inheritance in genetics

From Big Medical Encyclopedia

INHERITANCE in genetics — transfer of genetic information at the level of a cell of a pla of a complete organism from parents or ancestors to children or descendants respectively; it is carried out by a feather of giving genes, i.e. material substrate of heredity — molecules DNA, from one generation to another. Reproduction of genes is connected with ability of DNA to doubling (replication) with the participation of specific proteins — enzymes. Rare «errors» of replication are fixed in structure of genes and further are reproduced in a new, mutant form (see. Mutation ).

All known modes of inheritance of signs — to dominant, autosomal and dominant, autosomal and recessive and linked to a floor are characteristic of the person (with X-chromosome). It belongs to inheritance discrete or the Mendelian characters caused by alleles of one locus — so-called monogenic inheritance. If variability of characteristic is continuous, then speak about multi - or polygenic inheritance.

At autosokhmno-dominant type H. when parents are homozygous, one — on the gene controlling development of a dominant character, and another — on the gene controlling development of a recessive character in firstgeneration from couple of alternative signs is shown only one — dominant. This phenomenon was called domination, and later — the first law of Mendel (see. Mendel laws ). So, the gene defining brown color of eyes at the person is dominant and suppresses effect of the recessive gene controlling blue color. If one of parents brown-eyed and gomozigoten on this gene (a genotype of AA), and another blue-eyed and gomozigoten on the recessive gene controlling blue color of eyes (a genotype aa), then all children in such family are heterozygous on the gene defining color of eyes and, therefore, brown-eyed (a genotype of Aa). If one of parents brown-eyed, but geterozigoten on the gene controlling color of eyes (a genotype of Aa), and another gomozigoten on a recessive gene and, therefore, blue-eyed (a genotype aa), then on average a half of children from such marriage is brown-eyed (a genotype of Aa) and a half — blue-eyed (a genotype aa). The probability of the birth and that, and other children is equal to 1:1. Thus, effects of a dominant gene are shown always, regardless of whether there is a gene in homozygous (AA) or heterozygous (Aa) a state. Recessive genes show the action only in a homozygous state.

By 70th 20 century at the person were known apprx. 700 normal and patol. signs, development to-rykh is controlled by this or that autosomal dominant gene. In particular, many hereditary anomalies of a skeleton are described, transfer to-rykh is from generation to generation caused by autosomal dominant genes (e.g., an achondroplasia, or the disproportional dwarfism which is characterized by strong shortening of extremities). Family trees with an autosomal and dominant mode of inheritance of a polydactylia (polydactylism, additional fingers) are known, to a brachydactyly (korotkopalost) and other anomalies of development.

The signs controlled by dominant genes rather easily are found, and it is easy to track transfer of genes in several consecutive generations. To reveal transfer to posterity of autosomal recessive genes much more difficultly since these genes show the effects only in a homozygous state. An example of an autosomal and recessive mode of inheritance of signs is albinism. From marriage between two albinos only children albinos since parents are homozygous on a gene of albinism are born (a genotype ss). From marriage between an albino and the person with normal pigmentation children albinos usually are not born. However if someone from spouses albinos geterozigoten on a gene of albinism (the genotype With c), then in a family on average will be a half of children albinos and a half — with normal pigmentation (a ratio 1:1).

More than 500 hereditary diseases are known, to-rye are controlled by autosomal recessive genes. If for it is prepotent the inherited diseases defeat of in a row several generations of one family is characteristic (N. down), then at recessive N. one or several children of healthy parents quite often suffer. At related marriages of externally healthy family members, in a cut the case, inherited on recessive type took place, the probability hereditarily of the caused diseases at children is high since both parents can be the hidden carriers of a mutant gene. At the same time the closer relationship between marrying, the is more danger of emergence in children various hereditary diseases (see). At to a dominant mode of inheritance at children both signs from parents are shown at the same time (see. Codominance ).

The inheritance of characters linked to a floor is caused by the genes localized in gonosomes — X or Y. An example of the disease linked to a floor is hemophilia. The recessive gene causing an incoagulability of blood is localized in X-chromosome.

At women it can be shown only in a homozygous state when both X-chromosomes have no normal dominant gene that happens seldom. Men have no second X-chromosome owing to what this recessive mutant gene shows the action and there is hemophilia. Daltonism, or color-blindness when people are not capable to distinguish nek-ry colors, mostly — red and green is other example of inheritance of the sign linked to a floor. It is caused by the mutant recessive gene localized in X-chromosome. At the person it is described apprx. 100 various diseases controlled by the genes localized on X-chromosome.

There is a number of the signs caused by genes, to-rye can be on autosomes or gonosomes of both floors, but these signs are shown only at one of them. Such signs are called the signs limited to a floor. Lactescence and fat content of milk at cattle belong to such signs, e.g. Genes controlling secondary sexual characteristics at the person and animals are available both at men's, and for female individuals, but their manifestation depends on effect of the corresponding sex hormones.

The nature of domination of nek-ry signs depends on a floor. So, the gene causing baldness is located in an autosome and is dominant at men, but recessive at women.

Such signs of the person as growth, weight, a constitution, resistance to diseases, heart rate, arterial and venous pressure, longevity (quantitative characters), are inherited as a result of total action of many genes of various loci. Most often the genes forming polygenic system separately give weak effect, and their total action is rather strong. E.g., the singing voice at the person substantially is defined by polygenic system what points the analysis of genealogical famous singers to. Formation of a singing voice depends on anatomic features of a thorax, force of respiratory muscles, lung volume as well as features in a structure of lips, language and a nasal cavity, to-rye are under control of the hereditary factors forming the polygenic system defining emergence of a beautiful singing voice in nek-ry cases.

Extent of manifestation of gene effects at polygenic N. in many respects depends on environmental factors. E.g., the individual having predisposition to leanness at excess food can surpass in weight the person with hereditarily the caused obesity if the last adheres to the corresponding diet.

There was an opinion that if the person made great progress in any area of the activity, then the abilities developed as a result of persistent work can be transferred it to children. However such views were not based on the scientific facts and had simplistic character; the personal qualities of the person acquired by it during life to be transferred but to inheritance cannot. The numerous facts of transfer from parents to children of certain bents, habits and skill in any area are connected with imitation of the child parents and the directed his education.

See also Genetics , Variability , Medical genetics , Heredity .

Bibliography: Barashnev G. I. and Veltishchev Yu. E. Hereditary diseases of a metabolism at children, JI., 1978, bibliogr.; N. P. Genetik's tanks of the person, M., 1978; Davidenko va E. F. and JI and e r m and N I. S. Klinicheskaya of the geneticist, JI., 1975, bibliogr.; Dubinin N. P. General genetics, M., 1976; McKusick V. Mendelian inheritance in man, Baltimore, 1978.

B. V. Konyukhov.