LINKAGE, CROSSING-OVER, AND THE GERJM PLASM 419 



Confirmatory evidence of the validity of the theory of linkage 

 comes from the comparative study of other species of Drosophila, some 

 of which have the same number of chromosomes as has D. melanogaster, 

 others of which have a larger number. In one species that has four 

 pairs of chromosomes, like the original species, only four linkage groups 

 have been found, while in other species in which an extra pair of 

 chromosomes has been found there is a fifth linkage group. Compara- 

 tive studies upon the linkage groups and the kinds of genes in these 

 linkage groups have revealed a striking parallelism between the differ- 

 ent species and a beautiful conformity between the numbers of chromo- 

 somes and the number of linkage groups. Also it should be said that 

 the relative numbers of genes discovered is in a rather definite propor- 

 tion to the size of the chromosomes. 



CROSSING-OVER 



All of our studies of the mechanism of heredity up till now have 

 led to the conclusion that chromosomes are very definite and individual 

 structures that continue from generation to generation intact and are 

 passed as wholes from parent to offspring. We have spoken of the 

 process of pairing of homologous chromosomes in synapsis as though 

 this pairing were no more ultimate than a mere temporary embrace. 

 We have spoken as if, during the reduction division to form gametes, 

 the homologous chromosomes merely part company and proceed intact 

 to opposite poles of the dividing cell and enter separate gametes un- 

 affected by having associated in the embrace of synapsis. That this 

 is far from true has been revealed by an exact numerical study of the 

 varying degrees of linkage in the characters whose genes are supposed 

 to be located in a single member of a given chromosome pair. On the 

 basis that a chromosome is an inviolable body proceeding as a whole 

 from generation to generation, we should, of course, expect any two 

 characters that were once represented by genes in the same chromo- 

 some to stay together perfectly, i.e., always to appear together in the 

 same individual. The fact that this result was not realized led to 

 further advances in our understanding of the complex heredity ma- 

 chine. Let us see just how linkage works out with certain genes in 

 the X-chromosome. Remember that each of the characters was 

 located in the X-chromosome because each one by itself followed the 

 mode of heredity of a sex-linked character. 



The mode of linkage of two sex-linked genes. — The body color 

 called "yellow" and the eye color called "white" have already been 



