The CHROMOSOME MAP (Greek
chroma color, coloring - f soma a body) — the graphic representation of a chromosome with designation of the sequence of an arrangement on it of genes and relative distances between them. Process of creation of chromosome maps is called mapping of chromosomes.
As the size of separate genes (see the Gene) is very small, they are represented on X. to. points. Such points are called loci (genetic loci). The locus can be occupied with any of alleles of this gene (see Alleles). Because genes are located on chromosomes linearly, represent a chromosome in the form of a piece of a straight line with the scale put on it (usually uneven) corresponding to location of the known genes. Such X. to. reflects not the actual distances between genes in a chromosome but only the results of assessment of their vzai-mopolozheniye and relative intervals between them received by genetic methods therefore such X. to. call genetic maps of chromosomes (see fig. 14 to the Art. of Vakteriya, t. 2, Art. 486). In cases, when the provision of genes on X. to. it is correlated with morfol. features of a chromosome it is also designated on its diagrammatic representation, such by X. to. call cytologic cards of chromosomes (see fig. 5 to St. Karyotype, t. 10, Art. 170).
Genetic mapping of chromosomes — complex, multi-stage process. Initial premises for creation of genetic maps of chromosomes of any biol. a look creation of its collection hereditarily of the differing forms (breeds, grades, versions, strains, lines) is. On the basis of such genetic collection determine consistent patterns of inheritance of each of signs by means of the analysis of posterity. Signs, inheritance to-rykh in posterity submits to Mendel's laws (see Mendel laws, Heredity), are selected for mapping of the genes controlling them. Then series of crossings between the individuals differing on two and more signs are carried out, as a result to-rykh groups come to light it is linked heritage. The signs belonging to one linkage group are controlled by the genes localized in the same couple homologous (structurally - and dentichn ykh) x ro mo with about m. Respectively the signs relating to different linkage groups, i.e. inherited independently from each other are controlled by the genes localized in different chromosomes. Total number of linkage groups at given biol. a look it is equal to chromosome number of haploid (unary) set. Distribution of set of ancestral features on linkage groups completes the first stage of creation of genetic maps of chromosomes. After that the issue of compliance of each of linkage groups to a certain couple of chromosomes of diploid chromosomal complement is resolved (see). It is the simplest to make it for the signs inherited it is linked to a floor since the genes controlling them are localized in gonosomes. Compliance to certain chromosomes of other linkage groups usually is solved by establishment of parallelism between transfer of the studied ancestral feature and transfer of marker (marked) chromosomes. As chromosomal markers (tags) in this case secondary banners of chromosomes, their satellites and other constant structural features of chromosomes can serve. The problem of marking not only the whole chromosomes, but also their certain sites significantly became simpler as a result of development of methods of differential coloring of chromosomes (see Chromosomes). Sometimes in quality tsitol. markers of chromosomes use their restructurings, especially interchromosomes-nye exchanges — translocations (see). In such cases the transition of ancestral features from one linkage group in another traced parallel to transfer of material of one chromosome on another allows to establish unambiguously localization of the genes controlling these signs on the corresponding chromosomes.
Informative was a tracing of inheritance and manifestation of signs at persons with an aneuploidy — lack of separate chromosomes or their excess quantity. Deviations in inheritance of these or those signs at such patients allow to draw a conclusion on localization of the corresponding genes on those chromosomes, to-rye are present at set in excess or insufficient quantity.
At genetic mapping of signs of the person especially useful was a method of the distant hybridization of somatic cells. For example, at hybridization of somatic cells of the person and a mouse from kernels of hybrid cells in the course of cellular divisions eliminirutsya gradually chromosomes of the person (disappear). If in culture of somatic cells of the person it is possible to reveal any signs hereditary bio-chemical (e.g., activity of certain enzymes, availability of species-specific proteins, etc.) * that disappearance of such signs at elimination of the corresponding chromosomes unambiguously indicates localization of the studied genes on these chromosomes. This and some other methods allowed to establish at the person everything 24 (22 autosomal, X and Y) the linkage groups corresponding to the identified couples of chromosomes defined cytologic and distribution on these groups apprx. 800 of about 2500 ancestral features described at the person. At the same time use of molecular and genetic methods provides mapping of many genes within narrow segments of chromosomes.
The final stage of genetic mapping of chromosomes is establishment of interposition of genes and relative (genetic) distances between them. Assessment of a vzaimopo-lozheniye of genes and distances between them by results of crossing is most convenient.
As measure of distance between genes on the genetic map serves the frequency of exchange of sites of homologous chromosomes (crossing-over) during the crossing (see Meiosis) expressed as a percentage. As unit of distance between genes use length of the site of a chromosome, in limits to-rogo probability of a crossing-over makes 1% (unit of the chromosome map, unit of a crossing-over, Morgan, a morganida). Apply also smaller unit to short distances — santimor-gan (0,01 Morgan).
After creation of genetic maps of chromosomes their comparison to cytologic cards can be carried out. Most in detail it is made for cards of chromosomes of a drosophila. In a crust, time mapping of chromosomes of the person to a certain extent approaches it. On genetic and cytologic maps of chromosomes the same order of genes remains, but genetic distances usually do not match with cytologic since the probability of a crossing-over (see the Recombination, chromosomes) on different sites of chromosomes is various (the crossing-over is usually complicated in heterochromatinic — considerably a spira - lizovanny regions of chromosomes).
The CHROMOSOMAL THEORY of HEREDITY 131
Recombination (exchange) of linked genes (see the Recombination) can take place and in a mitosis, though is considerable less than in meiosis. It allows to build mitotic cards of chromosomes, on to-rykh the same order of genes is observed, but distances between them differ from the values received in case of meiotic, and cytologic cards.
The research of especially big selections (tens and hundreds of thousands of descendants) allows to estimate the frequency of a recombination not only between genes, but also in separate genes and to define genetic distances. Recombinational cards of fine structure of genes are supplemented with their complementation maps, creation to-rykh is based on assessment of interallelic complementation (functional complementarity) of a series of alleles of one locus.
In medical genetics definition of localization of genes on the chromosome map allows to differentiate clinically similar, but differently inherited and having different probability repetitions in posterity of a form of hereditary pathology.
Creation of X. to. has great theoretical value for studying of the organization of genetic material and mechanisms of variability (see Variability). At selection presence of X. to. gives the chance to purposefully combine genetic material, receiving new breeds, grades, strains with required properties.
See also Bacteria, genetics of bacteria.
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B. B. Mechanisms of a genetic
tion of a rekombin, L., 1971; With e r e r about in with to and y A. S. Genetic analysis, M., 1970; Genetic variants and strains of the laboratory mouse, ed. by M. G. Green, Stuttgart — N. Y., 1981; McKusickV. A. The human gene map 1 December 1984, Clin. Genet., v. 27, p. 207, 1985. V. I. Ivanov.