In 1906, W. Batson and R. Punnett, crossing sweet pea plants and analyzing the inheritance of pollen shape and flower color, discovered that these characteristics do not give independent distribution in the offspring; hybrids always repeated the characteristics of the parent forms. It became clear that not all traits are characterized by independent distribution in the offspring and free combination.

Each organism has a huge number of characteristics, but the number of chromosomes is small. Consequently, each chromosome carries not one gene, but a whole group of genes responsible for the development of different traits. He studied the inheritance of traits whose genes are localized on one chromosome. T. Morgan. If Mendel conducted his experiments on peas, then for Morgan the main object was the fruit fly Drosophila.

Drosophila produces numerous offspring every two weeks at a temperature of 25 °C. The male and female are clearly distinguishable in appearance - the male's abdomen is smaller and darker. They have only 8 chromosomes in a diploid set and reproduce quite easily in test tubes on an inexpensive nutrient medium.

By crossing a Drosophila fly with a gray body and normal wings with a fly having a dark body color and rudimentary wings, in the first generation Morgan obtained hybrids with a gray body and normal wings (the gene that determines the gray color of the abdomen dominates the dark color, and the gene that determines development of normal wings, - above the gene of underdeveloped wings). When carrying out an analytical crossing of an F 1 female with a male who had recessive traits, it was theoretically expected to obtain offspring with combinations of these traits in a ratio of 1:1:1:1. However, individuals with characteristics of the parental forms clearly predominated in the offspring (41.5% - gray long-winged and 41.5% - black with rudimentary wings), and only a small part of the flies had a combination of characters different from those of the parents (8.5% - black long-winged and 8.5% - gray with rudimentary wings). Such results could only be obtained if the genes responsible for body color and wing shape are located on the same chromosome.

1 - non-crossover gametes; 2 - crossover gametes.

If the genes for body color and wing shape are localized on one chromosome, then this crossing should have resulted in two groups of individuals repeating the characteristics of the parental forms, since the maternal organism should produce gametes of only two types - AB and ab, and the paternal organism - one type - ab . Consequently, two groups of individuals with the genotype AABB and aabb should be formed in the offspring. However, individuals appear in the offspring (albeit in small numbers) with recombined traits, that is, having genotypes Aabb and aaBb. In order to explain this, it is necessary to recall the mechanism of formation of germ cells - meiosis. In the prophase of the first meiotic division, homologous chromosomes are conjugated, and at this moment an exchange of regions can occur between them. As a result of crossing over, in some cells, sections of chromosomes are exchanged between genes A and B, gametes Ab and aB appear, and, as a result, four groups of phenotypes are formed in the offspring, as with the free combination of genes. But, since crossing over occurs during the formation of a small part of gametes, the numerical ratio of phenotypes does not correspond to the ratio 1:1:1:1.

Clutch group- genes localized on the same chromosome and inherited together. The number of linkage groups corresponds to the haploid set of chromosomes.

Chained inheritance- inheritance of traits whose genes are localized on the same chromosome. The strength of linkage between genes depends on the distance between them: the further the genes are located from each other, the higher the frequency of crossing over and vice versa. Full grip- a type of linked inheritance in which the genes of the analyzed traits are located so close to each other that crossing over between them becomes impossible. Incomplete clutch- a type of linked inheritance in which the genes of the analyzed traits are located at a certain distance from each other, which makes crossing over between them possible.

Independent inheritance- inheritance of traits whose genes are localized in different pairs of homologous chromosomes.

Non-crossover gametes- gametes during the formation of which crossing over did not occur.

Non-recombinants- hybrid individuals that have the same combination of characteristics as their parents.

Recombinants- hybrid individuals that have a different combination of characteristics than their parents.

The distance between genes is measured in Morganids- conventional units corresponding to the percentage of crossover gametes or the percentage of recombinants. For example, the distance between the genes for gray body color and long wings (also black body color and rudimentary wings) in Drosophila is 17%, or 17 morganids.

In diheterozygotes, dominant genes can be located either on one chromosome ( cis phase), or in different ( trans phase).

1 - Cis-phase mechanism (non-crossover gametes); 2 - trans-phase mechanism (non-crossover gametes).

The result of T. Morgan's research was the creation of chromosomal theory of heredity:

1. genes are located on chromosomes; different chromosomes contain different numbers of genes; the set of genes of each of the non-homologous chromosomes is unique;
2. each gene has a specific location (locus) on the chromosome; allelic genes are located in identical loci of homologous chromosomes;
3. genes are located on chromosomes in a specific linear sequence;
4. genes localized on the same chromosome are inherited together, forming a linkage group; the number of linkage groups is equal to the haploid set of chromosomes and is constant for each type of organism;
5. gene linkage can be disrupted during crossing over, which leads to the formation of recombinant chromosomes; the frequency of crossing over depends on the distance between genes: the greater the distance, the greater the magnitude of crossing over;
6. Each species has a unique set of chromosomes - a karyotype.

Chromosomal level of organization of hereditary material. Chromosomes as gene linkage groups.

It follows from the principles of genetic analysis that independent combination of traits can only be carried out under the condition that the genes that determine these traits are located in different pairs of chromosomes. Consequently, in each organism, the number of pairs of characters for which independent inheritance is observed is limited by the number of pairs of chromosomes. On the other hand, it is obvious that the number of characteristics and properties of an organism controlled by genes is extremely large, and the number of pairs of chromosomes in each species is relatively small and constant. It remains to be assumed that each chromosome contains not one gene, but many. If this is so, then it should be recognized that Mendel’s third rule concerns only the distribution of chromosomes, and not genes, i.e. its action is limited. Analysis of the manifestation of the third rule showed that in some cases new combinations of genes were completely absent in hybrids, i.e. complete linkage was observed between the genes of the original forms and a 1:1 split was observed in the phenotype. In other cases, a combination of traits was observed with less frequency than expected from independent inheritance.

In 1906, W. Betson described a violation of the Mendelian law of independent inheritance of two characters. Questions arose: why are not all traits inherited and how are they inherited, how are genes located on chromosomes, what are the patterns of inheritance of genes located on the same chromosome? The chromosomal theory of heredity, created by T. Morgan, in 1911, was able to answer these questions.

T. Morgan, having studied all the deviations, proposed to call the joint inheritance of genes, limiting their free combination, linkage of genes or linked inheritance.

Patterns of complete and incomplete coupling. Clutch groups in humans.

Research by T. Morgan and his school has shown that genes are regularly exchanged in a homologous pair of chromosomes. The process of exchange of identical sections of homologous chromosomes with the genes they contain is called chromosome crossing or crossing over. Crossing over occurs in meiosis. It provides new combinations of genes located on homologous chromosomes. The phenomenon of crossing over, like gene linkage, is characteristic of animals, plants, and microorganisms. The exceptions are male fruit flies and female silkworms. Crossing over ensures the recombination of genes and thereby significantly increases the role of combinative variability in evolution. The presence of crossing over can be judged by taking into account the frequency of occurrence of organisms with a new combination of characteristics. The phenomenon of crossing over was discovered by Morgan in Drosophila.

Recording the genotype of a diheterozygote with independent inheritance:

A IN

Recording the genotype of a diheterozygote with linked inheritance:

Gametes with chromosomes that have undergone crossing over are called crossover, and those that have not undergone are called non-crossover.

AB, AB AB, AB

Non-crossover gametes. Crossover gametes.

Accordingly, organisms that arise from a combination of crossover gametes are called crossovers or recombinants, and those arising from a combination of non-crossover gametes - non-crossovers or non-recombinants .

The phenomenon of crossing over, as well as the linkage of genes, can also be considered in the classic experiment of T. Morgan when crossing Drosophila.

	Sign	P♀ B.V. x♂ bv
	gray body color
	black body color
	normal wings
	vestigial wings
Analysis cross 1. Complete linkage of genes. 2. Incomplete linkage of genes.		1. Full grip P♀ bv x♂ B.V. F 2 bv bv splitting – 1:1
2. Incomplete traction (crossing over) P:♀ B.V. x♂ bv G: BV bv Bv bV bv non-crossover crossover F 2 B.V. bv Bv bV non-crossovers – 83% crossovers – 17%

To measure the distance between genes by test crossing, you can use the formula:

Where:

X– distance between genes in % crossing over or in morganids;

A– number of individuals of the 1st crossover group;

V– number of individuals of the 2nd crossover group;

n– total number of hybrids in the experiment;

100% – coefficient for conversion to percentage.

Based on a study of linked inheritance, Morgan formulated a thesis that was included in genetics under the name Morgan's rule : genes localized on the same chromosome are inherited linked, and the strength of linkage depends on the distance between them.

Linked genes are arranged in a linear order and the frequency of crossing over between them is directly proportional to the distance between them. However, this thesis is typical only for genes that are close to each other. In the case of relatively distant genes, some deviation from this dependence is observed.

Morgan proposed expressing the distance between genes as the percentage of crossing over between them. The distance between genes is also expressed in morganids or centimorganids. Morganidae is the genetic distance between genes where crossing over occurs with a frequency of 1%.

The frequency of crossing over between two genes can be used to infer the relative distance between them. So, if between genes A And IN crossing over is 3%, and between genes IN And WITH– 8% crossing over, then between A And WITH crossing over should occur at a frequency of either 3+8=11% or 8-3=5%, depending on the order in which these genes are located on the chromosome.

A ─ ─ ─ B ─ ─ ─ ─ ─ ─ ─ ─ C B ─ ─ ─ A ─ ─ ─ ─ ─ ─ ─ ─ C

Task 1. Cataracts and polydactyly are inherited as dominant autosomal traits. The woman inherited cataracts from her father and polydactyly from her mother. The genes are linked, the distance between them is 3M. What are the genotypes and phenotypes of the children from the marriage of this woman and a man normal for these characteristics? What is the probability of having healthy children?

cataract		P♀ aB x ♂ aw
polydactyly		X = AB = 3 Morgue.
P♀ aB x ♂ aw		Answer: the probability of having a healthy child is 1.5%, having one characteristic is 48.5%, having both characteristics is 1.5%
G: (аВ) (Ав) (ав)
F1 aB Av aw AB aw aw aw aw 48,5% 48,5% 1,5% 1,5%

Genetic map chromosomes is a diagram showing the order of genes at their relative distance from each other. The distance between linked genes is judged by the frequency of crossing over between them. Genetic maps of all chromosomes have been compiled for the most genetically studied organisms: Drosophila, chickens, mice, corn, tomatoes, Neurospora. Genetic maps of all 23 chromosomes have also been compiled for humans.

After establishing the linear discreteness of chromosomes, the need arose to compile cytological maps in order to compare them with genetic maps compiled on the basis of taking into account recombinations.

Cytological card is a map of a chromosome that determines the location and relative distance between genes on the chromosome itself. They are constructed based on the analysis of chromosomal rearrangements, differential coloring of polytene chromosomes, radioactive labels, etc.

To date, genetic and cytological maps have been constructed and compared for a number of plants and animals. The reality of this comparison confirms the correctness of the principle of the linear arrangement of genes on a chromosome.

In humans, some cases of linked inheritance can be named.

The genes that control the inheritance of ABO blood groups and nail and patella defect syndrome are inherited linked.

The genes for the Rh factor and the oval shape of red blood cells are linked.

The third autosome contains the genes for the Lutheran blood group and the secretion of antigens A and B with saliva.

The genes for polydactyly and cataracts are inherited linked.

The X chromosome contains the genes for hemophilia and color blindness, as well as the genes for color blindness and Duchenne muscular dystrophy.

Autosome 6 contains subloci A, B, C, D/DR of the HLA system, which control the synthesis of histocompatibility antigens.

Inheritance of X-linked and holandric traits.

Traits controlled by genes located on the sex chromosomes are called adhered to the floor. More than 60 sex-linked diseases have been described in humans, most of which are inherited recessively. Genes on sex chromosomes can be divided into 3 groups:

Genes partially linked to sex. They are located in paired segments X And Y chromosomes . Partially sex-linked diseases include: hemorrhagic diathesis, convulsive disorders, retinitis pigmentosa, xeroderma pigmentosa, and general color blindness.

Genes are completely sex-linked. They are located in the area X chromosome , for which there is no homologous region in Y chromosome (heterological). These genes control diseases: optic atrophy, Duchenne muscular dystrophy, color blindness, hemophilia, and the ability to smell hydrocyanic acid.

Genes located in the region Y chromosomes , for which there is no homologous locus in X chromosome are called holandric . They control symptoms: syndactyly, hypertrichosis of the auricle.

The color blindness gene occurs in 7% of men and 0.5% of women, but 13% of women are carriers of this gene.

Sex-linked inheritance was described by T. Morgan using the example of inheritance of the eye color trait in Drosophila.

Several patterns of inheritance of sex-linked traits have been noted:

passed cross to cross (from father to daughter, from mother to son);

the results of direct and back crossings do not coincide;

in the heterogametic sex, the trait manifests itself in any state (dominant or recessive).

Basic provisions of the chromosomal theory of heredity.

The main provisions of the chromosomal theory of heredity can be formulated as follows:

Genes are located on chromosomes. Each gene on a chromosome occupies a specific locus. Genes are arranged linearly on chromosomes.

Each chromosome represents a group of linked genes. The number of linkage groups in each species is equal to the number of pairs of chromosomes.

Allelic genes are exchanged between homologous chromosomes—crossing over.

The distance between genes on a chromosome is proportional to the percentage of crossing over between them. Knowing the distance between genes, you can calculate the percentage of genotypes in the offspring.

The law of independent distribution of traits (Mendel's third law) is violated if the genes that determine different traits are located on the same chromosome. Such genes are usually inherited together, i.e. chained inheritance. The phenomenon of linked inheritance was studied by Thomas Morgan and his associates and is therefore called Morgan's law.

T. Morgan's law can be formulated as follows: genes located on the same chromosome form a linkage group and are often inherited together, while the frequency of joint inheritance depends on the distance between genes (the closer, the more often).

The reason why linked inheritance is disrupted is crossing over, which occurs in meiosis during the conjugation of chromosomes. In this case, homologous chromosomes exchange their sections, and thus previously linked genes can end up on different homologous chromosomes, which determines the independent distribution of traits.

For example, gene A is linked to gene B (AB), and the homologous chromosome contains recessive alleles of the corresponding genes (ab). If, during the process of crossing over, homologous chromosomes almost never exchange sections so that one gene moves to another chromosome, while the other remains in the same one, then such an organism forms gametes of only two types: AB (50%) and ab (50%). If an exchange of the corresponding sections occurs, then a certain percentage of the gametes will contain the Ab and aB genes. Usually their percentage is less than with independent distribution of genes (when A and B are on different chromosomes). If, with an independent distribution of all types of gametes (AB, ab, Ab, aB), there will be 25% each, then in the case of linked inheritance of gametes Ab and aB there will be less. The fewer there are, the closer the genes are located to each other on the chromosome.

Sex-linked inheritance is especially distinguished when the gene under study is located on the sex (usually X) chromosome. In this case, the inheritance of one trait is studied, and the second is gender. If an inherited trait is sex-linked, then it is inherited differently during reciprocal crosses (when the trait is first possessed by a female parent, then by a male parent).

If the mother has the aa genotype, and the father exhibits a dominant trait (there is definitely one gene A), then in the case of sex linkage, all daughters will have a dominant trait (in any case, they will receive his only X chromosome from the father, and all sons will have a recessive one) (from the father one gets the Y chromosome, which does not contain the corresponding gene, and from the mother, in any case, gene a). If the trait were not linked to sex, then among both sexes of children there could be owners of a dominant trait.

When the genes under study are linked in an autosome, such linkage is called autosomal. Linkage is called complete if the parental combinations of alleles are not disrupted from generation to generation. This happens very rarely. Usually, incomplete linked inheritance is observed, which violates both Mendel's third law and Morgan's law (in its abbreviated form: genes located on the same chromosome are inherited together).

Genes on a chromosome are arranged linearly. The distance between them is measured in centimorgans (cm). 1 cm corresponds to the presence of 1% of crossover gametes. By conducting various crosses and statistically analyzing the descendants, scientists identify linked genes, as well as the distance between them. Based on the data obtained, genetic maps are constructed, which reflect the localization of genes on chromosomes.

02-Sep-2014 | No comments | Lolita Okolnova

Chained inheritance

After the discovery, they began to notice that these laws do not always work.

For example: crossed a diheterozygous female Drosophila with gray body and normal wings with a male with black body and shortened wings .

A gray body and normal wings are dominant characteristics.

According to Mendel's laws, the crossing scheme is as follows:

But the practical result of crossing is different.

As a rule, a 1:1 split is observed in the offspring,

offspring phenotypes: gray body, normal wings And black body, short wings.

Does not work . Why is this so? Do Mendel's laws really not work? Of course not, the laws of nature can only be “violated” if another law allows it (an exception to the rule).

Let's find out...

• information about each trait is carried by a specific gene;
• genes are located on chromosomes.

Naturally, the number of chromosomes is much smaller than the number of genes, so several genes are encoded on one chromosome.

Genes located on the same chromosome are inherited together, that is linked .

And genes located on different chromosomes are inherited independently,

since during gametogenesis chromosomes are distributed randomly, therefore, two unlinked genes may end up together in one gamete, or they may not.

Genes located on the same chromosome will necessarily end up in the same gamete.

In the example we looked at earlier, we can notice:gray bodyinherited along withnormal wings, and the black body inherited along withshortened wings.

The genes for body color and wing length are located on the same chromosome.

Female Diheterozygous, there are two homologous chromosomes:

genes are encoded on one of the homologous chromosomes gray body and normal wings,

in the other - genes

But only two types of gametes are obtained - the characteristics of body color and wing size are “indivisible”

Paternal specimen Digomozygous for these characteristics:

genes on one homologous chromosome black body and shortened wings,

and in another homologous chromosome the same.

All characteristics encoded on one chromosome form the so-calledclutch group .

Traits from the same linkage group are inherited together.

And as you might guess,

quantity clutch groups equal to the number of chromosomes in the haploid set.

Examples of problems

Task 1:

Slightly different design: linked traits are written on “sticks”, for example, the genotype of the female from our problem should be written like this:

• rods mean homologous chromosomes in which genes are localized
• letters on one side of the sticks represent genes that are linked to each other.

That is, the entry says:

AB signs are linked to each other; signs ab are also linked to each other

• position of genes in the genotype 1) called cis position: AB \\ ab (dominant traits on one chromosome, recessive on the other)
• position 2) called trans position: Ab\\aB.

Let's look at an example:

1) In the problem statement, all the characteristics are immediately indicated; let’s fill out the table:

2) The first plant is diheterozygous; it is said that the dominant traits are localized on one chromosome, that is, linked. Moreover, dominant traits are located on one homologous chromosome, therefore, recessive traits (cis-position) are located on another homologous chromosome. Genotype of the first plant: AB \\ ab.

We get only two types of gametes (since the characters are linked):

AB andab.

3) Since the second plantshowedberecessivesigns, we conclude that it is dihomozygous. And its genotype: ab\\ab.Only one type of gametes is formed:ab.

4) Finally, let's draw up a crossing scheme:

And let’s answer the last question of the problem - about the law:

The law of concatenated inheritance appears, it says:genes localized on the same chromosome form a linkage group and are inherited together .

But it happens that even genes from the same linkage group (localized on the same chromosome) are inherited separately, that is, “uncoupled.”

For example, let's take the crossing from the previous problem.

With the same crossing, 4 phenotypic groups (instead of the required 2) can be obtained in the offspring, as with independent inheritance. This is due to the possibility crossing over between homologous chromosomes (for those who do not understand what we are talking about, I advise you to read the articlecrossing over ).

Let’s say if an individual has signsABare linked, then during the formation of gametes, if crossing over occurs, there is a possibility that the section of the chromosome in which one of the genes is encoded will “jump” to another homologous chromosome, and the linkage will be broken. Using our problem as an example, in the case of crossing over, the crossing will be as follows:

A diheterozygous plant produces two more varieties of gametes due to crossing over. Gametes, during the formation of which, crossing over occurred (in this problem this isAb And aB) are called crossover . Statistically, the percentage of crossover gametes is lower than non-crossover ones.

And accordingly, the closer the genes are located to each other on a chromosome, the more likely they are to separate.

This dependence of the probability of gene separation by crossing over and the distance between genes turned out to be so “convenient” that the distance between genes is measured as a percentage of the probability of their separation by crossing over. According to the formula:

Where:

• x – probability of gene disconnection as a percentage,
• a is the number of individuals formed from crossover gametes, n is the number of all individuals.
• And 1% probability of gene separation was taken as a unit of distance between these genes.

This unit is called Morganida. The unit was named after the famous geneticist who studied this phenomenon

1 morganid = 1% probability that linked genes, as a result of crossing over, will end up on different homologous chromosomes

Task 2:

1) Let's create a table of features

2) Since the condition says that an analyzing cross was carried out, it means that the second plant is dihomozygous for recessive traits, its genotype: ab \\ ab.

3) The offspring received 4 phenotypic groups. Since the characteristics are linked, crossing over obviously took place. Also, the appearance of four phenotypic groups during an analytical cross indicates the diheterozygosity of the first plant. This means his genotype is either AB \\ ab, or Ab \\ aB.

To determine in what position the genes are - cis or trans, you need to look at the ratio in the offspring. PThe percentage of crossover gametes is less than non-crossover gametes, therefore there are more individuals resulting from non-crossover gametes.

These individuals: 208 tall plants with smooth endosperm, 195 short plants with rough endosperm

In them, a dominant trait is inherited with a dominant one, and a recessive trait with a recessive one. Therefore, the genes in the parental diheterozygous individual are in cis position: AB \\ ab.

4) Crossing scheme:

Crossover gametes and individuals obtained from them are marked in red. There are fewer of these individuals, since fewer crossover gametes are formed. If the parent diheterozygous individual had genes in a trans position, the offspring, on the contrary, would produce more individuals with the following characteristics: tall, rough, and short, smooth.

5) Let's determine the distance between genes.

To do this, we calculate the probability that the features will be separated as a result of crossing over.

According to the formula:

x= 9 + 6208+ 195+ 9 + 6 ×100%= 15418 ×100%=3.59%

That is, the distance between genes = 3.59 morganids.

Linked inheritance of genes

At the beginning of the 20th century, when geneticists began to conduct many experiments on crossing on a wide variety of objects (corn, tomatoes, mice, Drosophila flies, chickens, etc.), it was discovered that the patterns established by Mendel did not always appear. For example, dominance is not observed in all pairs of alleles. Instead, intermediate genotypes arise in which both alleles participate. There are also many pairs of genes that do not obey the law of independent inheritance of genes, especially if the pair of allelic genes is located on the same chromosome, that is, the genes are linked to each other. Such genes came to be called linked.

The mechanism of inheritance of linked genes, as well as the location of some linked genes, was established by the American geneticist and embryologist T. Morgan. He showed that the law of independent inheritance, formulated by Mendel, is valid only in cases where genes carrying independent characteristics are localized on different non-homologous chromosomes. If the genes are located on the same chromosome, then the inheritance of traits occurs jointly, i.e. linked. This phenomenon came to be called linked inheritance , and law of adhesion or Morgan's law .

The law of linkage states: linked genes located on the same chromosome are inherited together (linked).

There are many known examples of linked inheritance of genes. For example, in corn, the color of seeds and the nature of their surface (smooth or wrinkled), linked to each other, are inherited together. U sweet pea (Lathyrus odoratus) Flower color and pollen shape are inherited concatenated.

All genes on one chromosome form a single complex - clutch group. They usually end up in the same sex cell - the gamete - and are inherited together.

Clutch group- all genes on one chromosome. The number of linkage groups is equal to the number of chromosomes in the haploid set. For example, humans have 46 chromosomes - 23 linkage groups, peas have 14 chromosomes - 7 linkage groups, and the fruit fly Drosophila has 8 chromosomes - 4 linkage groups.

Genes included in the linkage group do not obey Mendel's third law of independent inheritance. However, complete gene linkage is rare. If genes are located close to each other, then the probability of chromosome crossing is low and they can remain on the same chromosome for a long time, and therefore will be inherited together. If the distance between two genes on a chromosome is large, then there is a high probability that they can diverge on different homologous chromosomes. In this case, genes obey the law of independent inheritance.

Incomplete gene linkage. When analyzing the inheritance of linked genes, it was discovered that sometimes linkage can be broken as a result of crossing over, which occurs during meiosis during the formation of germ cells.

If the site of chromosome breakage during the exchange of sections is located between genes A (a) and B (b), then gametes will appearAb And aB, and four groups of phenotypes are formed in the offspring, as with unlinked inheritance of genes. The difference is that the numerical ratio of phenotypes will not correspond to the ratio 1:1:1:1, as in a dihybrid test cross.

The farther the genes are from each other on a chromosome, the higher the probability of crossover between them, the greater the percentage of gametes with recombined genes, and therefore the greater the percentage of individuals different from their parents. This phenomenon is calledincomplete gene linkage.

On the image - Inheritance with incomplete linkage of genes(using the example of crossing two lines of Drosophila, where A- normal wings,A- rudimentary wings,IN- gray body color,V- black body color).

Complete gene linkage. The closer the genes are to each other on a chromosome, the less likely there is to crossover between them.If genes are located very close to each other (side by side), then crossover between them is usually not observed. In this case they talk aboutcomplete gene linkage.