DUNN 



rate may become associated. This 

 change in the relationships of genes is 

 known as "crossing-over" and it pro- 

 vides a quantitative measure of the 

 strength of the tendency toward asso- 

 ciation. In terms of the chromosome 

 hypothesis it is interpreted as an inter- 

 change of parts and of the genes which 

 the parts carry, between two members 

 of a chromosome pair, so that two 

 genes originally resident in one chro- 

 mosome may come to lie in two chro- 

 mosomes and may thence be distrib- 

 uted to separate gametes and exhibit 

 their effects (unit characters) in differ- 

 ent individuals. For any two characters 

 the number of times crossing-over 

 occurs is found to have a characteristic 

 value and this value is stated as the 

 percentage of times crossing-over 

 occurs as evidenced by the frequency 

 of individuals possessing the two char- 

 acters in the new combination. One 

 other important aspect of these meas- 

 urable breaks in linkage is that from 

 the linkage strength may be inferred 

 the proportional distance apart of 

 linked genes. From cytological evi- 

 dence crossing-over is supposed to 

 take place between homologous chro- 

 mosomes in the hybrid at the time 

 when these chromosomes are inti- 

 mately twisted one about the other. 

 Breaks resulting in a separation of 

 characters are then supposed on mere 

 physical grounds to be more frequent 

 between genes located far apart in the 

 chromosomes than between those lo- 

 cated near together. The bulk of the 

 evidence indicates that the loci of 

 genes are on the same straight line in 

 any chromosome. Numerical strength 

 of linkage may then be a measure of 

 the exact localization of the genes in 

 the germ plasm, and it is to a consider- 

 ation of this point that our whole 

 discussion has led. For if the genes for 

 unit characters can be thus localized, 



95 



a direct comparison of species in 

 which similar variations occur can be 

 made on this point alone, even though 

 the species cannot be crossed. 



The study of localization of the 

 genes for unit character variations is 

 attended bv^ numerous limitations. It 

 can only be prosecuted through the 

 experimental breeding of large num- 

 bers of organisms, exhibiting numer- 

 ous variations. It is dependent even 

 under these conditions on the occur- 

 rence of linkage, which is by no means 

 common. It is a corollary of the loca- 

 tion of genes in chromosomes, that the 

 numbers of groups of linked genes be 

 equal to the number of chromosome 

 pairs present. Where the number of 

 chromosomes is large, and the number 

 of unit variations known is small, the 

 chances are few that any two char- 

 acters will be found to be localized in 

 one chromosome pair. Even under 

 such limitations, linked genes have 

 been studied in several insects (chiefly 

 Drosophila) and plants, and most re- 

 cently in mammals. The general results 

 of these studies have been to confirm 

 the chromosome theory and to in- 

 crease our knowledge of the localiza- 

 tion of genes. 



Correlative evidence has come from 

 a brilliant series of cytological inves- 

 tigations on the germ cells of several 

 organisms. It has been established that 

 in the cells of each species are to be 

 found a definite number of chromo- 

 somes, characteristic for the species. 

 This number in germ cells is half the 

 number found in the somatic cells, due 

 to the intervention of reducing cell 

 divisions. The chromosomes them- 

 selves are in general arranged in pairs 

 of homologues in the somatic cells and 

 in the primordial germ cells, one mem- 

 ber of each pair having come from 

 each parent, and this duality^ again 

 becomes evidenced in the passage of 



