Induced Chromosomal Changes and Their Significance in Growth and Development

Abstract

OF all the structures of the cell the chromosomes have unquestionably attracted the major attention of investigators principally because it was early demonstrated that they are in some way associated with heredity. The idea that the hereditary units were contained in the chromosomes had been presented in the writings of Weismann, Boveri, Strasburger and Hertwig before the rediscovery of Mendel's Laws in 1900, and was developed further by Cannon and especially by Sutton. Sinee then, overwhelming evidence has accumulated in support of the chromosome theory of heredity. Boveri, and especially Morgan and Bridges, have proved that every chromosome is an individual in the sense that it carries a definite complex of genes. The theory of the linear arrangement of the genes within the chromosomes, formulated by Sturtevant, has become a fundamental concept of modern genetics and cytology. Every individual, race or species has a relatively constant genotype, that is, the sum total of the hereditary potentialities, inherited from its ancestors. The stability of the genotype is responsible for the persistence of the visible traits of the organism. The genes, however, are carried in the chromosomes and every organism is characterized by having a definite number of chromosomes, and every chromosome bears a definite complex of genes arranged in a fixed linear order. These stable structural units constitute the karyotype. A chromosome, as well as a gene, is a self-duplicating body and is potentially able to duplicate itself exactly an indefinite number of times. Nevertheless, the constancy of the karyotype, like that of the genotype, is subject to certain variations. The chromosome structure is subject to change, and an altered