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scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
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genomethe overall genetic constitution of an organism/species as constituted by the chromosomal makeup of every member (the term arises from a combination of the terms ‘gene’ and ‘chromosome’). See also GENETICS, HUMAN GENOME PROJECT.
a haploid chromosome set; the sum total of the genes localized in the individual set of chromosomes of a given organism. The term was introduced in 1920 by the German biologist H. Winkler. The genome is usually understood to mean the sum of the genes concentrated in the chromosomes, without taking into account the hereditary determinants connected with the structures of cytoplasm. In the gametes of diploid organisms, as well as in the cells of haploid organisms, there is one genome; in the somatic cells of diploid organisms there are two genomes. With an increase in ploidy of the cells, the number of genomes grows. In fertilization, the genomes of the paternal and maternal gametes unite. As a rule, genomes obtained from paternal and maternal gametes are homologous. Homology between all or some genomes exists in all but distant hybrids. Absolute homology of two genomes may be defined as the coinciding of the linear arrangement of the genes in every chromosome. This coinciding ensures the possibility of the normal conjugation of chromosomes in meiosis.
Changes in the number of chromosomes (for example, polyploidy or an increase or loss of individual chromosomes) are called genomic mutations. An organism in which one and the same genome is repeated several times is called au-topolyploid. An organism in which different genomes are combined is called allopolyploid. Wheats may serve as an example of the allopolyploids. The haploid number of chromosomes in hard wheat is 14, in soft wheat, 21; and in einkorn, seven. By hybridization and the study of the conjugation of chromosomes in meiosis, it was ascertained that in all wheats there is one common genome (genome A) made up of seven chromosomes. In hard and soft wheat there are two common genomes (genomes A, B), each having seven chromosomes. Finally, in soft wheat, there is still another special genome (genome D), also made up of seven chromosomes. Thus, the genomic formula for einkorn is AA and for hard wheat it is AABB. Soft wheat, which has arisen in the process of evolution by the crossing of three different wild cereals and by the doubling of the number of chromosomes in the hybrids, has the genomic formula AABBDD.
In the experiments of the Soviet geneticist G. D. Kar-pechenko, radish and cabbage genomes were combined for the first time in a hybrid organism. By means of corresponding crossings and cytological analysis, the origin of separate genomes can be established. For example, V. A. Rybin obtained (resynthesized) a cultivated plum by crossing the cherry plum and the sloe; thus it was established that the genome of a plum includes cherry plum and sloe genomes. Of great significance in understanding the structure and functioning of the genome have been the establishment of the structure of the molecules of nucleic acids (DNA and RNA) and of the mechanisms of their replication, and the establishment of the means of “tracing” and transmitting genetic information.
V. N. SOIFER and V. V. KHVOSTOVA