"chromosome" are words that are familiar to every schoolchild. But the idea of this issue is quite general, since delving into the biochemical jungle requires special knowledge and the desire to understand all this. And even if it is present at the level of curiosity, it quickly disappears under the weight of the presentation of the material. Let's try to understand the intricacies in a scientific-polar form.
A gene is the smallest structural and functional piece of information about heredity in living organisms. Essentially, it is a small piece of DNA that contains knowledge about a certain sequence of amino acids for building a protein or functional RNA (with which the protein will also be synthesized). The gene determines those characteristics that will be inherited and transmitted by descendants further along the genealogical chain. In some single-celled organisms, there is gene transfer that is not related to the reproduction of their own kind; it is called horizontal.
Genes bear a huge responsibility for how each cell and the organism as a whole will look and work. They control our lives from the moment of conception to the very last breath.
The first scientific step forward in the study of heredity was made by the Austrian monk Gregor Mendel, who in 1866 published his observations on the results of crossing peas. The hereditary material he used clearly showed patterns of transmission of traits such as the color and shape of peas, as well as flowers. This monk formulated the laws that formed the beginning of genetics as a science. Inheritance of genes occurs because parents give their child half of all their chromosomes. Thus, the characteristics of mom and dad, mixing, form a new combination of existing characteristics. Fortunately, there are more options than there are living creatures on the planet, and it is impossible to find two absolutely identical creatures.
Mendel showed that hereditary inclinations do not mix, but are transmitted from parents to descendants in the form of discrete (separate) units. These units, presented in pairs (alleles) in individuals, remain discrete and are transmitted to subsequent generations in male and female gametes, each of which contains one unit from each pair. In 1909, the Danish botanist Johansen called these units genes. In 1912, a geneticist from the United States of America, Morgan, showed that they are located in chromosomes.
More than a century and a half has passed since then, and research has advanced further than Mendel could have imagined. At the moment, scientists have settled on the opinion that the information found in genes determines the growth, development and functions of living organisms. Or maybe even their death.
Classification
The structure of the gene contains not only information about the protein, but also instructions on when and how to read it, as well as empty sections necessary to separate information about different proteins and stop the synthesis of the information molecule.
There are two forms of genes:
- Structural - they contain information about the structure of proteins or RNA chains. The sequence of nucleotides corresponds to the arrangement of amino acids.
- Functional genes are responsible for the correct structure of all other sections of DNA, for the synchronicity and sequence of its reading.
Today, scientists can answer the question: how many genes are on a chromosome? The answer will surprise you: about three billion pairs. And this is only in one out of twenty-three. The genome is the smallest structural unit, but it can change a person's life.
Mutations
A random or targeted change in the sequence of nucleotides included in a DNA chain is called a mutation. It may have virtually no effect on the structure of the protein, or it may completely distort its properties. This means there will be local or global consequences of such a change.
Mutations themselves can be pathogenic, that is, manifest themselves in the form of diseases, or lethal, preventing the body from developing to a viable state. But most of the changes go unnoticed by humans. Deletions and duplications occur constantly within DNA, but do not affect the course of life of any individual.
A deletion is the loss of a section of a chromosome that contains certain information. Sometimes such changes are beneficial for the body. They help him protect himself from external aggression, such as the human immunodeficiency virus and the plague bacteria.
Duplication is the doubling of a section of a chromosome, which means that the set of genes it contains also doubles. Due to the repetition of information, it is less susceptible to selection, which means it can quickly accumulate mutations and change the body.
Gene properties
Each person has a huge Genes - these are functional units in its structure. But even such small areas have their own unique properties that make it possible to maintain the stability of organic life:
- Discreteness is the ability of genes not to mix.
- Stability - preservation of structure and properties.
- Lability is the ability to change under the influence of circumstances, to adapt to hostile conditions.
- Multiple allelism is the existence within DNA of genes that, while encoding the same protein, have different structures.
- Allelicity is the presence of two forms of one gene.
- Specificity - one trait = one gene, inherited.
- Pleiotropy is the multiplicity of effects of one gene.
- Expressiveness is the degree of expression of a trait that is encoded by a given gene.
- Penetrance is the frequency of occurrence of a gene in a genotype.
- Amplification is the appearance of a significant number of copies of a gene in DNA.
Genome
The human genome is all the hereditary material that is found in a single human cell. It contains instructions about the construction of the body, the functioning of organs, and physiological changes. The second definition of this term reflects the structure of the concept, not the function. The human genome is a collection of genetic material packaged in a haploid set of chromosomes (23 pairs) and belonging to a specific species.
The basis of the genome is a molecule well known as DNA. All genomes contain at least two types of information: encoded information about the structure of messenger molecules (called RNA) and protein (this information is contained in genes), as well as instructions that determine when and where this information is manifested during the development of the organism. The genes themselves occupy a small part of the genome, but at the same time they are its basis. The information recorded in genes is a kind of instruction for the production of proteins, the main building blocks of our body.
However, to fully characterize the genome, the information contained in it about the structure of proteins is insufficient. We also need data on the elements that take part in the work of genes and regulate their expression at different stages of development and in different life situations.
But even this is not enough to completely determine the genome. After all, it also contains elements that contribute to its self-reproduction (replication), compact packaging of DNA in the nucleus, and some still unclear areas, sometimes called “selfish” (that is, supposedly serving only for themselves). For all these reasons, at the moment, when we talk about the genome, we usually mean the entire set of DNA sequences present in the chromosomes of the cell nuclei of a certain type of organism, including, of course, genes.
Genome size and structure
It is logical to assume that the gene, genome, chromosome differ in different representatives of life on Earth. They can be either infinitely small or huge and contain billions of pairs of genes. The structure of the gene will also depend on whose genome you are studying.
Based on the relationship between the size of the genome and the number of genes included in it, two classes can be distinguished:
- Compact genomes with no more than ten million bases. Their set of genes strictly correlates with size. Most characteristic of viruses and prokaryotes.
- Large genomes consist of more than 100 million base pairs, with no relationship between their length and the number of genes. More common in eukaryotes. Most nucleotide sequences in this class do not code for proteins or RNA.
Research has shown that the human genome contains about 28 thousand genes. They are unevenly distributed along the chromosomes, but the meaning of this feature still remains a mystery to scientists.
Chromosomes
Chromosomes are a way of packaging genetic material. They are found in the nucleus of every eukaryotic cell and consist of one very long DNA molecule. They can easily be seen in a light microscope during the division process. A karyotype is a complete set of chromosomes that is specific to each individual species. Mandatory elements for them are a centromere, telomeres and replication points.
Chromosome changes during cell division
A chromosome is a series of links in a chain of information transmission, where each next one includes the previous one. But they also undergo certain changes during the life of the cell. For example, in interphase (the period between divisions), chromosomes in the nucleus are arranged loosely and take up a lot of space.
As a cell prepares for mitosis (the process of dividing into two), the chromatin compacts and coils into chromosomes so that it can be seen under a light microscope. In metaphase, the chromosomes resemble rods, closely spaced and connected by a primary constriction, or centromere. It is she who is responsible for the formation of the spindle, when groups of chromosomes line up. Depending on the location of the centromere, there is the following classification of chromosomes:
- Acrocentric - in this case, the centromere is located polar to the center of the chromosome.
- Submetacentric, when the arms (that is, the areas located before and after the centromere) are of unequal length.
- Metacentric if the centromere divides the chromosome exactly in the middle.
This classification of chromosomes was proposed in 1912 and is used by biologists to this day.
Chromosome abnormalities
As with other morphological elements of a living organism, structural changes can also occur with chromosomes that affect their functions:
- Aneuploidy. This is a change in the total number of chromosomes in a karyotype due to the addition or removal of one of them. The consequences of such a mutation can be lethal to the unborn fetus and can also lead to birth defects.
- Polyploidy. Manifests itself in the form of an increase in the number of chromosomes, a multiple of half their number. Most often found in plants, such as algae, and fungi.
- Chromosomal aberrations, or rearrangements, are changes in the structure of chromosomes under the influence of environmental factors.
Genetics
Genetics is a science that studies the patterns of heredity and variability, as well as the biological mechanisms that provide them. Unlike many other biological sciences, from its inception it has strived to be an exact science. The entire history of genetics is the history of the creation and use of more and more accurate methods and approaches. The ideas and methods of genetics play an important role in medicine, agriculture, genetic engineering, and the microbiological industry.
Heredity is the ability of an organism to provide a number of morphological, biochemical and physiological characteristics and characteristics. In the process of inheritance, the main species-specific, group (ethnic, population) and family features of the structure and functioning of organisms, their ontogenesis (individual development) are reproduced. Not only certain structural and functional characteristics of the body are inherited (facial features, some features of metabolic processes, temperament, etc.), but also the physicochemical features of the structure and functioning of the main biopolymers of the cell. Variability is the variety of characteristics among representatives of a certain species, as well as the ability of descendants to acquire differences from their parent forms. Variability, together with heredity, are two inseparable properties of living organisms.
Down syndrome
Down syndrome is a genetic disorder in which a person's karyotype consists of 47 chromosomes instead of the usual 46. This is one of the forms of aneuploidy discussed above. In the twenty-first pair of chromosomes, an additional one appears, which introduces extra genetic information into the human genome.
The syndrome received its name in honor of the doctor, Don Down, who discovered and described it in literature as a form of mental disorder in 1866. But the genetic basis was discovered almost a hundred years later.
Epidemiology
At the moment, a karyotype of 47 chromosomes in humans occurs once per thousand newborns (previously the statistics were different). This became possible thanks to the early diagnosis of this pathology. The disease does not depend on the mother's race, ethnicity or social status. Age has an effect. The chances of having a child with Down syndrome increase after thirty-five years of age, and after forty the ratio of healthy children to sick children is already 20 to 1. The father's age over forty also increases the chances of having a child with aneuploidy.
Forms of Down syndrome
The most common option is the appearance of an additional chromosome in the twenty-first pair along a non-hereditary path. It is due to the fact that during meiosis this pair does not separate along the spindle. Five percent of patients have mosaicism (an additional chromosome is not found in all cells of the body). Together they make up ninety-five percent of the total number of people with this congenital pathology. In the remaining five percent of cases, the syndrome is caused by hereditary trisomy of the twenty-first chromosome. However, the birth of two children with this disease in one family is insignificant.
Clinic
A person with Down syndrome can be recognized by characteristic external signs, here are some of them:
Flattened face;
- shortened skull (the transverse size is larger than the longitudinal one);
- skin fold on the neck;
- a fold of skin that covers the inner corner of the eye;
- excessive joint mobility;
- decreased muscle tone;
- flattening of the back of the head;
- short limbs and fingers;
- development of cataracts in children over eight years of age;
- anomalies in the development of teeth and hard palate;
- congenital heart defects;
- possible presence of epileptic syndrome;
- leukemia.
But it is, of course, impossible to make an unambiguous diagnosis based only on external manifestations. Karyotyping is necessary.
Conclusion
Gene, genome, chromosome - it seems that these are just words, the meaning of which we understand in a generalized and very distant way. But in fact, they greatly influence our lives and, by changing, force us to change. A person knows how to adapt to circumstances, no matter what they turn out to be, and even for people with genetic abnormalities there will always be a time and place where they will be irreplaceable.
Allele. Any chromosomal locus may have a different structure in different cases, and therefore different genes are located in it, which are called allelic. If the number of such alleles is more than two, then they form a system of multiple alleles. Each chromosome contains only one of the alleles, but any individual can contain several (usually two) alleles because it contains two or more homologous chromosomes, each of which carries a given locus.
Antimutagen- a substance that prevents or counteracts the mutagenic effects of other substances.
Autosome- a regular, non-sex chromosome.
Recovery- reunification of the two broken ends of the fragmented chromosome into the original structure.
Gene expression- the external effect of a gene, which can vary depending on various external influences or different gene environments, and sometimes is completely absent.
Gamete- sex cell.
Haploid- an organism whose cells contain only one genome. So-called polyhaploids (individuals containing half the number of chromosomes and developing from polyploid species) contain several genomes. The term haploid is also used to emphasize that gametes (in diploid species) contain only one chromosome of each type. Thus, gametes are haploid, and somatic cells are diploid.
Haplophase- a stage of development in different organisms at which cell nuclei contain the same number of chromosomes as gametes. This number is half that found in the diplophase. As a rule, each type of chromosome is represented in one copy during haplophase.
Gene- a small section of a chromosome that has a specific biochemical function and has a specific effect on the properties of an individual (see also Allele).
Genotype- the sum of all genes of an organism; hereditary constitution.
Heterogametic- sex that produces two types of gametes that influence sex determination (for example, containing an X or Y chromosome). The sex that forms only one type of gamete (for example, with the X chromosome) is called homogametic.
Heterozygote- an individual that produces several types of genetically different germ cells; this is due to the fact that the corresponding loci of its homologous chromosomes contain different alleles.
Heterosis- increase in the size and power of hybrids compared to parental forms.
Hybrid- an individual obtained as a result of crossing between genetically different parental types. In the broad sense of the word, each heterozygote is a hybrid.
Hybrid power- (see Heterosis).
Homozygosity- the case when a given chromosome or chromosomal region is represented in the singular; there is no homo- or heterozygosity. For example, in Drosophila, males are homozygous on the X chromosome for those loci that are absent on the Y chromosome.
Homologous chromosomes. Diploid organisms usually have two chromosomes of each type (polyploid organisms have more than two). These chromosomes are considered homologous even if they differ in several genes. If there are structural differences between the chromosomes, then they are considered partially homologous. Homology, in addition to external similarity, is also manifested in a good ability to conjugate in meiosis.
Clutch group- the totality of all genes localized on one chromosome.
Deletion- loss of one of the internal (non-terminal) sections of the chromosome.
Dihybrid- an individual heterozygous for two pairs of alleles.
Diploid- an organism with two homologous sets of chromosomes in somatic cells. This term is also used to designate an individual with double the number of chromosomes (2 n), formed as a result of fertilization, in contrast to the haploid set of chromosomes ( n), contained in gametes.
Additional chromosome- a supernumerary chromosome that is not homologous to any of the normal chromosomes and the absence of which does not reduce the viability of the individual.
Domination- a phenomenon in which one of the alleles of a heterozygote (dominant allele) has a stronger effect on the corresponding trait of an individual than the other allele (recessive). Dominance can be absolute if the heterozygote Aa for this trait does not differ from a homozygote A.A.. Dominance will be incomplete if, according to the expression of this trait, Aa closer to A.A. than to aa.
Duplication- a structural change in a chromosome in which one of the regions is represented more than once in the chromosome set.
Zygospore- spore; in algae and fungi it corresponds to the zygote.
Zygote- a cell formed by the fusion of two gametes.
Isogenic- having the same genotype.
Inbreeding- self-pollination or crossing between related individuals in those organisms that normally undergo cross-fertilization.
Intersex- an individual occupying an intermediate position between a female and a male.
Carcinogenic- causing malignant growth.
Karyotype- a set of features of the chromosomal complex relating to the number and shape of chromosomes.
Cell line- successive generations of cells developing from a specific original organ and genotypically adapted to life in tissue culture. Cell lines usually become malignant, acquiring the properties of malignant cells.
Clone- the totality of all descendants obtained from one original individual through vegetative propagation or apomictic seed formation.
Complementary genes- two dominant genes that individually have no effect, but together cause the development of a certain trait.
Crossing over- exchange between homologous regions of homologous chromosomes.
Lethal gene- a gene, the presence of which (especially in the homozygous state) leads to the death of the organism.
Lysis- dissolution of a bacterial cell after penetration of a phage particle into it and the formation of new phages in it.
Locus- the place on the chromosome in which the gene is located (see also Allele and Gene).
Mass selection- a method of plant breeding that involves selecting plants with desired properties from a general population and then making crosses between the selected plants, sometimes in the presence of the original population.
Maternal inheritance- transmission of a trait exclusively through the female line, caused by cytoplasmic factors.
Meiosis- reduction division; the process of nuclear division leading to the formation of a haploid phase in which the number of chromosomes is reduced by half compared to the diploid phase. During meiosis, the nucleus divides twice, but the chromosomes divide only once. Meiosis is a necessary prerequisite for a very important mechanism of genetic recombination.
Mendelian segregation- (see Cleavage).
Mendelism- a field of research focused on the study of gene effects and segregation rates.
Mitosis- nuclear fission leading to the formation of two daughter nuclei. During this process, each chromosome is duplicated. Chromosome duplication occurs before nuclear division, and already in prophase one can see that each chromosome is double and consists of two chromatids. In anaphase, these chromosomes move to different poles.
Monohybrid- an organism heterozygous for one pair of alleles.
Mutagen- factor causing mutation.
Mutagenicity- ability to cause mutations.
Mutant- an organism that differs from the original type by an individual deviation resulting from mutation.
Mutation- a hereditary change not caused by gene recombination. In a strict sense, a mutation implies a chemical change in a gene or a minor structural change in a chromosome.
Nondisjunction- the case when two homologous chromosomes or chromatids move away during anaphase to the same pole.
Unstable gene- a gene with a high mutation rate.
a lack of- loss of a chromosomal region, especially often occurring at one end of the chromosome (see Deletion).
Back mutation- a mutation, as a result of which the mutant allele again turns into the original allele. In such cases, the recessive allele usually mutates into a dominant wild-type allele.
Backcrossing- a cross between a hybrid and one of the parent forms.
Parthenogenesis- development of an embryo from an unfertilized egg.
Penetrance- frequency of gene manifestation. Penetrance is incomplete if certain individuals carrying a dominant gene or homozygous for one of the recessive genes do not exhibit the characteristics that the gene usually causes; this may be due to genotype or environmental influences.
Cross inheritance- sons inherit the properties of their mother, and daughters inherit the properties of their father. This type of inheritance is determined by the genes located on the sex chromosome, which one sex has in the singular and the other in the double. In these cases, sons receive their only chromosome of this kind from their mother, and daughters receive one of their two such chromosomes from father.
Pleiotropy- the ability of a gene to simultaneously influence several traits of an organism.
Polyhaploid- (see Haploid).
Polyploidy- the presence within a species of forms with different numbers of chromosomes, multiples of one basic number.
Sex chromosome- a chromosome that determines sex and is usually represented differently in the two sexes.
Population- a collection of a certain number of individuals of a given species belonging to different biotypes.
Gene manifestation- (see Penetrance).
Race- a group of biotypes that have certain common properties or a certain average genetic constitution.
Split- the emergence of clearly distinguishable categories of individuals with specific characteristics in the offspring of heterozygotes. Segregation, which is characterized by certain offspring ratios, is ultimately due to the fact that alleles belonging to the same pair are separated from one another during meiosis.
Reduction division- (see Meiosis).
Reduced gamete- a sex cell with half the number (i.e., half the somatic number) of chromosomes.
Recombination- rearrangement of genes during the formation of gametes in a hybrid, leading to new combinations of characteristics in the offspring.
Recessive- (see Dominance).
Reciprocal crosses- a cross between two parental types A and B, in one of which A serves as the maternal form, and in the other as the paternal form (QAXtfB or?BXcM).
Somatic- relating to body cells (not gametes).
Spore- a cell that serves for reproduction and is not a reproductive cell. Spores in flowering plants, ferns, mosses, etc. are a product of meiosis and contain half the number of chromosomes. When spores germinate, they begin haplophase.
Sterility- reduction or inhibition of the ability to sexually produce offspring.
Clutch- a connection between genes that excludes the possibility of their independent inheritance. Linkage is caused by the localization of genes on the same chromosome.
Point mutation- a mutation affecting a minimal region of the chromosome.
Transduction- transfer of parts of the bacterial chromosome by bacteriophages infecting a bacterial cell to other bacteria, which as a result are genetically changed.
Translocation- the transition of any part of a chromosome to a new position in the same chromosome or, more often, in another non-homologous chromosome. Translocations are almost always reciprocal, that is, different regions change places with one another.
Transformation- a genotypic change in a bacterial strain due to the absorption of nucleic acid (DNA) from bacteria of another strain.
F 1 - the first generation obtained from crossing two parental types. The next generations represent P 2 , P 3, etc.
Phenotype- the sum of the properties of an individual at a certain stage of development. The phenotype is the result of the interaction between the genotype and the environment.
Chromosome- a body located in the cell nucleus and, with appropriate coloring, visible in mitosis and meiosis. The chromosome has a definite shape and is differentiated by its length in relation to its chemical structure.
Cytoplasm- the contents of a living cell, with the exception of the nucleus. Often, only the seemingly homogeneous ground substance is called cytoplasm.
Purebred breeding- a method of breeding domestic animals with the goal of preserving the characteristics and valuable properties of a particular race.
Clean line- the sum of all individuals that constantly self-pollinate and all descend from one homozygous individual.
Position effect- a change in the action of a gene that, as a result of chromosomal rearrangement, has changed its position in the chromosome.
X chromosome- a chromosome that determines the development of the female sex in species with male heterogamety or determines sex in haplophase.
Y chromosome- a sex chromosome that is found only in males, if the male is heterogametic. The Y chromosome also determines sex in haplophase.
Lesson objectives: repetition of basic concepts and laws of genetics.
Equipment: tables on the basic laws of genetics, cards.
Lesson Plan
1. Educational game “Race for the Leader”.
2. Problem solving.
3. Work using cards.
4. Grading.
5. Homework assignment.
Teacher. Today we have a lesson-seminar on the topic “Fundamentals of Genetics”. We will repeat the basic laws of genetics. First, we will conduct an educational game in which we will remember the basic genetic concepts, and then a workshop on problem solving. At the end of the lesson we will work with cards.
1. Educational game “Race for the Leader”
The teacher calls those who want to participate in the game (4–5 people).
Part 1. The teacher reads out correct and incorrect definitions of genetic concepts. Students must agree or disagree with it by answering “yes” or “no.” The jury evaluates the correctness of the answers.
1. A gene is a specific section of DNA molecules responsible for the synthesis of one protein molecule. ( Yes.)
2. Genotype is a set of properties and characteristics of an organism. ( No. This is the totality of genes of an organism.)
3. Phenotype is the totality of all genes of a given organism. ( No. This is the totality of all the properties and characteristics of an organism.)
4. Gamete is a reproductive cell that carries one gene from an allelic pair. ( Yes.)
5. A zygote is a diploid cell formed by the fusion of two gametes. ( Yes.)
6. Homozygote - a zygote that has the same alleles of a given gene. ( Yes.)
7. Heterozygote - a zygote that has two different alleles of a given gene. ( Yes.)
8. Allelic genes – genes located in identical regions of homologous chromosomes. ( Yes.)
9. Homologous chromosomes are paired chromosomes that are identical in size, shape, and gene structure, but have different origins. ( Yes.)
10. Sex chromosomes are chromosomes that distinguish the male sex from the female sex. ( Yes.)
11. Autosomes are non-sex chromosomes, different in males and females. ( No. These are non-sex chromosomes that are the same in males and females..)
12. Karyotype is the totality of all genes of a given organism. ( No. This is the totality of all the chromosomes of a given organism.)
13. Chromosome locus - the region of the chromosome where the gene is located. ( Yes.)
Part 2. The teacher asks to answer questions and give definitions of genetic concepts. The jury evaluates the correctness of the answers. In case of incorrect answers, the teacher turns to the class, who helps correct the wording.
1. What is chained inheritance? ( Simultaneous inheritance of genes localized on the same chromosome.)
2. Define the concept of a linkage group. ( A linkage group is formed by genes located in one region (locus) of the chromosome.)
3. What are alternative signs? ( These are mutually exclusive, contrasting characteristics.)
4. Which trait is called dominant? ( The predominant trait that appears in the first generation of offspring of heterozygous individuals.)
5. What is a recessive trait? ( This is a trait that is inherited without appearing in the first generation of heterozygous descendants.)
6. What is a monohybrid cross? ( Monohybrid crossing is the crossing of forms that differ from each other in one characteristic, i.e. having one pair of alternative characteristics.)
7. How does a dihybrid cross differ from a monohybrid cross? ( Dihybrid crossing is the crossing of forms that differ from each other in pairs of alternative characters.)
8. What cross is called polyhybrid? ( This is the crossing of forms that differ from each other in many ways..)
9. What is backcrossing? ( Backcrossing is the crossing of a new genotype of the F1 descendant with one of the parental forms.)
10. What is analytical crossing and why is it carried out? ( Test crosses are performed to determine the resulting unknown genotype of the F1 offspring. To do this, it is crossed with a recessive parent form.)
11. What is a hybrid? ( This is an organism obtained by crossing genetically dissimilar parental forms..)
2. Problem solving
One student solves the problem at the board, the rest - in notebooks.
1. Phenylketonuria (a disorder of amino acid metabolism leading to brain disorders) is inherited as a recessive trait ( R). A healthy woman marries a sick man.
1) What kind of children can they have on this basis?
2) What can children be like in a family where parents are heterozygous for this trait?
2. Gray rabbits were crossed with white ones. In a generation F1 Only gray rabbits appeared. When crossing them with each other, 198 gray and 72 white rabbits were obtained. How many heterozygotes are there among the resulting offspring?4. Grading
The teacher comments on the work of the class.
5. Homework
The cells of each organism contain a certain number of chromosomes. There are a lot of genes in them. Humans have 23 pairs (46) of chromosomes, about 100,000 genes. Genes are located on chromosomes. Many genes are localized on one chromosome. A chromosome with all the genes it contains forms a linkage group. The number of linkage groups is equal to the haploid set of chromosomes. Humans have 23 linkage groups. Genes located on the same chromosome are not absolutely linked. During meiosis, during chromosome conjugation, homologous chromosomes exchange parts. This phenomenon is called crossing over, which can occur in any part of the chromosome. The farther the loci are located from each other on the same chromosome, the more often an exchange of sections can occur between them (Fig. 76). In the Drosophila fly, the genes for wing length (V - long and v - short) and body color (B - gray and b - black) are in one pair of homologous chromosomes, i.e. belong to the same clutch group. If you cross a fly with a gray body color and long wings with a black fly with short wings, then in the first generation all flies will have a gray body color and long wings (Fig. 77). As a result of crossing a diheterozygous male with a homozygous recessive female fly will look like their parents. This happens because genes located on the same chromosome are inherited linked. The male Drosophila fly has complete cohesion. If you cross a diheterozygous female with a homozygous recessive male, then some of the flies will look like their parents, but Rice. 76. Crossing over. 1 - two homologous chromosomes; 2 — their crossover during conjugation; 3 - two new combinations of chromosomes. On the other part, a recombination of characteristics will occur. Such inheritance occurs for genes of the same linkage group, between which crossing over can occur. This is an example of incomplete gene linkage. Basic provisions of the chromosomal theory of heredity. Genes are located on chromosomes. Genes on a chromosome are arranged linearly. Rice. 77. Linked inheritance of genes for body color and the state of wings in a fruit fly. The gene for gray color (B) dominates the gene for black body color (b), the gene for long wings (V) dominates over the gene for short wings (v). B and V are on the same chromosome. a - complete linkage of genes due to the absence of chromosome crossing in Drosophila males: PP - a gray female with long wings (BBVV) crossed with a black short-winged male (bbvv); F1 - gray long-winged male (BbVv) crossed with black short-winged female (bbvv); F2 - since crossing over does not occur in the male, two types of offspring will appear: 50\% - black short-winged and 50\% - gray with normal wings; b - incomplete (partial) linkage of characters due to chromosome crossing in female Drosophila: PP - a female with long wings (BBVV) crossed with a black short-winged male (bbvv); F1 - a gray female with long wings (BbVv) crossed with a black short-winged male (bbvv). F2 - since crossing over of homologous chromosomes occurs in the female, four types of gametes are formed and four types of offspring will appear: non-crossovers - gray with long wings (BbVv) and black short-winged (bbvv), crossovers - black with long wings (bbVv), gray short-winged (Bbvv) ). . Each gene occupies a specific place - a locus. Each chromosome represents a linkage group. The number of linkage groups is equal to the haploid number of chromosomes. Allelic genes are exchanged between homologous chromosomes. The distance between genes is proportional to the percentage of crossing over between them. Questions for self-control 1. Where are the genes located?2. What is a clutch group?3. What is the number of clutch groups?4. How are genes linked on chromosomes?5. How are the traits of wing length and body color inherited in the Drosophila fly?6. What characteristics will be produced in offspring when crossing a homozygous female with long wings and a gray body color with a homozygous black male with short wings?7. Offspring with what characteristics will appear when crossing a diheterozygous male with a homozygous recessive female?8. What kind of gene linkage occurs in a male Drosophila?9. What kind of offspring will be produced when a diheterozygous female is crossed with a homozygous recessive male?10. What gene linkage occurs in a female Drosophila?11. What are the main provisions of the chromosomal theory of heredity? Key words of the topic “Chromosomal theory of heredity” genes linkage group length cells conjugation crossing over wings linear locus place fly heredity exchange color organism pair recombination generation position descendants distance result parents male female crossing body theory region chromosome color part human number Chromosomal sex determination mechanism Phenotypic differences between individuals of different sexes are determined by genotype. Genes are located on chromosomes. There are rules of individuality, constancy, pairing of chromosomes. The diploid set of chromosomes is called karyotype. There are 23 pairs (46) of chromosomes in the female and male karyotype (Fig. 78). 22 pairs of chromosomes are the same. They are called autosomes. 23rd pair of chromosomes - sex chromosomes. In the female karyotype there is one Rice. 78. Karyotypes of different organisms. 1 - person; 2 - mosquito; 3 skerdy plants. sex chromosomes XX. In the male karyotype, the sex chromosomes are XY. The Y chromosome is very small and contains few genes. The combination of sex chromosomes in the zygote determines the sex of the future organism. When sex cells mature as a result of meiosis, gametes receive a haploid set of chromosomes. Each egg contains 22 autosomes + an X chromosome. The sex that produces gametes that are identical on the sex chromosome is called homogametic sex. Half of the sperm contains 22 autosomes + X chromosome, and half contains 22 autosomes + Y. A sex that produces gametes that are different on the sex chromosome is called heterogametic. The sex of the unborn child is determined at the moment of fertilization. If the egg is fertilized by a sperm having an X chromosome, a female organism develops, if a Y chromosome - a male organism (Fig. 79). Rice. 79. Chromosomal mechanism of sex formation. The probability of having a boy or a girl is 1:1 or 50\%:50\%. This determination of sex is typical for humans and mammals. Some insects (grasshoppers and cockroaches) do not have a Y chromosome. Males have one X chromosome (X0), and females have two (XX). In bees, females have a 2n set of chromosomes (32 chromosomes), and males have a n set (16 chromosomes). Women have two sex X chromosomes in their somatic cells. One of them forms a clump of chromatin, which can be noticeable in interphase nuclei when treated with the reagent. This lump is a Barr body. Men do not have a Barr body because they have only one X chromosome. If during meiosis two XX chromosomes enter the egg at once and such an egg is fertilized by a sperm, then the zygote will have a larger number of chromosomes. For example, an organism with a set of chromosomes XXX (trisomy X chromosome) by phenotype - girl. Her gonads are underdeveloped. Two Barr bodies are distinguished in the nuclei of somatic cells. An organism with a set of chromosomes XXY (Klinefelter syndrome) by phenotype - boy. His testes are underdeveloped and he has physical and mental retardation. There is a Barr body. Chromosomes XO (monosomy on the X chromosome)- determine Shereshevsky-Turner syndrome. An organism with this set is a girl. She has underdeveloped sex glands and is short in stature. No Barr body. An organism that does not have an X chromosome, but contains only a Y chromosome, is not viable. Inheritance of traits whose genes are located on the X or Y chromosomes is called sex-linked inheritance. If genes are located on sex chromosomes, they are inherited in a sex-linked manner. A person has a gene on their X chromosomes that determines the trait of blood clotting. A recessive gene causes the development of hemophilia. There is a gene (recessive) on the X chromosome that is responsible for the manifestation of color blindness. Women have two X chromosomes. A recessive trait (hemophilia, color blindness) appears only if the genes responsible for it are located on two X chromosomes: XhXh; XdXd. If one X chromosome has a dominant gene H or D, and the other has a recessive gene h or d, then there will be no hemophilia or color blindness. Men have one X chromosome. If she has the H or h gene, then these genes will definitely manifest their effect, because the Y chromosome does not carry these genes. A woman can be homozygous or heterozygous for genes localized on the X chromosome, but recessive genes appear only in the homozygous state .If the genes are on the Y chromosome (Holandric inheritance), then the signs determined by them are passed on from father to son. For example, ear hairiness is inherited through the Y chromosome. Men have one X chromosome. All genes contained in it, including recessive ones, are manifested in the phenotype. In the heterogametic sex (male), most of the genes localized on the X chromosome are located in hemizygous condition, i.e. they do not have an allelic pair. The Y chromosome contains some genes that are homologous to the genes of the X chromosome, for example, genes for hemorrhagic diathesis, general color blindness, etc. These genes are inherited through both the X and Y chromosomes . Questions for self-control 1. What are the rules of chromosomes?2. What is a karyotype?3. How many autosomes does a person have?4. Which chromosomes in humans are responsible for the development of sex?5. What is the probability of having a boy or a girl?6. How is sex determined in grasshoppers and cockroaches?7. How is the sex of bees determined?8. How is sex determined in butterflies and birds?9. What is a Barr body?10. How can you determine the presence of a Barr body? 11. How can you explain the appearance of more or less chromosomes in a karyotype? 12. What is sex-linked inheritance? 13. What genes in humans are inherited in a sex-linked manner?14. How and why do recessive genes linked to sex in women manifest their effect?15. How and why do recessive genes linked to the X chromosome manifest their effect in men? Keywords of the topic “Chromosomal sex determination” autosomesbutterfliesprobability ear hairgametesgenotype genesheterogametic sex lump of chromatinhomogametic sexcolorboymeiosismammalmomentmonosomymalesettreatmentfertilizationorganismspeciespairspairssexgadevelopment differences birth growth blood clotting testes Down syndrome Klinefelter syndrome Shershevsky-Turner syndrome blindness maturation condition combination spermatozoa son cockroaches body Barratrisomy Y-chromosomephenotypechromosome X-chromosome human nucleus ovum
