CONTINUITY OF THE RACE 

 I 



585 



Fig. 24-4. The linear arrangement of the genes on the chromosomes has been worked out for a few animals, among 

 them Drosophila. This has been accomplished in two ways, one by employing cross-over technics and the other by 

 actual observation, the latter method being possible only because of the tremendous (1000 to 2000 times normal) 

 size of the chromosomes in the salivary gland cells. A portion of the left end of the X-chromosome is shown here 

 with a few genes indicated as to observable position on the chromosome and as computed by cross-over studies. 



new combinations that were not present 

 in either parents or grandparents, that is, 

 rough white and smooth black. This is the 

 Law of Independent Assortment. 



The significance of Mendel's work is dif- 

 ficult to estimate because it was tlie starting 

 point from which geneticists were able to 

 delve into the more precise nature of the 

 mechanism of inheritance. Let us now view 

 these discoveries in the lio;ht of informa- 

 tion that has come to us since Mendel's 

 time, for we now understand what he did 

 not know, namely, what goes on within the 

 cell that is responsible for these ratios. 



PHYSICAL BASIS OF 

 INHERITANCE-THE GENE 



From the very beginning of serious study 

 of genetics, scientists concentrated on the 

 nuclei of sex cells for an explanation of in- 

 heritance. In 1902, W. S. Sutton and C. E. 

 McClung in this country, and Theodore 

 Boveri in Germany, decided that hereditary 

 factors within the chromosomes were re- 

 sponsible for heredity. In 1911, another 

 American scientist, Thomas Hunt Morgan, 

 conceived the idea of genes, lying in a 

 linear fashion on the chromosomes. He was 

 convinced of this because whenever he 

 saw irregular behavior in the inheritance of 



certain traits in his Drosophila (fruit fly) 

 cultures he also noted irregularities in their 

 cliiomosomes and, conversely, any devia- 

 tion in the chromosome pattern was re- 

 flected in abnormal flies. From this informa- 

 tion Morgan and his associates were able 

 to construct chromosome "maps" showing 

 rather definitely where the genes lay with 

 respect to one another on the four chromo- 

 somes in Drosophila. Today, maps have 

 been constructed for several species of 

 Drosophila (Fig. 24-4), for several plants, 

 the most complete of which is corn, and for 

 certain chromosomes of mice and poultry. 

 Even the sex chromosomes of man have 

 been mapped. 



Even in very early studies many efforts 

 were made to see the genes within the 

 chromosomes. As early as 1881, Balbiani in 

 Italy pointed out the giant chromosomes in 

 the salivary glands of certain flies, although 

 at that time he had no interest in the me- 

 chanics of inheritance. In recent years, 

 however, these cells have been studied 

 again in the light of present knowledge of 

 Si;enetics. The chromosomes in these cells 

 appear under the light microscope as rib- 

 bons of alternately dark and light discs or 

 bands (Fig. 24-5). Because of their great 

 size in comparison to most chromosomes, it 

 is possible to distinguish the characteris- 



