crossing in genetics

From Big Medical Encyclopedia

CROSSING in genetics — receiving posterity from parents of the known and (or) studied genotype for the purpose of definition of patterns of inheritance of the studied signs, localization of the genes controlling these signs, patterns of manifestation of genes, their interaction with other genes of a role of environmental factors in formation of signs, etc. S. is an obligatory element of all types of the genetic analysis of the organisms breeding sexually or having any analogs of a syngenesis (see. Genetic analysis ). The term «crossing» on sense is close to the term «hybridization». Sometimes it is used as a synonym of the term «cultivation» (animals or plants), meaning at the same time the types and systems crossing used for obtaining set of organisms with certain genetic features. In this sense closely related cultivation — inbreeding (see) — call closely related crossing, and outbreeding — unrelated crossing. Selective S. phenotypical of similar individuals in experimental genetics and selection call positive, and dissimilar — negative * assortativny crossing.

In all fields of modern experimental biology and medicine (genetics, immunology, embryology, oncology, pharmacology, etc.) S. in combination with artificial selection (see) — selection — is the most important, almost only method of receiving homogeneous experimental material with the set hereditary properties.

Development and improvement of methods C. of plants in 18 — 19 centuries formed a basis for formation modern geneticists (see). The main merits in this area belong to J. G. Kolreuter, G. Mendel, etc. At the beginning of 20 century with development of biometrics by mathematicians and biologists by Pearson (To. Pearson), R. A. Fisher, Meyter (To. Mather), etc. were developed by Yu. A. Filipchenko, A.S. Serebrovsky the main methods of the analysis of results of crossing.

S.'s use in to genetics (see) demands exact knowledge of belonging of all individuals to certain generations. Initial generation usually call parent and parents designate a letter P (Latin of parentes). The direct posterity of parental generation is called hybrids of the 1st generation and designate F 1 (Latin filia children). The succeeding generations received from crossing among themselves of individuals of hybrids of the previous generation designate F2, F3 etc. (see. Hybridization ).

At S. it is possible to trace manifestation in generations of different number of signs. Depending on it distinguish crossings mono - di - three - and polyhybrid (see. Mendel laws ).

When the purpose C. is studying of inheritance of a sign (or signs), the studied genotype inherent to all individuals (to the line, grades, breeds), groups of the studied individuals with group of individuals of the testerny line apply S., without subdividing posterity on parent couples (so-called mass crossing). Is called the Testerny line or the line tester genetically homogeneous, usually inbredny, set of the individuals possessing signs (markers), convenient for the genetic analysis, patterns of inheritance to-rykh are well-known. If the studied sign is available (or it is supposed) for separate individuals, resort to individual S. and the separate analysis of posterity from each couple of parents.

For definition of dependence of inheritance of characters on a floor experiments on S. put in two directions, e.g. AA X aa and aa the X AA. Such inverse crossings call reciprocal, at the same time conditionally designate one of S. as direct, and another — as the return.

Nek-ry problems of the genetic analysis are easier solved in the analysis of the studied signs given about manifestation not in hybrid generations of F2, F3 etc., and in posterity from returnable S. of hybrids (a so-called bek-cross-country) with a parent form. The posterity from returnable S. is designated by Fb (English back back). At the same time most informatively S. of hybrids with a recessive parent form which received the name of analyzing.

All individuals of each generation (F1, F2, Fb, etc.) who showed the studied signs in certain combinations are completely considered, and the observed number of all classes of posterity is compared with the number expected them according to this or that hypothesis of inheritance of these signs (see. Mutational analysis , Recombination analysis ). At the same time only after direct S. of homozygotes (see. Genotype ) all individuals in posterity will be uniform on the corresponding signs. It is the first law of Mendel, the law of domination or uniformity of hybrids. In all other generations observe splitting of posterity on the studied signs in certain numerical ratios. E.g., during the crossing of mice albinos (the recessive character, is shown only at inheritance of a gene of albinism — with from both parents, i.e. a genotype of albinos ss) with mice of gray coloring of wild type (a genotype of CC) all individuals of Fx will have a genotype With with and respectively gray coloring. At an accidental combination of genes the Dream in equal shares a genotype of CC, Ss, ss and ss will occur among their posterity (F2) that on average there correspond to the relation 3 gray: 1 albino. In F b from Ss of X analyzing skreshchivashsh ss a half of individuals will have a genotype of Ss, and other half — ss.

It is essential that numerical ratios of classes at filial segregation in S.'s generations belong to statistical regularities, i.e. are observed rather precisely only in the big and not displaced (accidental) selections. Practically in any selection of the average sizes (several tens — several cells of individuals) observed the number of classes during the splitting always a little deviate from the expected chislennost. For check of the statistical importance of such deviations usually use criterion of X 2 (see. Biometrics ).

In pilot studies often there is a need to transfer a gene from one genetic background on another, napr, for receiving so-called kongenny lines of the animals containing different in the genotypes alleles (see) certain a gene, but extremely close on all other genotype. In such cases resort to a series of repeated so-called absorption crossings of the received posterity with the same repeatedly (recurrently) crossed parent form.

During the studying of heredity of the person (see. Heredity ) systematic S. ethically is unacceptable. Serves as an analog of a method C. in genetics of the person genealogical method (see), i.e. search in population of marriages with the combinations of signs interesting the researcher at parents and tracing of such signs in posterity.

Bibliography: Gaysinovich A. E. Origin of genetics, M., 1967, bibliogr.; Gershenzon S. M. Fundamentals of modern genetics, Kiev, 1983, bibliogr.

V. I. Ivanov.