Augustinian monastery in Brünn, taking the name Gregor. With funds from this monastery, he studied at the University of Vienna (1851–1853). Returning to Brunn, he taught physics and biology at school. His experiments with peas date back to this period (1856–1863). In 1868, Mendel became abbot of the monastery and retired from science.
G. Mendel's success was determined by a combination of circumstances.
- Mendel studied mathematics and probability theory at one time. Therefore, he understood that when assessing the results of crosses, it was necessary to operate with large numbers in order to minimize the effect of “sampling error.”
- Mendel was lucky with his choice of experimental subject. There are many varieties of peas, all of which are self-pollinating plants. The structural features of the legume flower made it possible to carry out artificial pollination and made cases of pollination with foreign pollen rare. In addition, peas have a short development period, numerous offspring and a large number of clearly visible alternative characters. For seven years, Mendel carefully examined more than 10 thousand plants and tens of thousands of seeds, in which he studied the inheritance characteristics of seven different characters (seed surface, seed color, flower color, bean shape, bean color, flower position, stem length).
Mendel's experiments were carefully thought out. He began his research by crossing varieties that differed in one trait, that is, with monohybrid crossing. With such crossing, the patterns of inheritance of only two variants of the trait are traced, and all other characteristics of the organism are not taken into account. A classic example of a monohybrid cross is crossing pea varieties with green and yellow seeds.
If you cross pea plants with yellow and green seeds, then all the resulting hybrids will have yellow seeds. Hence, in first generation hybrids, only one of each pair of alternative characters develops. That's what it is Mendel's first law, called law of uniformity of first generation hybrids. The second symptom seems to disappear without appearing phenotypically.
Mendel called the phenomenon of predominance of the trait of one of the parents in hybrids dominance(from lat. dominantis– dominant).
A trait that appears in first-generation hybrids and suppresses the development of another trait is called dominant. Dominant genes are indicated by capital letters ( A). The opposite, that is, suppressed sign, is called recessive(from lat. recessus– retreat, removal). Recessive genes are designated by lowercase (small) letters ( A).
If in the genotype of an organism (zygote) there are two identical allelic genes - both dominant or both recessive ( AA or ahh), – then such an organism is called homozygous(from Greek homos– identical, equal and zygotos– connected together) for a given pair of alleles. If of a pair of allelic genes one is dominant and the other is recessive ( Ahh), then such an organism is called heterozygous(from Greek heteros– different, different and zygotos).
7. Genetic symbolism. Crossing schemes
Let's consider the notations that are usually used in genetics when describing crosses.
To designate the genes responsible for the trait in question, they use, as Mendel himself did, letters of the Latin alphabet.
The parent generation is designated by the letter P (from lat. parentale- parents), and generations of descendants (first, second, third, etc.) - with the letter F (from lat. filial– children) with the corresponding index (F 1, F 2, F 3, etc.). The icon is used to record a cross. The symbol ♀ denotes the female gender (the mirror of Venus), and ♂ the male gender (the shield and spear of Mars).
The procedure for drawing up crossbreeding schemes is as follows:
1) the phenotypes of the parent organisms are written on the first line, with the maternal organism being written first in the diagram;
2) on the second line – genotypes of the parent organisms. The dominant allele ( Ahh, but not aA);
3) on the third line - the types of gametes that parents can give;
4) on the fourth line – genotypes of zygotes of the hybrid generation;
5) on the fifth line – phenotypes of the hybrid generation.
Let's draw up a crossing scheme for G. Mendel's first law:
Phenotypes of parents |
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♀ Yellow |
♂ Green |
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Parents' genotypes |
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Genotypes of zygotes |
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Hybrid phenotypes |
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Yellow (100%) |
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II. Consolidation of knowledge
General conversation as you learn new material.
III. Homework
Study the textbook paragraph (the subject and significance of genetics, the hybridological method of studying heredity, heredity and its material structures, genotype and phenotype, G. Mendel’s first law, genetic symbolism).
Lesson No. 3–4. G. Mendel's second law. Incomplete dominance. Codominance. Analyzing, return and reciprocal crossings
Equipment: tables on general biology, schemes of genetic crossings illustrating Mendel's second law, incomplete dominance, codominance, analyzing, return and reciprocal crossings.
During the classes
I. Test of knowledge
Oral knowledge test:
– subject of genetics;
– hybridological method of studying heredity, mono-, di- and polyhybrid crossing;
– heredity and its material structures; allelic genes;
– genotype and phenotype;
– G. Mendel’s first law – the law of uniformity of first generation hybrids;
– the procedure for drawing up crossing schemes.
II. Learning new material
1. Mendel's second law
G. Mendel's second law is called law of splitting. It reads: in the offspring obtained from crossing first-generation hybrids, a splitting phenomenon is observed: a quarter of individuals from second-generation hybrids carry a recessive trait, three quarters carry a dominant one.
Consequently, the recessive trait did not disappear in F1 hybrids, but was only suppressed and appeared in F2. Let us present the crossing scheme according to Mendel’s second law:
♀ Yellow |
♂ Yellow |
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2. Law of purity of gametes
To answer the question: why stable hybrids are not formed, let us consider the cytological basis of splitting - the law (rule) of gamete purity.
G. Mendel explained the splitting of offspring when crossing heterozygous individuals by the fact that gametes are genetically pure, that is, they carry only one gene from an allelic pair. Law of gamete purity can be formulated as follows: When germ cells are formed, only one gene from an allelic pair enters each gamete. Why is this happening?
To understand the cytological basis of inheritance, it is necessary to recall the main phenomena occurring in meiosis. As a result of two divisions of meiosis, cells are formed that carry a haploid set of chromosomes (n), that is, containing one chromosome from each pair of homologous chromosomes. Subsequently, gametes are formed from these cells. The fusion of haploid gametes during fertilization leads to the formation of a diploid (2n) organism.
The original parental forms in the experiment under consideration by G. Mendel were homozygous, that is, they carried two identical genes. Obviously, both parents are capable of producing gametes of only one variety, with plants having two dominant genes AA, produce gametes carrying only the gene A, and plants with two recessive genes ahh form germ cells with a gene A. All first generation hybrids are heterozygous and have yellow seeds, since the dominant gene A suppresses the action of a recessive gene A green color of seeds.
♀ Yellow |
♂ Green |
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A| |A |
A| |A |
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A| |A |
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A| |A |
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Yellow (100%) |
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Heterozygous plants Ahh capable of producing two types of gametes carrying genes A And A. During fertilization, four types of zygotes arise: AA + +Ahh + aA + ahh, which can be written like this: AA + 2Aa+ + ahh(1:2:1). Since in the experiment the seeds of heterozygous plants are also colored yellow, in F 2 the ratio of yellow to green seeds is equal to 3:1.
♀ Yellow |
♂ Yellow |
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A| |A |
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Thus, the dominant and recessive alleles, which end up in the same zygote and, consequently, in the organism that develops from it, do not change the nature of each other. Therefore, the properties of the alleles remain constant regardless of which zygote they are in before. Each gamete always carries only one allele of a gene.
The cytological basis of the law of gamete purity and the law of segregation is that homologous chromosomes and the allelic genes located in them are distributed in meiosis among different gametes, and then during fertilization they are reunited in the zygote. In the processes of divergence in gametes and union into a zygote, allelic genes behave as independent, integral units.
3. Incomplete dominance
Heterozygous organisms do not always exactly correspond in phenotype to the parent who is homozygous for the dominant gene. Mendel himself was faced with the fact that when crossing large-leaved peas with small-leaved peas, the first generation hybrids did not repeat the trait of any of the parent plants. All of them had leaves of medium size, that is, the severity of the trait in the hybrids was intermediate.
Later it turned out that the intermediate manifestation is characteristic of many characteristics of plants and animals. This is precisely the nature of the inheritance of flower color in snapdragons, plumage color in chickens, wool in cattle and sheep, etc. Cases where heterozygous offspring have an intermediate phenotype are called incomplete dominance, or intermediate manifestation of the trait.
As an example, consider the inheritance of flower color in a night beauty. When crossing homozygous red-flowered and white-flowered varieties, the entire first generation of hybrids has pink flowers. When crossing hybrids, we obtain a split in the ratio of 1/4 red-flowered, 2/4 pink-flowered, 1/4 white-flowered. It is characteristic that with incomplete dominance, splitting by phenotype corresponds to splitting by genotype, since heterozygotes are phenotypically different from homozygotes.
To explain the phenomenon of incomplete dominance, let us remember that genes encode proteins, often enzyme proteins. One parent plant has both alleles ( AA) encode enzymes responsible for the synthesis of red pigment. A lot of enzymes are produced, and the color of the flowers is red. The second parent plant has the allele A contains a mutation, as a result of which the enzyme loses activity - the color of the flowers is white. In plants with a genotype Ahh only one allele codes for a working enzyme. Few enzymes are produced, and the color of the flowers is pink.
4. Co-dominance
Upon further study of the nature of inheritance of many traits, it turned out that in F 1 the traits of both parents can appear. Such signs are called codominant(from lat. co- together and dominantis). Codominance is a special case of dominance when both alleles participate equally in the formation of a trait in a heterozygous individual.
A classic example of codominance is the interaction of alleles that determine the blood group system AB0.
As is known, at the beginning of the 20th century. human blood groups were discovered. It turned out that there are four blood groups, which are designated 0 , A, IN And AB. In 1924, the German doctor F. Bernstein showed that the blood groups of the system AB0 determined by three alleles of one gene: I A, I B, i. Genes I A And I B are codominant. Information about possible genotypes and phenotypes for the blood group system AB0 are given in the table.
Table. Information about genotypes and phenotypes of blood groups of the AB0 system
Blood type |
Genotype |
I A IA And I A i |
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I B I B And I B i |
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AB(IV) |
Thus, we studied several possible types of interaction of allelic genes: complete dominance, incomplete dominance and codominance.
5. Test crossbreeding
It is not always possible to determine its genotype based on the phenotype of an individual. In self-pollinating plants, the genotype can be determined in the next generation. For species that use other systems of sexual reproduction, so-called analytical crossing is used.
Crossing a hybrid individual with an individual homozygous for recessive alleles is called analyzing. In test crossing, an individual whose genotype is to be determined is crossed with individuals having the genotype ahh. Analyzing crossing is one of the main methods that allows one to establish the genotype of an individual; for this reason it is widely used in genetics and breeding.
Let's look at analytical crossing using an example. Let individuals with genotypes AA And Ahh have the same phenotype. Then, when crossed with an individual that is recessive for a certain trait and has the genotype aa F 1
Aa aa
From these examples it is clear that individuals homozygous for the dominant gene do not produce cleavage in F1, but heterozygous individuals, when crossed with a homozygous recessive individual, produce cleavage in F1.
6. Backcrossing
Crossing a first generation hybrid with one of the parental forms or a form similar in genotype is called returnable.
♀AA |
♂ahh |
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♀AA |
♂Ahh |
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AA Aa |
7. Reciprocal crosses
Two crosses that are characterized by a mutually opposite combination of the analyzed trait and sex in the forms taking part in these crosses are called reciprocal(from lat. reciprocus– mutual).
So, if in one crossing of animals the female had a dominant trait, and the male had a recessive one, then in the second crossing, reciprocal to the first, the female should have a recessive trait, and the male should have a dominant one.
Reciprocal crosses are used in genetic analysis to identify hereditary factors localized on the X chromosome.
III. Consolidation of knowledge
1. General conversation while learning new material.
2. Solving the problem.
In oats, immunity (immunity) to smut dominates over susceptibility. Taking this into account, the breeder decided to cross the homozygous form of oats with a plant affected by smut. Determine and indicate the genotypes of the original forms. Explain how the breeder determined the homozygosity of the parent if the heterozygous form is phenotypically indistinguishable from the homozygous one?
Given:
A– immunity to smut
A
– susceptibility to smut
R -?
Solution:
| 1) | ♀ Immunity |
♂ Sensitivity |
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2) To determine the homozygosity of a parent resistant to smut, it was necessary to conduct an analytical cross. With the participation of a homozygous plant, the results of the crossing will be the same as in the above scheme.
Answer: R – ♀ AA ♂ ahh; To determine the genotype of an individual that is a carrier of a dominant trait, it is necessary to conduct an analyzing cross.
IV. Homework
1. Study the textbook paragraph and notes made in class (Mendel’s second law and its cytological basis, incomplete dominance, codominance, analyzing crosses, reciprocal crosses, reciprocal crosses).
2. Solve the problems at the end of the textbook paragraph.
