How often does monohybrid crossing occur in nature? Splitting of characters by genotype and phenotype during dihybrid crossing. What is genotype and phenotype

One of the features of Mendel's method was that he used pure lines for experiments, that is, plants in the offspring of which, during self-pollination, no diversity in the studied trait was observed. (In each of the pure lines, a homogeneous set of genes was preserved). Another important feature of the hybridological method was that G. Mendel observed the inheritance of alternative (mutually exclusive, contrasting) characters. For example, plants are short and tall; flowers are white and purple; seed shape is smooth and wrinkled, etc. An equally important feature of the method is an accurate quantitative accounting of each pair of alternative characteristics in a series of generations. Mathematical processing of experimental data allowed G. Mendel to establish quantitative patterns in the transmission of the studied characteristics. It was very significant that G. Mendel followed an analytical path in his experiments: he observed the inheritance of diverse traits not immediately in the aggregate, but only of one pair of alternative traits.

The hybridological method underlies modern genetics.

Uniformity of the first generation. Dominance rule. G. Mendel conducted experiments with peas, a self-pollinating plant. He chose for the experiment two plants that differed in one characteristic: the seeds of one variety of peas were yellow, and the other were green. Since peas, as a rule, reproduce by self-pollination, there is no variability in seed color within a variety. Taking this property into account, G. Mendel artificially pollinated this plant by crossing varieties that differed in the color of the seeds. Regardless of which variety the mother plants belonged to, the first generation hybrid seeds (Fi) were only yellow. Consequently, in hybrids only one trait appears; the trait of the other parent seems to disappear. G. Mendel called such a predominance of the trait of one of the parents dominance, and the corresponding traits dominant. He called traits that did not appear in first-generation hybrids recessive. In experiments with peas, the trait of yellow seed color dominated over green color. Thus, G. Mendel discovered uniformity in color in the first generation hybrids, i.e. all hybrid seeds had the same color. In experiments where crossed varieties differed in other traits, the same results were obtained: uniformity of the first generation and dominance of one trait over another.

Splitting of characters in second generation hybrids. From hybrid pea seeds, G. Mendel grew plants that, through self-pollination, produced second-generation seeds. Among them were not only yellow seeds, but also green ones. In total, in the second generation he received 6022 yellow and 2001 green seeds, i.e. 3/4 of the hybrids were yellow and 1/4 were green. Consequently, the ratio of the number of descendants of the second generation with a dominant trait to the number of descendants with a recessive trait turned out to be close to 3:1 . He called this phenomenon splitting of signs. G. Mendel was not embarrassed that the ratios of descendants that he actually discovered deviated slightly from the 3:1 ratio. Further, by studying the statistical nature of the laws of inheritance, we will be convinced that Mendel is right.

Numerous experiments on genetic analysis of other pairs of traits gave similar results in the second generation. Based on the results obtained, G. Mendel formulated the first law - the law of splitting. In the offspring obtained from crossing first-generation hybrids, the phenomenon of splitting is observed: a quarter of individuals from second-generation hybrids have a recessive trait, three quarters have a dominant one.

Analyzing crossing. With complete dominance among individuals with dominant characteristics, it is impossible to distinguish homozygotes from heterozygotes, and this often becomes necessary (for example, to determine whether a given individual is purebred or hybrid). For this purpose, an analytical cross is carried out, in which the studied individual with dominant characteristics is crossed with a recessive homozygous one. If the offspring from such a cross turns out to be homogeneous, then the individual is homozygous (its genotype is AA). If the offspring contains 50% of individuals with dominant traits, and 50% with recessive traits, then the individual is heterozygous.

Intermediate nature of inheritance. Sometimes Fi hybrids do not exhibit complete dominance; their characteristics are intermediate (Aa). This type of inheritance is called intermediate or incomplete dominance.

The rule of gamete purity, established by Mendel, was the first to demonstrate the property of gene discreteness, the immiscibility of alleles with each other and with other genes. Mendel was the first to show that the hereditary factors in the gametes of a first-generation hybrid remain exactly the same as in the parents. They do not mix and do not undergo changes after being together in a hybrid organism.

QUESTION 2. TASK.

TICKET#20

QUESTION 1.

Dihybrid crossing. Having established the patterns of inheritance of one trait (monohybrid crossing), Mendel began to study the inheritance of traits for which two pairs of allelic genes are responsible. A cross involving two pairs. alleles is called a dihybrid cross. Mendel conducted a dihybrid cross in which homozygous parents differed from each other in two characteristics: seed color (yellow and green) and seed shape (smooth and wrinkled). The appearance of individuals with yellow smooth seeds indicates the dominance of these characters and the manifestation of the rule of uniformity in Fi hybrids. During the formation of gametes in Fi individuals, four combinations of two pairs of alleles are possible. Alleles of the same gene always end up in different gametes. The divergence of one pair of genes does not affect the divergence of the genes of the other pair.

If in meiosis the chromosome with gene A moved to one pole, then to the same pole, i.e. the same gamete can contain a chromosome with both gene B and gene B. Therefore, with equal probability, gene A can appear in the same gamete with both gene B and gene B. Both events are equally likely. Therefore, as many AB gametes there will be, there will be the same number of AB gametes. The same reasoning is valid for gene a, i.e. the number of gametes aB is always equal to the number of gametes ab. As a result of the independent distribution of chromosomes in meiosis, the hybrid forms four types of gametes: AB, AB, aB and ab in equal quantities. This phenomenon was established by G. Mendel and called the law of independent splitting, or Mendel's second law. It is formulated as follows: splitting for each pair of genes occurs independently of other pairs of genes.”

Punnett grid. Independent splitting can be depicted as a table. After the geneticist who first proposed this table, it is called the Punnett grid. Since in a dihybrid cross with independent inheritance four types of gametes are formed, the number of types of zygotes formed by the random fusion of these gametes is 4x4, i.e. 16. Exactly so many cells in the Punnett lattice. Due to the dominance of A over a and B over b, different genotypes have the same phenotype. Therefore, the number of phenotypes is only four. For example, in 9 cells of the Punnett grid out of 16 possible combinations there are combinations that have the same phenotype - yellow smooth seeds. The genotypes that determine this phenotype are: 1ААВВ: 2ААББ: 2АаВВ: 4АаББ,

The number of different genotypes formed during dihybrid crossing is 9. The number of phenotypes in Fa with complete dominance is 4. This means that dihybrid crossing is two independently occurring monohybrid crossings, the results of which seem to overlap each other. Unlike the first law, which is always valid, the second law applies only to cases of independent inheritance, when the genes under study are located in different pairs of homologous chromosomes.

Question 1. Do you think dihybrid crosses often occur in nature?
G. Mendel analyzed the inheritance of two, three or more pairs of characters in peas. Hybrids that are obtained from crossing organisms that differ in two pairs of alternative characters are called dihybrids, three pairs are called trihybrids, etc.
For dihybrid crosses, Mendel used homozygous pea plants that differed in color and type of seed surface: the mother plant had yellow and smooth seeds; both traits were dominant. The father plant had green and wrinkled seeds; both traits were recessive. If we designate the dominant and recessive alleles that determine the color of the seed, respectively, with the letters A and a, and the alleles that determine the shape of the surface of the seed with the letters B and b, then the genotypes of the homozygous parental forms will look like this: the mother plant AABB and the paternal plant aabb. In the first case, the gametes will carry alleles A and B (AB), in the second - a and b (ab). The fusion of two such gametes will lead to the appearance of a dihybrid zygote AaBb. According to the phenotype, such plants, with complete dominance, will have two dominant characteristics: their seeds will be yellow and smooth.
In fact, in nature, organisms differ from each other in many pairs of alternative characteristics. In nature, no one selects traits for analysis. And it is incorrect to talk about how often dihybrid crossing occurs in nature.

Question 2. How many types of gametes are formed in first-generation hybrids during dihybrid crossing?
When crossing first-generation hybrids during dihybrid crossing, each parent produces four varieties of gametes. This happens because the genes responsible for these traits are located in different pairs of homologous chromosomes. During meiosis, each gamete contains one chromosome from a pair. Chromosomes (and therefore genes) can be combined in different combinations. Two pairs of chromosomes give four variants of gene combinations: AB, Av, aB, av. For dihybrid crosses, Mendel used homozygous pea plants that differed in color and type of seed surface: the mother plant had yellow and smooth seeds; both traits were dominant. The father plant had green and wrinkled seeds; both traits were recessive. If we designate the dominant and recessive alleles that determine the color of the seed, respectively, with the letters A and a, and the alleles that determine the shape of the surface of the seed with the letters B and b, then the genotypes of the homozygous parental forms will look like this: the mother plant AABB and the paternal plant aabb. In the first case, the gametes will carry alleles A and B (AB), in the second - a and b (ab). The fusion of two such gametes will lead to the appearance of a dihybrid zygote AaBb. According to the phenotype, such plants, with complete dominance, will have two dominant characteristics: their seeds will be yellow and smooth. To find out how many varieties of gametes form such a dihybrid, Mendel carried out an analytical cross: he crossed F 1 hybrid plants with plants homozygous for two recessive traits (that is, having green and smooth seeds; genotype aabb). The offspring produced 4 classes of seeds in a ratio close to 1:1:1:1: 55 yellow smooth (AaBb); 51 green smooth (aaBb); 49 yellow wrinkled (Aabb) and 53 green wrinkled (aabb). Thus, Mendel showed that the dihybrid forms 4 varieties of gametes in equal proportions and is heterozygous for both allelic pairs.

Monohybrid crossing is a crossing, which is characterized by the difference between the parent forms from each other in the presence of one pair of alternative, contrasting characteristics. A sign is any feature of an organism, any of its properties or qualities by which it is possible to distinguish individuals. In plants, such a property is, for example, the shape of the corolla (asymmetrical or symmetrical), its color (white or purple), etc. The characteristics also include the speed of maturation (late ripening or early ripening), as well as resistance or susceptibility to certain diseases .

All properties in their entirety, starting from external ones and ending with certain features in the functioning or structure of cells, organs, and tissues, are called a phenotype. This concept can also be used in relation to one of the available alternative characteristics.

The manifestation of properties and characteristics is carried out under the control of existing hereditary factors - in other words, genes. Together, genes form a genotype.

Monohybrid crossing according to Mendel is represented by crossing peas. In this case, there are such quite clearly visible alternative properties as white and green and yellow coloring of unripe beans, wrinkled and smooth surface of seeds, and others.

Carrying out a monohybrid crossing, G. Mendel, an Austrian botanist of the 11th century, found out that in the first generation (F1) all hybrid plants had flowers of a purple hue, but white color did not appear. This is how the first conclusion was drawn about the uniformity of the first generation samples. In addition, the scientist found that in the first generation all samples were homogeneous in all seven characteristics he studied.

Thus, monohybrid crossing assumes for individuals of the first generation the presence of alternative characteristics of only one parent, while the properties of the other parent seem to disappear. G. Mendel called the predominance of properties dominance, and the characteristics themselves - dominant. The scientist called qualities that do not manifest themselves recessive.

Carrying out monohybrid crossing, G. Mendel self-pollinated the grown hybrids of the first generation. The scientist sowed the seeds that had formed in them again. As a result, he received the next, second generation (F2) of hybrids. In the obtained samples, cleavage according to alternative characteristics was noted in an approximate ratio of 3:1. In other words, three quarters of the second generation individuals had dominant properties, and one quarter had recessive properties. As a result of these experiments, G. Mendel concluded that the recessive trait in the samples was suppressed, but did not disappear, appearing in the second generation. This generalization is called the “Law of Segregation” (Mendel’s second law).

The scientist carried out further monohybrid crossing in order to identify how inheritance would occur in the third, fourth and subsequent generations. He grew specimens using self-pollination. As a result of experiments, it was revealed that plants whose characteristics are recessive (white flowers, for example), in subsequent generations reproduce offspring only with these (recessive) properties.

Plants of the second generation behaved somewhat differently, the properties of which were called dominant by G. Mendel (owners, for example, of purple flowers). Among these samples, the scientist, analyzing the offspring, identified two groups with absolute external differences in each specific trait.

For individuals that differ in two characteristics, problems are by definition relatively simple; when solving them, Mendel's laws are applied.

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§ 30. Dihybrid crossing. Law of independent inheritance of characteristics

1. Can two organisms of the same species differ in only one characteristic?

All organisms of the same species usually differ from each other in many ways.

If two individuals differ from each other in two characteristics, then crossing between them is called dihybrid, if three – trihybrid etc. Crossing of individuals that differ in many characteristics is called polyhybrid.

Having established the patterns of inheritance of one trait, G. Mendel studied the nature of splitting when crossing two pure lines of peas, differing in two characteristics: the color of the seeds (yellow or green) and the shape of the seeds (smooth or wrinkled). In such a cross, the traits are determined by different pairs of genes: one allele is responsible for the color of the seeds, the other for the shape. Yellow color of peas ( A) dominates the green (a), and the smooth form ( IN) over wrinkled ( b).

In the first generation (F 1), all individuals, as it should be according to the rule of uniformity of first-generation hybrids, had smooth yellow peas. In order to understand how all possible types of gametes will be combined when crossing two first-generation hybrids, the American geneticist Punnett proposed the so-called Punnett grid, which allows you to visualize all types of gene combinations in gametes and the results of their fusion. Since dihybrid crossing produces 4 types of gametes: AB, Ab, aB And ab, then the number of types of zygotes that can arise from the random fusion of these gametes is 4 × 4, i.e. 16. This is the number of cells in the Punnett lattice (Fig. 58). The figure shows that with this crossing the following 9 types of genotypes arise: AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBB, aaBb And aabb, since there are repetitions in 16 combinations. These 9 genotypes appear as 4 phenotypes: yellow smooth, yellow wrinkled, green smooth and green wrinkled. The numerical ratio of these phenotypic variants is as follows:

9zh:3zhm:3zg:1zm.

If the results obtained by G. Mendel are considered separately for each of the characteristics (color and shape), then for each of them the 3:1 ratio characteristic of monohybrid crossing will be maintained. Hence, G. Mendel concluded that during dihybrid crossing, the genes and traits for which these genes are responsible are combined and inherited independently of each other. This conclusion is called law of independent inheritance of characteristics, which is valid for those cases when the genes for the traits under consideration lie on different chromosomes.

Rice. 58. Pattern of inheritance of characters in dihybrid crossing

Dihybrid crossing. Polyhybrid crossing. Punnett grid. Law of independent inheritance of characteristics

Solving dihybrid crossing problems

1 . Transfer (redraw) the algorithm for solving the genetic problem presented below into your notebook. Analyze the pattern of inheritance of traits during dihybrid crossing presented in the textbook and fill in the gaps in the algorithm.

Algorithm for solving the dihybrid crossing problem

2. Determine the genotypes of the parents, types of gametes and write down the crossing scheme.

3. Let's create a Punnett lattice F 2:.

2. Based on the analysis of the results obtained, answer the questions.

1) How many types of gametes are formed by the parent plant with yellow smooth seeds; with green wrinkled seeds?

2) What is the probability of the appearance of F 1 plants with yellow seeds as a result of the first crossing; with green seeds?

3) What is the probability of the appearance of F 1 plants with yellow smooth seeds as a result of crossing; with yellow wrinkled ones; green smooth; green wrinkled?

4) How many different genotypes can there be among the first generation hybrids?

5) How many different phenotypes can there be among the first generation hybrids?

6) How many types of gametes does the F 1 plant with yellow smooth seeds form?

7) What is the probability of the appearance of F 2 plants with yellow seeds as a result of self-pollination; with green seeds?

8) What is the probability of the appearance of F 2 plants with yellow smooth seeds as a result of crossing; with yellow wrinkled ones; with green smooth ones; with green wrinkled ones?

9) How many different genotypes can there be among the second generation hybrids?

10) How many different phenotypes can there be among second generation hybrids?

3. Solve dihybrid crossing problems.

1. In humans, right-handedness dominates left-handedness, and brown eye color dominates over blue. A right-handed, brown-eyed man, whose mother was a blue-eyed left-hander, and a right-handed, blue-eyed woman, whose father was left-handed, marry. 1) How many different phenotypes can their children have? 2) How many different genotypes can there be among their children? 3) What is the probability that this couple will have a left-handed child (expressed as %)?

2. Black coat color and a floppy ear in dogs dominate over the brown color and erect ear. Purebred black dogs with floppy ears were crossed with dogs with brown coat coloring and erect ears. Hybrids crossed with each other. 1) What proportion of F 2 puppies should be phenotypically similar to the F 1 hybrid? 2) What part of F 2 hybrids should be completely homozygous? 3) What proportion of F 2 puppies should have a genotype similar to that of F 1 hybrids?

3. Black coloring in cats dominates over fawn, and short hair dominates over long hair. Purebred Persian cats (black long-haired) were crossed with Siamese (fawn short-haired). The resulting hybrids were crossed with each other. What is the probability of receiving in F 2: a) a purebred Siamese kitten; b) a kitten phenotypically similar to a Persian; c) long-haired fawn kitten (express in parts)?

Questions

1. Do you think monohybrid crosses often occur in nature?

2. How many types of gametes are formed in first-generation hybrids during dihybrid crossing?

§ 31. Genetics of sex. Sex-linked inheritance

1. Give examples of bisexual animals.

2. Which chromosomes are called sex chromosomes?

The vast majority of animals are represented by individuals of two species - male and female, and gender splitting occurs in a 1:1 ratio. Such splitting in the offspring is observed in cases where heterozygous ( Ahh) and homozygous ( ahh) parental individuals.

G. Mendel suggested that one of the sexes is heterozygous, and the second is homozygous for the gene that determines the sex of the organism.

This assumption was confirmed at the beginning of the 20th century, when T. Morgan and his colleagues were able to establish that males and females differ in the set of chromosomes. All chromosomes, except one pair, are the same in male and female organisms and are called autosomes. But in one pair of chromosomes, males and females are different. These chromosomes are called sexual. For example, female Drosophila in each cell have three pairs of autosomes and one pair of identical sex chromosomes, the so-called X chromosomes, and males have the same three pairs of autosomes and two different sex chromosomes - X And Y(Fig. 59). When gametes are formed during meiosis, females will produce one type of gamete: 3 autosomes + X sex chromosome. In males, two types of gametes will be formed in equal quantities: 3 autosomes + X sex chromosome or 3 autosomes + Y sex chromosome.

Rice. 59. Gender segregation scheme

If, during fertilization, a sperm with an X chromosome merges with an egg, a female will develop from the zygote, and if with a Y chromosome, then a male. Thus, the sex of the future individual is determined during fertilization and depends on which set of sex chromosomes is formed at this moment.

Since female Drosophila are capable of producing only one type of gamete (with X sex chromosome), they are called homogametic sex. Male fruit flies produce two types of gametes (with X or Y sex chromosomes), and they are called heterogametic sex. In many mammals (including humans), worms, crustaceans, many amphibians, fish and most insects, the female sex is also homogametic ( XX), and the male is heterogametic ( XY). In women, each cell has 22 pairs of autosomes and two X sex chromosomes, and in men the same 22 pairs of autosomes, as well as X And Y sex chromosomes. However, in many species, chromosomal sex determination looks different. In birds, reptiles and some fish, males are homogametic ( XX), and females are heterogametic ( XY). In some insects (bees, grasshoppers), females are homogametic ( XX), and males have only one sex chromosome in their chromosome set ( XO).

Inheritance of sex-linked traits. There are many different genes located on the sex chromosomes, and not all of them determine sex-related traits. If the genes responsible for a particular trait are located on autosomes, then inheritance occurs regardless of who is the carrier of the gene - father or mother. After all, the autosomes in males and females are the same. The location of a gene on the sex chromosome is called gene-sex linkage, and the gene itself is sex-linked.

For example, in humans, a dominant gene is located on the X chromosome ( N), which determines normal blood clotting. A recessive variant of this gene ( h) leads to decreased blood clotting, called hemophilia. The Y chromosome does not have an allelic pair for this gene, and the trait (non-clotting of blood) appears in men even though the gene h recessive.

In the same way, color blindness is inherited - the inability to distinguish between red and green colors.

Autosomes. Sex chromosomes. Homogametic and heterogametic sex. Gene-sex linkage

Do practical work.

Solving problems on the inheritance of sex-linked traits

1 . Copy the algorithm for solving the genetic problem presented below into your notebook. Read the conditions of the problem on the inheritance of sex-linked traits and fill in the gaps in the algorithm.

In humans, the gene causing hemophilia is recessive and is located on the X chromosome, while albinism is caused by an autosomal recessive gene. Parents normal according to these characteristics gave birth to an albino and hemophilic son. What is the likelihood that their next son will exhibit these two abnormal features? What is the probability of having healthy daughters?

Algorithm for solving the problem of inheritance of sex-linked traits

1. Write down the object of study and the designation of genes.

2. According to the conditions of the problem, the son’s genotype is aaX b Y. Therefore, according to the first characteristic, the parents must be heterozygous, since the son receives his X chromosome from the mother, which means that she is heterozygous for the second characteristic. Let's write down the crossing (marriage) scheme and create a Punnett lattice.

3. Write down the answer. The probability of displaying signs of an albino and hemophiliac (genotype ________) in the next son ______; probability of having healthy daughters (genotype) _________.

2. Solve problems on the inheritance of sex-linked traits.

1. In humans, the absence of sweat glands depends on a recessive sex-linked gene. In a family, father and son have this anomaly, but the mother is healthy. 1) What is the probability that the son will inherit the above trait from his father? 2) What is the probability of having a daughter with no sweat glands in this family (in%)?

2. In cats, the red and black genes are allelic and localized on the X chromosome. They are transmitted independently, and therefore heterozygotes have a variegated (tricolor) coloration. 1) How many different phenotypes can be obtained by crossing a three-haired cat with a black cat? 2) What is the probability of a calico cat appearing (in%)?

Questions

1. How many sex chromosomes are there in human somatic cells; cats; camel?

2. How many sex chromosomes are contained in the egg; in the sperm?

3. What determines the sex of a human child: the chromosomes of the egg or the chromosomes of the sperm?

4. How many autosomes does Drosophila have? in a person?

5. What signs are called sex-linked?

§ 32. Patterns of variability: modification variability. Reaction rate

1. What is called heredity?

2. What is variability?

One of the properties of living organisms is their variability. If you obtain several currant bushes from one bush by vegetative means (for example, by propagating stems by cuttings), then, naturally, the genotype of all new bushes will be the same. However, the phenotypes of these bushes can differ greatly from each other - in size and number of branches, leaves, yield, etc. These differences in a number of traits in plants with the same genotype are due to the fact that the manifestation of the action of individual genes and the entire genotype of the organism depends from environmental conditions. For example, the illumination for one of these bushes turned out to be greater than for the others. Either the soil under one of them is better fertilized, or one of the bushes receives more moisture, etc. Such changes in the body that do not affect its genes and therefore are not passed on from generation to generation are called modifications, and this variability is modification.

Rice. 60. Modification variability

Most often, quantitative traits are subject to modification: height, weight, fertility, etc. (Fig. 60).

Various traits and properties of organisms are characterized by greater or lesser dependence on environmental conditions. For example, in humans, the color of the iris and blood type are determined only by genes, and living conditions cannot influence these signs in any way. But height, weight, physical strength and endurance strongly depend on external conditions, for example, on the quantity and quality of nutrition, physical activity, etc. The limits of modification variability of any trait are called reaction norm. The variability of a trait is sometimes very large, but it can never go beyond the limits of the reaction norm. For example, a person can run 100 m in 11 s, 10 s, 9 s, but he can never run in 5 s. For some traits, the reaction norm is very wide (for example, the shearing of wool from sheep, the weight of bulls, the milk production of cows), while other traits are characterized by a narrow reaction norm (for example, the color of the fur of rabbits).

A very important conclusion follows from what has been said. It is not the trait itself that is inherited, but the ability to manifest this trait under certain conditions, or we can say that the norm of the body’s reaction to external conditions is inherited.

So, we can list the following main characteristics of modification variability.

1. Modification changes are not passed on from generation to generation.

2. Modification changes occur in many individuals of the species and depend on the effect of environmental conditions on them.

3. Modification changes are possible only within the limits of the reaction norm, i.e., ultimately they are determined by the genotype.

Variability. Modifications. Modification variability. Reaction rate

Do the lab work.

Revealing the variability of organisms

Goal of the work: identify manifestations of modification variability in organisms.

Progress

1. Consider the objects offered to you.

2. Examine the appearance (phenotype) of each object (notice differences in size, shape, color, etc.).

3. Enter the results into the table.

4. Draw a conclusion.

Questions

1. What types of variability are inherent in organisms?

2. What variability is called modification? Give examples.

3. Which signs of the body are subject to modification and which are not?

4. What is the reaction norm?

5. What are the main reasons for the modification variability of organisms?

6. What are the main characteristics of modification variability?

Tasks

Think about which organism is better adapted to the conditions of existence - with a wide or narrow norm of reaction. Support your assumption with specific examples.

§ 33. Patterns of variability: mutational variability

1. Are modifications inherited?

2. What are genotype and phenotype?

Mutational variability. So, modification changes are not inherited. The main reason for the emergence of new characteristics and properties in living organisms is the manifestation of mutations - mutational variability. Mutations- these are changes in the genotype that occur under the influence of external or internal environmental factors (Fig. 61).

Mutations. The term “mutation” was first proposed in 1901 by the Dutch scientist Hugo de Vries, who described spontaneous mutations in plants. Mutations occur rarely, but lead to sudden abrupt changes in traits that are passed on from generation to generation.

Mutations can affect the genotype to varying degrees, and therefore they are divided into gene, chromosomal and genomic.

Genetic, or point mutations occur most frequently. They occur when one or more nucleotides within one gene are replaced by others. As a result, changes occur in the activity of the gene, a protein is synthesized with a changed sequence of amino acids and, therefore, with changed properties, and as a result, some sign of the organism will be changed or lost. For example, thanks to gene mutations, bacteria can become resistant to antibiotics or other drugs, change their body shape, color of colonies, etc.

Rice. 61. Mutations in the Drosophila fruit fly

Chromosomal mutations are significant changes in chromosome structure that affect several genes. For example, there may be a so-called loss when the end of a chromosome breaks off and some genes are lost. Such a chromosomal mutation in the 21st chromosome in humans leads to the development of acute leukemia - leukemia, leading to death. Sometimes the middle part of a chromosome is “cut off” and destroyed. This chromosomal mutation is called deletion. The consequences of a deletion can be different: from death or a severe hereditary disease (if that part of the chromosome that contained important genes is lost) to the absence of any disorders (if that part of the DNA is lost in which there are no genes that determine the properties of the organism).

Another type of chromosomal mutation is the doubling of some part of it. In this case, some genes will occur several times in the chromosome. For example, an eightfold repeating gene was found in one of the chromosomes of Drosophila. This type of mutation is duplication– less dangerous to the body than loss or deletion.

At inversions the chromosome breaks in two places, and the resulting fragment, turning 180°, is reinserted into the place of the break. For example, a section of a chromosome contains the genes A-B-C-D-E-F. There were gaps between B and C, D and E, and the IOP fragment turned over and became embedded in the gap. As a result, the chromosome region will have the structure A-B-D-D-C-E-F. Finally, it is possible to transfer a section of one chromosome to another, non-homologous.

At genomic mutation the number of chromosomes changes. Most often, such mutations occur if, during the formation of gametes in meiosis, the chromosomes of any pair diverge and both end up in one gamete, while in the other gamete one chromosome is missing. Both the presence of an extra chromosome and its absence most often lead to unfavorable changes in the phenotype. For example, when chromosomes do not disjunct, women may produce eggs containing two 21st chromosomes. If such an egg is fertilized, a child will be born with Down syndrome. These children have a very characteristic appearance, pathology of internal organs, and severe mental disorders. Unfortunately, children with Down syndrome are born quite often.

A special case of genomic mutations is polyploidy, i.e., a multiple increase in the number of chromosomes in cells as a result of a violation of their divergence in mitosis or meiosis. Somatic cells of such organisms contain 3 n, 4n, 8n etc. chromosomes, depending on how many chromosomes were in the gametes that formed this organism. Polyploidy is common in bacteria and plants, but very rare in animals. Many types of cultivated plants - polyploids(Fig. 62). Thus, three quarters of all cereals cultivated by humans are polyploid. If the haploid set of chromosomes ( n) for wheat is 7, then the main variety bred in our conditions - soft wheat - has 42 chromosomes, i.e. 6 n. Polyploids are cultivated beets, buckwheat, etc. As a rule, polyploid plants have increased viability, size, fertility, etc. Currently, special methods for obtaining polyploids have been developed. For example, a plant poison from autumn crocus - colchicine - is capable of destroying the spindle during the formation of gametes, resulting in gametes containing 2 n chromosomes. When such gametes fuse, the zygote will contain 4 n chromosomes.

The overwhelming number of mutations are unfavorable or even fatal for the body, since they destroy the integral genotype regulated over millions of years of natural selection.

Causes of mutations. All living organisms have the ability to mutate. Each specific mutation has some reason, although in most cases we do not know it. However, the total number of mutations can be dramatically increased by using different methods of influencing the body. Factors causing mutations are called mutagenic.

Rice. 62. Manifestation of polyploidy in grapes

Firstly, ionizing radiation has the strongest mutagenic effect. Radiation increases the number of mutations hundreds of times.

Secondly, mutations cause mutagenic substances, which act, for example, on DNA, breaking the chain of nucleotides. There are substances that act on other molecules, but also give mutations. Already mentioned above colchicine, leading to one of the types of mutations - polyploidy.

Thirdly, various physical influences, such as increased ambient temperature, also lead to mutations.

From what has been said, it becomes clear how important it is that in life we ​​are surrounded by as few factors as possible that cause mutations. And it is absolutely unreasonable to destroy your future children by using strong mutagens. For example, drug addicts, for a short-term loss of a sense of reality, take substances that cause irreparable damage to many cells of the body, including those primary germ cells from which eggs or sperm should then develop.

Thus, mutational variability has the following main characteristics.

1. Mutational changes occur suddenly, and as a result, the body acquires new properties.

2. Mutations are inherited and passed on from generation to generation.

3. Mutations are not directional, that is, it is impossible to reliably predict which gene is mutating under the influence of a mutagenic factor.

4. Mutations can be beneficial or harmful to the body, dominant or recessive.

Gene, chromosomal and genomic mutations. Loss. Deletion. Duplication. Inversion. Down syndrome. Polyploidy. Colchicine. Mutagenic substances

Questions

1. What are the main differences between modifications and mutations?

2. What types of mutations do you know?

3. How can you artificially increase the number of mutations?

4. Which mutations are more common - beneficial or harmful?

Tasks

Think about what the practical significance of studying the causes of mutations might be. Discuss this problem in class.