Before considering issues of gene interaction, it is necessary to familiarize yourself with the basic terms and definitions used in the study of this issue. We already know that hereditary traits are determined by genes.
Genes- separate sections of chromosome DNA responsible for the synthesis of one protein.
Locus– the location of the gene on the chromosome.
Each pair of homologous chromosomes contains two related genes that are responsible for the development of one trait
Loci of related genes are located in the same places on homologous chromosomes
Allele– one gene of a pair, located in a similar locus of homologous chromosomes and controlling the development of alternative traits. An allele is also called a form of a gene's state.
Homozygous or pure organisms - having in the same locus of homologous chromosomes genes identical in the nature of their action ( AA, aa, BB, bb).
Heterozygous or hybrid organisms - having genes in the same locus of homologous chromosomes that differ in the nature of their action (Aa, BB).
Punnett lattice– schematic representation of the crossing process.
Pleiotropy– multiple gene action, when one gene is responsible for a number of phenotypic effects.
Polygenic determination– the combined effect of several genes on one trait.
The patterns of independent segregation of characters in the offspring of hybrids established by G. Mendel apply to all cases when each individual gene determines the development of one hereditary trait. Along with this, numerous facts have accumulated indicating complex gene interactions. It turned out that the same gene can influence several different traits and, conversely, the same hereditary trait develops under the influence of many genes. Two types of gene interaction are known: allelic and non-allelic.
Three main forms of interaction between allelomorphic genes are known: complete dominance; incomplete dominance; and independent manifestation.
Complete Domination observed when the patterns of inheritance obey Mendel's laws, when the product of one gene is present in the phenotype of heterozygotes. When two homozygous individuals with genotype AA and aa are cross-pollinated in the first hybrid generation, all plants will be identical in phenotype, but heterozygous Aa in genotype.
Incomplete dominance– in which the phenotype of heterozygotes has an average value between dominant and recessive homozygotes.
The simplest example of allelic interaction of genes is incomplete dominance when crossing white and red flowers in snapdragons and resulting in pink flowers. In the second hybrid generation, splitting occurs: one red-flowered plant, two with pink flowers and one with white flowers. In this case, there is complete correspondence between the phenotype and the genotype - homozygotes AA have red flowers, heterozygotes Aa have pink flowers and homozygotes aa have white flowers. (create a Punnett lattice).
Codominance– interaction of allelic genes, in which heterozygotes have a product of both genes in their phenotype.
When codominant in heterozygous organisms, each of the allelomorphic genes causes the formation of the trait it controls, regardless of which of the other allelomorphic genes accompanies it. An example of codominance is the inheritance of the ABO blood group in humans. Blood type is controlled by a series of multiple alleles of a single gene. Three alleles form six genotypes OO - the first, AA or AO - the second, BB or VO - the third, AB - the fourth blood group.
Interaction of non-allelomorphic genes:
Genes located at different loci and responsible for the expression of one gene are called non-allelic.
Four forms of interaction are known: 1) completeness, in which the corresponding trait develops only in the presence of two specific non-allelomorphic genes; 2) epistasis, in which one of the genes completely suppresses the action of another non-allelomorphic gene; 3) polymerization, in which non-allelomorphic genes act on the formation of the same trait and cause approximately the same changes in it; 4) modification, in which some genes modify the action of others, suppressing, intensifying or weakening them.
Complementary genes– causing, when combined together, a new phenotypic manifestation of a trait. Cleavage – 9:3:3:1, 9:7, 9:3:4, 9:6:1
9:3:3:1 – each dominant gene has an independent phenotypic manifestation, the combination of these two genes in the genotype causes a new phenotypic manifestation, and their absence does not result in the development of the trait. For example, gene A determines the development of blue plumage in budgies, gene B causes yellow plumage, and parrots with the A_B_ genotype are green, and those with the aabb genotype are white.
9:7 – dominant and recessive alleles of complementary genes do not have independent phenotypic manifestation. For example, the purple color of the corolla of a sweet pea flower develops only when dominant genes A and B are combined in the genotype; in all other cases, there is no color, and the corolla turns out to be white
9:3:4 – dominant and recessive alleles of complementary genes have independent phenotypic manifestation. For example, color in rabbits is determined by two complementary genes: A - presence of color, a - absence, B - black color, b - blue color
9:6:1 - the combination of dominant alleles of complementary genes ensures the formation of one trait, the combination of recessive alleles of these genes - another, and the presence of only one of the dominant genes in the genotype - a third. For example, pumpkins with genotype A_B_ have a disc-shaped fruit, with genotype aabb - elongated, and with genotype A_bb or aaB_ - spherical
Epistasis– interaction of non-allelic genes, in which one of them suppresses the action of the other. A gene that suppresses the action of another non-allelic gene is called a suppressor or inhibitor, and is designated I or S. The gene that is suppressed is called hypostatic. Epistasis can be dominant or recessive.
Dominant epistasis is the suppression of a gene by a dominant allele of another gene.
Cleavage: 13:3 - observed when the dominant allele of an epistatic gene does not have its own phenotypic manifestation, but only suppresses the action of another gene, while its recessive allele does not affect the manifestation of the trait. For example, in some breeds of chickens, the presence of a dominant epistatic gene suppresses the development of plumage color; in its absence, chickens are colored
12:3:1 - observed when an individual homozygous for recessive traits has a special phenotype. For example, from crossing two heterozygous dogs, puppies with the I_aa genotype are white, and those with the iiA_ genotype are black, and those with the iiAA genotype are brown.
The interaction of non-allelic genes, in which the recessive allele of an epistatic gene in a homozygous state suppresses the action of another gene, is called recessive epistasis. In single recessive epistasis, the recessive allele of one gene suppresses the effect of another (aa suppresses B_). In double cases, the recessive allele of each gene in the homozygous state suppresses the effect of the dominant allele (aa suppresses B_, bb suppresses A_). Splitting 9:3:4 or 9:7
Polymerism– interaction of non-allelic genes that clearly influence the development of the same trait.
Such genes are called polymeric or multiple and are designated by the same letters with the corresponding index (A1, A2, A3). Most often, polymer genes control quantitative traits (height, weight, etc.)
Polymerism can be cumulative (summative, additive) and non-cumulative
With cumulative polymerization, the degree of manifestation of the trait depends on the number of dominant alleles of the corresponding polymer genes. For example, the more dominant alleles of genes responsible for skin color are contained in a person’s genotype, the darker his skin
With non-cumulative polymerization, the degree of development of a trait does not depend on the number of dominant alleles, but only on their presence in the genotype. For example, chickens with genotype a1, a2, a3 have unfeathered legs, in all other cases the legs are feathered
Modifying genes- genes that enhance or weaken the effect of other genes. Modifier genes themselves do not have their own manifestation. Theoretically, any gene, interacting with others, should modify the expression of another gene. However, there are groups of genes that clearly show their modifying effect on the expression of several genes. For such modifier genes, their independent effect on the individual is often not detected. We learn about their existence by their influence on other genes. According to the type of their action, modifier genes are represented in two categories: 1) genes that enhance the manifestation of a trait determined by another gene; 2) genes that weaken the effect of another gene.
Chained inheritance.
Mendel's third law - the rule of independent inheritance - has a significant limitation. It is valid only in cases where the genes are localized on different chromosomes. When non-allelomorphic genes are located on the same chromosome in a linear order, independent segregation is not observed, but joint inheritance of genes is observed, limiting their free combination; T. Morgan called this phenomenon linkage of genes or linked inheritance. If, according to Mendel's theory, when crossing AB and av, a hybrid AaBb is obtained, forming four varieties of gametes AB, Av, Ba, va. In accordance with this, in analyzing crossing a splitting of 1:1:1:1 is carried out, i.e. 25% each. However, as facts accumulate, deviations from such a split are observed. In some cases, new combinations of Av and Ba were completely absent - complete linkage was observed between the genes of the original forms, which manifested itself in equal quantities - 50% each. Genes were more often inherited in the original state (they were linked).
Each organism has a huge number of characteristics, but the number of chromosomes is small. Consequently, one chromosome carries not one gene, but a whole group of genes responsible for the development of different traits.
The outstanding American geneticist T. Morgan studied the inheritance of traits whose genes are localized on one chromosome.
The phenomenon of joint inheritance of characteristics is called linked. The material basis for the linkage of genes is the chromosome. Genes localized on the same chromosome are inherited together and form one linkage group . The number of linkage groups is equal to the haploid set of chromosomes . The phenomenon of joint inheritance of genes localized on the same chromosome is called linked inheritance. Linked inheritance of genes localized on one chromosome is called Morgan's law.
There are two options for the localization of dominant and recessive alleles of genes belonging to the same linkage group:
Cis position, in which dominant alleles are located in one of a pair of homologous chromosomes, and recessive alleles are in the other.
Trans position, in which the dominant and recessive alleles of a gene are located on different homologous chromosomes.
Genes on chromosomes have different strengths of cohesion. Linkage can be complete - if genes belonging to the same linkage group are always inherited together; incomplete if recombination is possible between genes belonging to the same linkage group.
Research by T. Morgan showed that gene exchange partially occurs in a homologous pair of chromosomes. The exchange process was called crossing over. Crossing over promotes a new combination of genes located on homologous chromosomes and thereby increases the role of combinative variability in evolution. The study of the phenomenon of crossing over, which disrupts the linkage of genes, confirmed the idea of a strictly fixed arrangement of genes along chromosomes. The principle of linear arrangement is known as T. Morgan's second law.
Gene linkage may be disrupted during crossing over; this leads to the formation of recombinant chromosomes. Depending on the characteristics of gamete formation, there are:
crossover gametes– gametes with chromosomes that have undergone crossing over;
non-crossover gametes– gametes with chromosomes formed without crossing over.
With linked inheritance of traits, the genes of which are localized on one chromosome, the ratio of phenotyric classes of the offspring obtained from crossing often differs from the classical Mendeleevian one. This is due to the fact that some of the gametes of the parental individuals are crossover, and some are non-crossover.
The probability of crossover occurring between genes depends on their location on the chromosome: the farther the genes are located from each other, the higher the probability of crossover between them. The unit of distance between genes located on the same chromosome is taken to be 1% crossing over. Its value depends on the strength of adhesion between genes and corresponds to the percentage of recombinant individuals (individuals formed with the participation of crossover gametes) from the total number of descendants obtained during crossing. The unit of distance between genes is named in honor of T. Morgan Morganida.
The percentage of crossing over between genes is calculated using the formula:
X = (a+b) x 100
P
where X is the percentage of crossing over, a is the number of crossover individuals of one class, b is the number of crossover individuals of another class, n is the total number of individuals obtained from the analyzing cross.
The crossover value does not exceed 50 % , if it is higher, then free combination between pairs of alleles is observed, indistinguishable from independent inheritance.
According to the chromosomal theory of heredity, genes are arranged linearly on chromosomes. Genetic map of a chromosome– schematic representation of the relative position of genes included in one linkage group.
The position of a gene in a linkage group is judged by the percentage of crossing over (the number of crossover individuals): the greater the percentage of crossing over or the number of crossover individuals in F a, the further away the analyzed genes will be located/
Problems on linked inheritance are solved similarly to problems on mono- and dihybrid crossing. However, with linked inheritance, the genes that control the development of the analyzed traits are localized on one chromosome. Therefore, the inheritance of these characteristics does not obey Mendel's laws.
The genotypes of crossed individuals and hybrids should be written in chromosomal form;
When recording genotypes, the location of genes on the chromosomes of a homologous pair (cis or trans position) should be taken into account. In cis, the dominant alleles of genes are on one chromosome, and the recessive alleles are on the other. In trans-position, a dominant allele of one gene and a recessive allele of another are located on the chromosome.
With complete linkage, an individual that is heterozygous for all the characteristics under consideration forms two types of gametes
In case of incomplete linkage, the formation of crossover and non-crossover gametes occurs.
The number of non-crossover gametes is always greater than crossover ones;
An organism always produces an equal number of different types of both crossover and non-crossover gametes;
The percentage of crossover and non-crossover gametes depends on the distance between genes;
If the distance between genes is known (in percent crossover or morganids), then the number of crossover gametes of a certain type can be calculated using the formula
P = % crossing over (2)
where is the number of crossover gametes of a certain type;
If the number of crossover individuals is known, then the percentage of crossing over between genes is calculated using formula (1)
If we consider traits whose genes are part of different linkage groups, then the probability of combining genes of different linkage groups in one gamete is equal to the product of the probabilities of each gene forming this gamete.
To determine the probability of the appearance of different varieties of zygotes, it is necessary to multiply the frequencies of the gametes that form this zygote.
In lefties, terry hair is maintained by one recessive gene. s, dominated by a simple flower S. Dominant allele S linked to a recessive allele l which causes pollen to die off , A recessive s with dominant L- normally developed pollen
Petunias have incomplete double dominance, so heterozygotes Gg form weakly double flowers compared to homozygotes GG, producing a plant with double flowers.
There is also a gene A- terry amplifier, which does not act independently in the presence of it ( AA or Ahh) it is possible to distinguish densely and weakly double plants, in the absence ( ahh)monohybrid segregation is observed 3:1
In Turkish cloves, doubleness is determined by one recessive gene, which simultaneously causes male sterility. Since double plants can only serve as a maternal form, 100% doubleness in the offspring is not feasible.
It is theoretically possible to obtain 50% when pollinating double plants ( her) simple ( Her
In garden carnation (forms Shabot, Margarita, Vienna, Grenadine), doubleness is determined by one gene with incomplete dominance. Homozygotes GG They are distinguished by a greater degree of terryness and almost complete male sterility. Heterozygotes Gg produce less double flowers.
The inheritance of doubleness is rarely the same and constant in different varieties and forms of the same species, much less family. It is especially unstable in species and varieties of the Asteraceae family and largely depends on the level of agricultural technology and weather conditions .
Doubleness in Asteraceae is determined by several or many genes, most of which have a dominant effect
This applies to asters, marigolds, marigolds and daisies.
To maintain doubleness, constant mass selection of double plants is necessary while maintaining a sufficient number of plants with a disk of bisexual tubular flowers in the center of the inflorescence as a source of pollen.
01. Allelic genes are located in
- identical loci on non-homologous chromosomes
- different loci on the same chromosome
- different loci of homologous chromosomes
- only in heterosomes
02. with codominant interaction of alleles
the phenotypic effect is due to
- manifestation of one of the alleles
- manifestation of only the dominant allele into a trait
- simultaneous manifestation of each of the alleles
- intermediate effect of two alleles
- suppression of one of the alleles
03. % of occurrence of Rh conflict in marriage rh - - mother and
homozygous Rh+ father
05. the ability of a gene to determine the development of several
signs are called
- dosage
- pleiotropy
- discreteness
- allelicity
- specificity
06. number of alleles of the gene responsible for blood groups of the AB0 system in a human somatic cell
|
07. according to Mendel's 2nd law in the second generation
there is a split in the ratio
- 1:2:1 by genotype
- 3:1 by genotype
- 1:1 by phenotype and genotype
- 2:1 by phenotype
08. segregation by genotype during dihybrid crossing in
relation 9 A-B; 3 A-bb; 3 aaB-; 1 aabb is noted in the offspring
parents
- digomozygous
- diheterozygous
- one homozygous for two pairs of genes and the other diheterozygous
- homozygous for the first pair of genes and heterozygous for the second
- heterozygous for the first pair of genes and homozygous for the second
09. Multiple allelism – presence in a population
several
- genes responsible for the formation of one trait
- genes responsible for the formation of various traits
- gene alleles responsible for the formation of several variants of one trait
- alleles interacting according to the type of codominance
- genotype variants
10. when crossing Aa x Aa% homozygous individuals in
offspring
11. to establish the genotype of an individual with a dominant
trait, an analyzing cross is carried out with an individual
- phenotypically similar
- having a recessive trait
- heterozygous
- from parent
- subsidiary
12. Phenotypic cleavage in a ratio of 9:7 is possible with
- co-dominance
- complete dominance
- overdominance
- polymers
13. the ability of a gene to exist in the form of several
options is called
- dosage
- pleiotropy
- discreteness
- polymer
- allelicity
14. when crossing heterozygotes in the case of complete
dominance is marked by splitting
- 1:1 by genotype and phenotype
- 1:2:1 by genotype and phenotype
- 1:2:1 by genotype and 3:1 by phenotype
- 2:1 by phenotype and genotype
15. when crossing diheterozygotes in the offspring of an individual with the genotype Aabb occur with frequency
16. an organism heterozygous for the first gene and homozygous for the second recessive gene ( Ааbb), forms gametes
- AB; Ab
- Aa; bb
- Ab; ab
- AB; Ab; aB; ab
17. The law of independent combination of traits is valid provided that the genes are located in
- sex chromosomes
- one pair of autosomes
- different pairs of chromosomes
- identical loci of homologous chromosomes
- only on the X chromosome
18. A child with blood group IV can be fathered by parents with
blood groups
- I; III
- III; III
- II; II
- IV; IV
19. the likelihood of Rh conflict in marriage
heterozygous Rh-positive parents as a percentage
20. epistasis is the interaction of genes
- non-allelic, in which the intensity of the trait’s expression depends on the number of doses of dominant alleles
- allelic, in which an intermediate variant of the trait is formed in heterozygotes
- allelic, in which heterozygotes exhibit only a dominant trait in their phenotype
21. number of alleles of the gene responsible for blood groups of the AB0 system in a human gamete
- four
- depends on blood type
22. in most human populations the number of alleles of a gene,
responsible for blood groups of the AB0 system,
- four
- depends on population size
23. when crossing individuals with genotypes Aa x Aa%
heterozygous individuals in the offspring
25. incomplete dominance in monohybrid crossing
manifests itself in the second generation by splitting
- 1:2:1 by genotype and phenotype
- 1:2:1 by genotype and 3:1 by phenotype
- 3:1 by genotype and 1:2:1 by phenotype
- 1:1 by genotype and phenotype
- 2:1 by phenotype
26. when crossing diheterozygotes, the offspring
split
- 1:1:1:1 by phenotype
- 1:2:1 by genotype
- 9:3:3:1 by phenotype
- 1:1:1:1 by genotype
- 1:2:1 by phenotype
27. complementarity is a type of gene interaction
- non-allelic dominant, in which the manifestation of one trait is enhanced
- non-allelic, in which in the presence of two dominant alleles from different
allelic pairs, a new variant of the trait is formed
- in which the gene of one allelic pair suppresses the manifestation of the gene of another allelic pair into a trait
- allelic, in which the phenotype of heterozygotes is due to the simultaneous expression of genes
28. polymer is a type of gene interaction
- non-allelic dominant, leading to the appearance of a new variant of the trait in the phenotype
- in which the gene of one allelic pair suppresses the expression of the gene of another allelic pair into a trait
- allelic, in which heterozygotes exhibit only the dominant allele in their phenotype
- non-allelic responsible for one trait, in which the intensity of the trait’s expression depends on the number of gene doses
- allelic, in which the phenotype of heterozygotes is due to the simultaneous expression of genes
29. the formation of a normal trait in an organism heterozygous for two mutant alleles is possible when
- complementary gene interaction
- co-dominance
- epistasis
- interallelic complementation
- overdominance
30. Parents with blood types cannot have a child with blood group III
Terminology
1. Allelic genes– genes located in identical loci of homologous chromosomes.
2. Dominant trait– suppressing the development of another.
3. Recessive trait- suppressed.
4. Homozygote- a zygote that has the same genes.
5.Heterozygote- a zygote with different genes.
6. Split– divergence of traits in the offspring.
7.Crossing over- chromosome overlap.
In the heterozygous state, the dominant gene does not always completely suppress the manifestation of the recessive gene. In some cases, the F 1 hybrid does not completely reproduce one of the parental characteristics and the expression of the trait is intermediate in nature with a greater or lesser bias towards a dominant or recessive state. But all individuals of this generation show uniformity in this trait. The intermediate nature of inheritance in the previous scheme does not contradict Mendel's first law, since all descendants of F 1 are uniform.
Incomplete dominance- a widespread phenomenon. It was discovered when studying the inheritance of flower color in snapdragons, the structure of bird feathers, the color of the wool of cattle and sheep, biochemical characteristics in humans, etc.
Multiple allelism.
Until now, we have analyzed examples in which the same gene was represented by two alleles - dominant (A) and recessive (a). These two gene conditions arise due to mutation. A gene can mutate repeatedly. As a result, several variants of allelic genes arise. The set of these allelic genes, which determine the diversity of variants of the trait, is called a series of allelic genes. The occurrence of such a series due to repeated mutation of one gene is called multiple allelism or multiple allelomorphism. Gene A can mutate into the state a 1, a 2, a 3, and n. Gene B, located in another locus, is in the state b 1, b 2, b 3, b n. For example, in the Drosophila fly, a series of alleles for the eye color gene is known, consisting of 12 members: red, coral, cherry, apricot, etc. to white, determined by a recessive gene. Rabbits have a series of multiple alleles for coat color. This causes the development of solid color or lack of pigmentation (albinism). Members of the same series of alleles may be in different dominant-recessive relationships with each other. It should be remembered that the genotype of diploid organisms can contain only two genes from a series of alleles. The remaining alleles of this gene in different combinations are included in pairs in the genotypes of other individuals of this species. Thus, multiple allelism characterizes the diversity of the gene pool, i.e. the totality of all genes that make up the genotypes of a certain group of individuals or an entire species. In other words, multiple allelism is a species trait, not an individual trait.
Mendel's Second Law - Law of Segregation
If the descendants of the first generation, identical in the studied trait, are crossed with each other, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 of the individuals will have a dominant trait, 1/4 will have a recessive one. According to the genotype in F 2, there will be 25% of individuals homozygous for dominant alleles, 50% of organisms will be heterozygous, and 25% of the offspring will be organisms homozygous for recessive alleles. The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation. Thus, Mendel’s second law (see Fig. 2) can be formulated as follows: when two descendants of the first generation are crossed with each other (two heterozygotes), in the second generation a splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1: 2:1.
Figure 2. Mendel's second law
With incomplete dominance in the offspring of F 2 hybrids, the segregation of genotype and phenotype coincides (1: 2: 1).
Law of gamete purity
This law reflects the essence of the process of gamete formation in meiosis. Mendel suggested that hereditary factors (genes) do not mix during the formation of hybrids, but are preserved unchanged. In the body of the F hybrid, from the crossing of parents that differ in alternative characteristics, both factors are present - dominant and recessive. The dominant hereditary factor is manifested in the form of a trait, while the recessive one is suppressed. The connection between generations during sexual reproduction is carried out through germ cells - gametes. Therefore, it must be assumed that each gamete carries only one factor from a pair. Then, during fertilization, the fusion of two gametes, each of which carries a recessive hereditary factor, will lead to the formation of an organism with a recessive trait that manifests itself phenotypically. The fusion of gametes carrying a dominant factor, or two gametes, one of which contains a dominant and the other a recessive factor, will lead to the development of an organism with a dominant trait. Thus, the appearance in the second generation (F 2) of a recessive trait of one of the parents (P) can only occur if two conditions are met:
1. If in hybrids the hereditary factors remain unchanged.
2. If the germ cells contain only one hereditary factor from an allelic pair.
Mendel explained the splitting of characteristics in the offspring when crossing heterozygous individuals by the fact that the gametes are genetically pure, i.e. carry only one gene from an allelic pair. The law of gamete purity can be formulated as follows: during the formation of germ cells, only one gene from an allelic pair (from each allelic pair) gets into each gamete. Cytological proof of the law of gamete purity is the behavior of the chromosome in meiosis: in the first meiotic division, homologous chromosomes end up in different cells, and in the anaphase of the second, daughter chromosomes, which, due to crossing over, may contain different alleles of the same gene. It is known that every cell of the body has exactly the same diploid set of chromosomes. Two homologous chromosomes contain two identical allelic genes.
The formation of genetically “pure” gametes is shown in the diagram in Figure 3.
Figure 3. Formation of “pure” gametes
When male and female gametes merge, a hybrid is formed that has a diploid set of chromosomes (see Fig. 4).
Figure 4. Hybrid formation
As can be seen from the diagram, the zygote receives half of the chromosomes from the father’s body, and half from the mother’s. During the formation of gametes in a hybrid, homologous chromosomes also end up in different cells during the first meiotic division (see Fig. 5).
Figure 5. Formation of two types of gametes
Two varieties of gametes are formed according to a given allelic pair. Thus, the cytological basis of the law of gamete purity, as well as the splitting of characteristics in offspring during monohybrid crossing, is the divergence of homologous chromosomes and the formation of haploid cells in meiosis.
1. Parents have brown eyes, their child has blue eyes. This trait is formed in the presence of two allelic genes. Allelic genes are:
A. Different states of genes resulting from mutations;
B. Genes located in identical loci of homologous chromosomes and responsible for the development of a certain trait;
C. Different states of the gene found in the population and responsible for the possibility of developing different variants of the trait;
D. Genes located on non-homologous chromosomes and responsible for the development of one trait;
E. Genes that determine the development of various hereditary inclinations.
2. There are two children in the family. The son has blue eyes, and the daughter has brown eyes. The genes that control the development of this trait (eye color) are located in:
A. Identical loci of homologous chromosomes;
B. Different loci of homologous chromosomes;
C. Different loci of non-homologous chromosomes;
D. Identical loci of non-homologous chromosomes;
E. Sex chromosomes.
3. Polydactyly, myopia and absence of small molars are transmitted as autosomal dominant traits. The genes for all three traits are located on different pairs of chromosomes. The number of hereditary factors (allelic genes) for each trait contained in gametes:
4. A woman with a high content of cystine in her urine marries a healthy man. Determine the probability of having healthy children from this marriage. It is known that urolithiasis (cystinuria) develops in a homozygous dominant state:
5. A brown-eyed woman, whose father has blue eyes and whose mother has brown eyes, has the genotype for this trait:
A. Homozygous;
B. Digomozygous;
C. Hemizygous;
D. Heterozygous;
E. Diheterozygous.
6. If both parents are heterozygous for two traits that are inherited independently, the ratio of phenotypes in the offspring will be:
7. Determine what genotype and phenotype the descendants of the first generation will have when crossing homozygous individuals with alternative traits.
A. Same for everyone;
B. Segregation by genotype and phenotype 3:1;
C. Segregation by genotype and phenotype 1:2:1;
D. Segregation by genotype and phenotype 1:1;
E. Segregation by genotype and phenotype 2:1.
8. The blood group according to the Rh system is determined by 3 different pairs of genes located sequentially on one chromosome, the allelic genes among them are:
A. Located in adjacent loci of the same chromosome;
B. Located in loci of the same chromosome at a distance of 1 morganid;
C. Determining the development of an individual trait;
D. Located in identical loci of homologous chromosomes;
E. Located in loci of identical arms (q or p) of homologous chromosomes.
9. Mother has curly hair, and father has straight hair. In F1 the hair is wavy. This phenotypic manifestation is the result of the interaction of allelic genes like:
A. Co-dominance;
B. Overdominance;
C. Epistasis;
D. Complementarity;
E. Incomplete dominance.
10. Mother and father have the fourth blood group of the ABO system. In this family it is impossible to have a child with this blood type (ignore the Bombay phenomenon):
11. In a family where parents have sickle cell anemia, 2 healthy boys were born, how many different phenotypes, determined by one pair of genes, can the offspring of two heterozygous organisms have with incomplete dominance?
1. Is G. Mendel’s III law always observed? In what cases are genes inherited independently, and in what cases are they inherited linked?
Mendel's third law is observed if non-allelic genes are located in different pairs of chromosomes. In this case, inheritance is called independent. If non-allelic genes are localized on the same pair of chromosomes, they are inherited together. This type of inheritance is called linked.
2. What is a clutch group? What is the number of linkage groups in the cells of different organisms?
A linkage group is a set of genes localized on the same chromosome.
Each pair of homologous chromosomes contains genes that control the same traits, so the number of linkage groups is equal to the number of pairs of chromosomes. For example, humans (2n = 46) have 23 linkage groups, and Drosophila (2n = 8) have 4 linkage groups.
3. Why does the frequency of crossing over between linked genes depend on the distance between them?
In prophase I of meiosis, during the conjugation of homologous chromosomes, the formation of crossovers between chromatids occurs arbitrarily, at any corresponding sites.
If the genes are relatively close to each other, then the probability that the crossing will occur precisely in the area separating these genes is small. If the genes are located at a considerable distance from each other, then the likelihood that the chromatids will cross at some point between them is much higher.
Thus, the greater the distance between linked genes, the more often crossing over occurs between them. Conversely, the closer genes are located to each other, the lower the frequency of crossing over between them.
4. What are genetic maps of chromosomes? What are the prospects for their use?
A genetic map of a chromosome is a diagram of the relative arrangement of genes located on a given chromosome, taking into account the distances between them.
Genetic maps are widely used in breeding; on their basis, the possibility of obtaining organisms with certain combinations of traits is predicted. Genetic maps of human chromosomes are used in medicine to diagnose and treat a number of hereditary diseases.
5. Formulate the main provisions of the chromosomal theory of heredity.
Basic provisions of the chromosomal theory of heredity:
● Genes on chromosomes are arranged linearly, in a certain sequence. Allelic genes are located in identical loci of homologous chromosomes.
● Genes located on the same chromosome form a linkage group and are inherited together. The number of linkage groups is equal to the number of pairs of chromosomes.
● Gene linkage can be disrupted as a result of crossing over, which occurs during the conjugation of homologous chromosomes in prophase I of meiosis.
● The frequency of crossing over is proportional to the distance between genes: the greater the distance, the higher the frequency of crossing over and vice versa.
● One morganide is taken as a unit of distance between linked genes - the distance at which crossing over occurs with a probability of 1%.
6. What types of gametes and in what percentage will form diheterozygous individuals AB//ab and Ab//aB, if it is known that the distance between genes A and B is 20 morganids?
A diheterozygous individual with the genotype AB//ab (the so-called cis-position of genes) will form four types of gametes: non-crossover AB and ab (40% each), as well as crossover Ab and aB (10% each).
A diheterozygous individual with the Ab//aB genotype (the so-called trans gene position) will form non-crossover gametes Аb and aB (40% each), as well as crossover gametes AB and ab (10% each).
7. Gray body color in Drosophila dominates over yellow, red eyes dominate over garnet. The genes responsible for these traits are localized in the first pair of chromosomes and are located at a distance of 44 morganids. We crossed pure lines of gray-bodied flies with garnet eyes and yellow-bodied flies with red eyes. From the resulting hybrids, a female was selected and subjected to analytical crossing. What will be the percentage of phenotypic classes in the offspring?
● Let’s introduce gene designations and indicate the distance between them:
A – gray body;
a – corpus luteum;
B – red eyes;
b – garnet eyes;
rf AB = 44% (44 morganids).
● Let's establish the genotypes of the parental forms. Individuals of a pure line of gray-bodied flies with garnet eyes have the genotype Ab//Ab, while individuals of a pure line of yellow-bodied flies with red eyes have the genotype aB//aB.
● As a result of crossing pure lines, the first hybrid generation was obtained:
● Therefore, the female that was subjected to the test cross has the genotype Ab//aB.
Let us write down an analyzing cross, taking into account that a diheterozygous female produces two types of non-crossover gametes (in equal proportions) and two types of crossover gametes (also in equal proportions). For convenience, we highlight crossover gametes and individuals with asterisks (*):
● Calculate the percentage of individuals in F a.
The distance between the genes is 44 morganids, which means that crossing over between them occurs with a probability of 44%. Consequently, the total number of crossover individuals will be 44% (and individuals of each phenotypic class - 22%). The total number of non-crossing individuals is: 100% – 44% = 56% (i.e. each phenotypic class – 28%).
Answer: the offspring will produce 28% of individuals with a gray body, garnet eyes and a yellow body, red eyes, as well as 22% of individuals with a gray body, red eyes and a yellow body, garnet eyes.
8. In one of the plant species, dissected leaves dominate over whole leaves, and the blue color of the flowers dominates over pink. As a result of testing crossing, offspring of four phenotypic classes were obtained:
1) 133 plants with dissected leaves and blue flowers;
2) 362 plants with dissected leaves and pink flowers;
3) 127 plants with whole leaves and pink flowers;
4) 378 plants with whole leaves and blue flowers.
Plants of the first phenotypic class were then crossed with plants of the second phenotypic class. What percentage of the resulting hybrids will have dissected leaves and pink flowers? Whole leaves and blue flowers?
● Let’s introduce gene designations:
A – dissected leaves;
a – whole leaves;
B – blue flowers;
b – pink flowers.
● As a result of analytical crossing, four phenotypic classes were obtained in unequal proportions. From this we can draw the following conclusions:
1) The genes that determine the type of leaves and the color of flowers are inherited linked.
2) The analyzed individual produced 4 types of gametes, i.e. was diheterozygous.
3) The descendants of the first and third phenotypic classes are crossover (there are approximately equal numbers of them and much fewer than individuals of the second and fourth phenotypic classes). The descendants of the second and fourth phenotypic classes are non-crossover (there are approximately equal numbers of them and much more than individuals of the first and third phenotypic classes).
4) The crossover frequency between genes A and B is: rf AB = sum of crossover individuals/total number of individuals × 100% = (133 + 127) : (133 + 362 + 127 + 378) × 100% = 260: 1000 × 100% = 26%.
● Let's establish the genotypes of individuals of the first and second phenotypic classes. Both of them inherited linked ab genes from the recessive homozygous parent ab//ab.
Individuals of the first phenotypic class have dissected leaves (A) and blue flowers (B), which means that they inherited linked AB genes from another parent (analyzed). Thus, individuals of the first phenotypic class have the genotype AB//ab.
Individuals of the second phenotypic class have dissected leaves (A) and pink flowers (b), therefore, they inherited linked Ab genes from the other parent (analyzed). Thus, individuals of the second phenotypic class have the genotype Ab//ab.
● Let’s write down the crossing of individuals of the first and second phenotypic classes; for convenience, we highlight crossover gametes and individuals with asterisks (*):
● In order to find the proportion of individuals in each cell of the Punnett lattice, you need to multiply the shares of the corresponding gametes by each other.
Therefore, the percentage of plants with dissected leaves and pink flowers is: (0.37 × 0.5 + 0.13 × 0.5 + 0.13 × 0.5) × 100% = (0.185 + 0.065 + 0.065) × 100% = 0.315 × 100% = 31.5%.
The percentage of plants with whole leaves and blue flowers is: 0.13 x 0.5 x 100% = 0.065 x 100% = 6.5%.
Answer: 31.5% of hybrids will have dissected leaves and pink flowers; 6.5% of hybrid plants will have whole leaves and blue flowers.
