Genetics. Basics of genetics

Structure and function of the cell.
Any dog, and all other animals, consists of an infinite number of cells, which are based on two major components. The main part of the cell cytoplasm occupies, consisting mainly of protein and contains a variety of structures. In the center of the cytoplasm is darker patch — the kernel. That there is the basic genetic material.
The nucleus contains a number of structures called chromosomes. The size and shape of chromosomes (their type) are a specific and constant factor for each species of animals and plants (this is the law of constancy of form of chromosomes). In any somatic cell of a living organism contains a constant and characteristic for him, as for his appearance, an even number of chromosomes (except sex cells) (law of the constancy of the chromosome number). The dog of 78, like all canines, including wolves and coyotes; jackal 74 at their in humans — 46, in feline — 38 horses — 64 and so forth. Therefore, different kinds of plants and animals differ not only in the number of chromosomes, but also on the nature of hereditary characteristics contained in them. The aggregate number, size and features of the structure of the chromosome set is called a karyotype (nuclear type). Detailed studies showed that regardless of the size and shape of all the chromosomes in somatic cells are arranged in pairs (the law of pairing of chromosomes), that is, we are talking about their diploid, unlike the haploid present in germ cells. Thus, correct to say that the dogs 78 chromosomes, and 39 pairs of them.
On each chromosome linearly arranged genes. For comparison, we can take a string of beads, in which the thread itself — chromosome, and strung beads on it — genes. Theoretically, this analogy is incorrect, since the genes — the most important part of the chromosome, and the beads exists in itself, regardless of the thread. But for clarity, this comparison is quite acceptable.
From the chemical point of view, the basis of the gene is DNA (deoxyribonucleic acid), which is passed through a huge mass of genetic information and instruction on which the inheritance. Gene — a unit of heredity. As chromosomes, genes are arranged in pairs. Paired chromosomes, one of which comes from the mother’s body, and the other from the father are called homologous. Typically, this homologous chromosome pair morphologically indistinguishable. Genes occupying homologous chromosomes in the same place (locus, or localization of the gene), responsible for one and the same sign / symptoms and are called alleles. As part of the homologous chromosome is inherited from the father and one from the mother, and the alleles are inherited in the same way.
If allelomorfnaya pair identical (homologous chromosomes in a pair of identical genes), i.e. both are responsible for the same characteristic expression, then this is called the individual is homozygous for this trait, and if each of those alleles responsible for the alternative (contrast) expression of one the same feature, the heterozygous thereon.

Mitotic cell division.
Puppy grows and increases in size by dividing somatic cells, called mitosis. Mitosis — Indirect somatic cell division, when there are complex changes in its nucleus and cytoplasm. After fertilization (ovogamii) by sperm (fusion of gametes or copulation) is formed zygote (oocyst) — a new organism, consisting of only one cell. The process of growth and development of a new organism begins with the first mitotic division of the cells (the parent) when there are two of her children (or rather, nursing), completely similar to her cell, and continues until death. During mitosis occurs:
doubling the substance of chromosomes;
change in the physical state and chemical organization of chromosomes;
divergence of sister chromosomes to the poles of the cell;
subsequent division of the cytoplasm and the full restoration of the two nuclei in the new cells.

In mitosis laid lifecycle of nuclear genes: doubling, distribution and function. The period between cell divisions is called interphase, during which take place in it the active processes of life and preparation for the next division. The whole cycle of changes occurring in the cell from one division to another it is called the mitotic cycle. The latter consists of two main periods — interphase and mitosis itself.
In mitosis of a single cell formed two having identical chromosome. Thus, mitosis ensures continuity and consistency in the number and dial, ie the qualitative specificity of chromosomes in successive generations of dividing cells.
We will not dwell on all the phases of mitosis. Let me just say one thing, that in interphase, the period between two successive cell division occurs in the nucleus replication (autoduplikatsiya or samoudvoenie) DNA, and thus the number of chromosomes in the cell (the formation of sister chromatids held together by the centromere, ie Taurus, performing the function mechanical center of the chromosome) and despiralization latter. In metaphase, or the central phase of nuclear fission, the chromosome consists of two chromatids, turns into two daughter chromosomes. In anaphase is the division and disagreement daughter chromosomes to the poles of the cell, that is, the restoration of the proper number. In telophase, the final stage of cell division, chromosomes acquire the same form as before the division, and the amount of DNA in each daughter nucleus is halved compared to the previous stage. Thus, both daughter cells contain the same amount of cytoplasm and identical sets of chromosomes and are ready to undergo mitosis.
Of course, not all the divided permanent somatic cells. During embryonic development, the differentiation of organs and tissues, developing on its specific, genetically incorporated the way. Therefore, some cells were converted into brain cells, and other — in blood cells and so forth. Moreover, some of them fall continuously, while others only at certain stages of development, or if necessary, in response, for example reparative (recovery) processes.

Meiotic cell division.
In sexual reproduction offspring resemblance with parents is provided through the germ cells. Despite its paltry compared with the body of the body size, they carry all the genetic information that predetermine the future course of development of the organism.
The basis of sexual reproduction is fertilization (syngamy), that is a fusion of two germ cells.
Physiological specialization of germ cells led to the features of their morphology and physiology, because of which the male and female sex cells are significantly different from each other. Ovum of the female body is not only responsible for the transmission of genetic information to offspring, but also the power of the embryo in the early stages of its development. Male germ cells, sperm, this function does not possess, and provides transmission of hereditary characteristics of an organism paternal descendants and stimulates the egg to develop.

At the heart of the development of germ cells is meiosis or reduction division of cells. It shall become immature germ cells have reached a certain differentiation. That is, it occurs during the formation of gametes, or gametogenesis.
If each sex cell has a diploid set of chromosomes, consisting in somatic cells, the number of chromosomes would be doubled in each successive generation. And as the karyotype of each species is constant, the number of chromosomes in the gametes to be haploid. For the reduction of the number of chromosomes in the gametes (their reduction) and meets meiosis.
The essence of meiosis is what happens two quick successive nuclear division and chromosome while reduplicated only once. The first division is called a pressure relief. Thus there is a reduction in the number of chromosomes in the nucleus twice through rapprochement of homologous chromosomes, called conjugation. However, they tend to mutually twisted and consist of two connected centromere chromatids. During conjugation of homologous chromosomes in places twisting can rupture and share with each other homologous regions. In this process, called crossing-over, involves only two (any) chromatids of four — one from each homologous chromosome. Crossover results in recombination of genetic material in the chromosomes, and this means that the gametes are not only the chromosome identical to the parent (non-exchanged regions), but a new combination with the chromosome material of the two homologous chromosomes. From each pair of homologous chromosomes in cells of the parent child misses one core, i.e. homologous chromosomes as opposed to non-homologous, which can be combined in different ways dependent on each other. The second division — commonly called equational, or equalization. In this case the centromere of each chromosome is divided and the daughter chromosomes (formerly chromatids) in an equal but of halving the number diverge to opposite poles of the cell. As a result, two meiotic divisions of a single parental diploid cells formed by four haploid daughter cells containing chromosome 39 (one of a pair of homologous chromosomes). After the formation of the zygote it would be 78 out of 39 homologous chromosome pairs.
The distribution of chromosomes in the gametes not have any laws, and is entirely coincidental. Theoretically, each gamete gets the same number of chromosomes of the mother and father. But we should not forget about the independent behavior of non-homologous chromosomes and crossing over of homologous chromosomes during meiosis, which is accompanied by recombination of genes and leads to a myriad of gametes that differ from the mother cell and interconnected by a set of chromosomes, not to mention the fact that any of the gametes can participate in the formation of the zygote.
Process which allows to control dog breeding at random distribution of chromosomes and genes located therein are unknown, making genetic predictions extremely challenging and fun. But this does not mean that you have to be fatalistic and count the fruit breeding pure coincidence. Of course, a certain element of chance in this case there is, but to achieve the specific goals of stabilization and improvement of the breed breeder must know the basics of genetics, conduct careful breeding, to be able to pick and choose breeding pairs, and keep records for statistical data processing. The stricter he leads the selection, the more aware of the specific genetic factors, the higher the chance of success.

Crossover and grip.
According to the theory of chromosomal genes in the chromosomes are located in a linear manner at certain locations (loci) and at a certain distance from each other. This is proved by using the phenomenon of crossing over. Genes located in one pair of homologous chromosomes inherited together, so that when they reach gametogenesis one gamete. This suggests that they are coupled together. Joint inheritance of genes limits their free combination, called linked inheritance.
Linkage group formed by all the genes are localized in the same chromosome, as each of them exhibits adhesion. The genes of one group cohesion inherited independently from other genes linkage groups. The number of linkage groups is equal to the haploid number of chromosomes.
The frequency of meiotic crossing over between two genes located in the same chromosome in certain environmental conditions, is constant, which is only possible with a linear arrangement of them. Thus the paternal chromosome genes can move into the motherboard. It has been established that the strength of adhesion between the genes depends on their distance from each other, and inversely proportional to it, the farther apart the genes located on chromosome, the less the force of adhesion between them and crossing-over occurs frequently, and vice versa. It is determined by calculating the percentage of crossovers (gametes with chromosomes that have undergone crossing-over), resulting from the crossing of the chromosomes using the formula:

Percentage chiasm = (number of crossovers / total number of offspring) x100.

What this power is, the less the force of adhesion.
Empirically it has been found that the overlap between chromosomes can be a single (single point) and multiple double (at several points simultaneously). Sometimes Perekrestov that occurred in the same chromosomal region, promotes or prevents the occurrence of another Perekrestov in nearby regions of the chromosome, which is called interference (interference).

Genetics of sex.
As has been said, the dog karyotype presented 39 pairs of chromosomes, one of which is different in males and females. Those chromosomes on which male and female do not differ (38 pairs) are called autosomes. Males has unequal pair of sex chromosomes (XY), and gametogenesis they formed sperm of two sorts — half with the X chromosome, and half a Y-chromosome. Females, however, both the same sex chromosomes (XX).
In birds, some fish and insects homogametic floors are male (ZZ), and heterogametic — females (ZW), poetomy sexing they do not occur at fertilization, and even when meiotic oogenesis (depending on what type of sex chromosome gets the egg ).
Dogs genotypic sex is determined at the time of fertilization and depends on how sperm (X-bearing or Y-chromosome) would be fertilized ovum (always carrying the X chromosome). According meiosis, among the descendants receive 50% females and 50% males (ratio 1: 1).
In the case where the genes are in the autosomes, reciprocal crosses give the same results as autosome in both sexes the same. If the genes are in the sex chromosomes, the inheritance of characters determined by them, will depend on the behavior of the sex chromosomes in meiosis during the formation of gametes and on the characteristics of these chromosomes. Y-chromosome is genetically inert, so it does not contain genes. This suggests that sex determination is not involved, and the genes are arranged in the male autosomes. Consequently, the genes that are in the X-chromosome alleles in the Y-chromosome, usually not. As a result, even recessive genes, while in the X-chromosome and not having allele in the Y-chromosome is shown as in homozygotes.
With an increase in the proportion of individuals of the genetic balance of autosomes in relation to the number of X chromosomes in the offspring is enhanced expression of male sexual characteristics, and with a corresponding increase in the share of the balance of the X chromosome is amplified expression of female characteristics. This is the essence of the theory of the balance of sex determination and violation ratio of 1: 1. This implies that the floor is determined by the predominance of individual genes from one floor above the other in the sheet and the genes of both sexes in the zygote are always present simultaneously (it always contains autosomes genes male and one or two X chromosome genes in these female ), that is, any organism is bisexual. Each zygote simultaneously laid the development potential of both women and in males. Everything depends on the nature of the interaction of male and female genes in gene balance, the ratio of the strength of their actions. Anyway, males (XY) has more than female (XX). But the nature of weeds due to early mortality and shorter lifespan, so that in the end, prevail still a female.

The interaction of genes — total domination.
We already know that genes are constructed from DNA — a substance is the repository of genetic information. We, as breeders are interested not so much going on with her biochemical reactions as their phenotypic (external) display. Gene — section of the DNA molecule, which contains information about the primary structure of a single chain of protein synthesized. One gene — one polypeptide chain. Genes themselves directly involved in protein synthesis is not accepted. They serve as a molecular matrix. Gene function consists in programming the synthesis of proteins in a cell. Their primary product in the nucleus are all types of RNA (ribonucleic acid) controlling protein synthesis in the cell. It is through these proteins genes controlling the synthesis of certain products in the cell, determine its function, affecting the body during ontogeny.
For example, a dog, for example, do not inherit the gene tan markings and a specific chain of the polynucleotide chain that is part of the DNA (the portion of amino acids arranged in the correct order and combination), which under appropriate conditions and in dependence on what happens in other areas of DNA leads to on wool spots tan markings. If any irregularities on the part of the chain or in the DNA itself, we do not get this color, or even the puppy. For simplicity, we will talk about genes as if heredity is a single line, not a complicated complex mechanism.
Let us turn to the study of the same type of interactions of allelic genes.
When complete domination, meets the first law of Mendel (uniformity first generation hybrids), one gene suppresses the expression of a pair allelomorfnoy other. In this case it is called dominant (from Lat. Dominantis — dominant, ruling), and one that could not be shown, remained dormant — recessive (from Lat. Recessus — retreat). From this it follows that the dominant gene appears regardless of whether it is in both the gametes, zygote is formed, i.e. homozygous, or one of them, that is in the heterozygous state and a recessive — a homozygous, i.e. if it exists in the maternal and paternal gametes.
The set of all genes of an organism interacting and environmental conditions, is called a genotype (from the Greek. genos — genus itypos — fingerprint pattern, that is, the gene or hereditary type of organism) and their external manifestation in conjunction with individual development — phenotype (from the Greek. phaino — or is the appearance of an organism).
Before proceeding to the further description, I will say a few words about the notation introduced in the genetics of Mendel, William Bateson and Saunders. Hybridization is indicated by a multiplication — x, dominant and controlling his hereditary factor — capital letter of the alphabet (eg, A) and recessive — lowercase (a), the alleles are denoted by the same letter (AA, Aa or aa); sign of dominance ->; When writing scheme of crossing the first place put the mother’s body, denotes the mirror of Venus — +, while the second — his father, a shield and a spear denotes Mars ->; initial parental forms denote the Latin letter — P (from Lat. parentale — parents), and the organisms collected from them, — F (from Lat. filii — children), and a digital subscript denoting hybrid generation and its serial number, for example F1, F2, F3, and so on. d.
For example, consider the mating females homozygous for the dominant black color — BB (from the English. black — black), with the recessive male liver (brown) color — bb.

In this case we are talking about Monohybrid cross, as the parental organisms differ in only one pair of contrasting alternative characters. As you can see, all the first generation hybrids have black coat color. The dominance of the hybrid of the first generation features of one of the parents called Mendel dominance. Because all hybrids F, the same (black), the law of dominance often called the law of uniformity of hybrids of the first generation (Mendel’s first law).
When crossing F1 hybrids between a second-generation individuals appear as black and brown colors, that is, with the signs of both parents . This is subject to the second law of Mendel — law splitting feature.

These data are statistically significant only in the analysis of at least one hundred pups. Otherwise, you will be hard to judge the genotype of both parents black color, as you can not be a litter of puppies liver color, despite the fact that one of them or both heterozygous. But birth from under them at least one pup liver color clearly indicates their heterozygosity.
Also Test cross, often use pedigree analysis, which is used in the preparation of special characters. Persona, which begins compiling genealogy, called the proband. If it comes down to posterity, it is called a family tree, and if it goes back to the ancestors — the ancestors of the table. In the analysis of pedigrees, you can determine whether we are interested in inherited trait, and if so, how — for a dominant or recessive manner. If you need to determine the quantitative relationships in the cleavage, analyzed several similar pedigrees.
The study of inheritance of recessive traits more difficult, as in this case, can jump through the generation of feature, that is the impression of his unexpected appearance.

Incomplete dominance — codominance.
Dominance is not always complete because the intermediate inheritance in which the heterozygous offspring on the severity of feature like no one parent, and between them occupy an intermediate position. In this case, the modified and the splitting of the second generation phenotype (instead of 3: 1, it will be 1: 2: 1).
For example, take two pure lines (homozygous parents) — mother with drooping ears — NN, and father — with standing — hh .

As can be seen from the diagram, with incomplete dominance, in contrast to the full, a second-generation cleavage phenotype and genotype of the same.
One of the most frequent causes of deviation from Mendelian laws of quantitative cleavage — unequal viability of zygotes of different genotypes (often because of the presence lethal genes).
Similar examples can be given on the basis of a series of gene inheritance aguti1 in rough collies. We are interested in this case, the two alleles — ay responsible for clean-sable color, and a ‘- for the black-and-tan (bicolor). The remaining genes are responsible for the appearance of these alleles with white spots on the fur, typical for collie, so we ignore them.
That’s what happens if we take the clean lines and Tie fawn bitch — auau with black and tan kobelёm a’a ‘.

1. Agouti (from the English. Agouti, from the Spanish. Aguti, borrowed from the language of the Guarani — acuti, — «golden hare») — sable, sable, «wild» color, which is found in nature, including wildlife family Canis. In this case, the hair is not painted over the entire length, and in the form of concentric regions, due to the uneven accumulation of melanin granules in it, because of which the hair has a silver base, gold (gray-fawn or brown) and black middle ends.

From the book «Genetics dogs» M.B.Uillis

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