From Big Medical Encyclopedia


(Greek chroma color, coloring - f soma a body; poJymorphos diverse) — features of a structure of chromosomes which are inherent to all cells of an organism distinguish one organism from another, are transferred to posterity and, as a rule, do not render pathological effect. X. and. is not beyond constancy of number, characteristic of each species, and morphology of chromosomes. Polymorphism is inherent in chromosomes of the most different types of plants and animals; especially in detail it is investigated at the person.

Molecular basis of X. the item is change of contents in a chromosome of DNA with repeatedly repeating nucleotide sequences (see. Deoxyribonucleic acid). The most part of such DNA is deprived of transkriptsionny activity, i.e. does not participate in implementation of genetic information (see the Transcription) and makes the grown poor structural genes (defining the sequence of amino acids) heterochromatinic regions (sites) of chromosomes (see Chromatin, Chromosomes). Variability of these regions of chromosomes is the main reason of X. the item, than also speaks lack of adverse influence of X. the item on a phenotype of an organism. As heterochromatinic areas contain in all chromosomes, polymorphism is potentially inherent in each of them, however most often it is found in chromosomes with rather large blocks of heterochromatin. X. the item causes unlimited number of optional versions of normal chromosomes and by that creates uniqueness of chromosomal complement (see) at each person. Homologous (structurally identical) chromosomes, occurring from different parents, often differ on certain regions of chromosomes (heteromorphism of homologs). By means of methods of genetic engineering (see) it is established that individuals differ from each other also on the nucleotide sequences of certain sites of a genome — polymorphism of DNA.

Ideas of the nature of X. items developed in process of introduction of new methods of a research of chromosomes. Initial data on variability of chromosomes of the person in size and a form were received in the late fifties the beginning of the 60th years when vper-

Fig. 1. Chromosomes: couple 1,

groups D (couple 13 — 15), G (couple 21 — 22) and U-hro-mosomy two unrelated individuals:

the individual And differs from an individual of B in presence of an okolotsentromerny banner at one of two chromosomes/, large satellites in one of D-chromosomes, long sputnichny threads in one of G-chromosomes and length of the U-chromosome (are specified by shooters).

to the Vyya began to study the chromosomes which are evenly painted on length by means of the main dyes (fig. 1) in metaphase of cellular division (see Cell division, Chromosomes). Much wider picture X. the item opens at special colourings of chromosomes. The main method is S-coloring at which Romanovsky's paint — Gimza is perceived only by structural heterochromatin (S-chromatin) of okolotsentromerny regions of all chromosomes (S-segments) and a long shoulder of a Y-chromosome. S-chromatin of chromosomes 1,9,13 — 16,21,22 and Y is available to the microscopic analysis, qualitative, semi-quantitative (points) or to direct measurement. In a Y-chromosome heterochromatin makes the main part of a long shoulder. S-segments of chromosomes of 1,9,16 and Y (fig. 2) differ in especially big variability. Variability of size of S-segments decides by change of amount of DNA in them on high repeatability of the nucleotide sequences, in particular satellite DNA. Apparently, from an individual to an individual also a set of such sequences can change. In some cases polymorphism of S-segments of chromosomes

of Fig. 2. Chromosomes of couples of O, Yu and U-chromosome of two individuals (S-coloring): the individual And differs from an individual of B in length of the U-chromosome, the sizes of the S-segment in homologous chromosomes couple 1 (are specified by shooters).

1,9 and 16 it is caused by different extent of movement of a segment from long in a short shoulder (partial and full inversions of an okolotsentromer-ny S-segment).

X. the item can be found also by means of coloring of chromosomes by fluorescent dyes (see Flyuorokhroma) which create a differential luminescence on all length of each chromosome — Q-coloring. Individuals can differ on fluorescence of the small okolotsentromerny site of chromosomes 3,4,13 — 15,21 and 22 and the large site of a long shoulder of a Y-chromosome. The difference in a luminescence is defined by the unequal maintenance of complex DNK-flyuo-rokhrom. Brightly fluorescent Y-chromosomes chromatin is available practically for all men. Polymorphism of this chromosome is caused by various size of its fluorescent part (fig. 3). X. the item of chromosomes 3, 4, 13—15, 21 and 22 is expressed in distinction at individuals of brightness of fluorescence of separate segments which at a considerable part of people can not be defined. Frequency fluorescing and not - fluorescent options on each of the chromosomes stated above the, and features between different populations on this or that chromosome are noted.

X. the item of autosomes has no sexual distinctions, there are no proofs of its change with age. Family genealogical and twin analyses (see. The twin method, the Genealogical method) show that the options of chromosomes which are available for parents are transferred to children invariable. However there are rare deviations testimonial of the fact that emergence of new chromosomal options and their transfer to posterity counting on a species in general do not stop. As origins of new options serves the unequal crossing-over (exchange of sites) of homologous chromosomes in the field of heterochromatinic areas (see Meiosis, the Recombination).

Biological value X. the item it is not clear as remains insufficiently found out a biological role of heterochromatin. There are bases to assume that this part of a genome is important as for normal implementation of the genetic program of an ontogeny of an organism from the moment of its origin, and for adaptation of a species to the environment.

The increased frequency of options of chromosomes with large blocks of heterochromatin can be found in a part live-born with the multiple inborn malformations which are not connected from chromosomes -

Fig. 3. U-chromosomes of 5 individuals (Q-coloring): all individuals distinguish

sya length of its fluorescent heterochromatinic part.

ny aberrations (see. Chromosomal diseases), and also at a Down syndrome (see Down a disease), Shereshevsky's syndrome — Turner (see Turner a syndrome) and some other. Mechanisms of pathological effect of options of chromosomes, extreme on content of heterochromatin, remain obscure. Demand further accumulation and deepening our knowledge of the nature of such effect the carriage of extreme options of chromosomes could be considered as risk factor at medicogenetic consultation (see. Medicogenetic consultation). The individual uniqueness of chromosomal complement which is created thanks to X. the item, is used for distinguishing of cells of one individual from cells of another, during the carrying out prenatal diagnosis of chromosomal and hereditary diseases (see). Unusual options of chromosomes are used as genetic markers by means of which establish a parent origin of chromosomes.

Bibliogrbochkov N. P., Zakharov A. F. and And in and N about in V. I. Meditsinskaya of the geneticist, M., 1984; 3 and x and r about in A. F., etc. Chromosomes of the person, Atlas, M., 1982; Perspectives of medical genetics, under the editorship of N. P. Bochkov, page 94, M., 1982; Polymorphism of chromosomes at the person, under the editorship of A. A. Prokofieva-Belgovskaya and

A. F. Zakharov, M., 1981, bibliogr.; About - kofyeva - Bie lgovsky A. A. Heterochromatic regions of chromosomes: structure and functions, Zhurn. general biol., t. 38, No. 5, page 735, 1977; VermaR. S. and. D about s i k H. Human chromosomal hetero-morphiosms, Int. Rev. Cytol., v. 62, p. 361, 1980. A. F. Zakharov.