36 THE THEORY OF THE GENE 



present between the members of certain pairs — differ- 

 ences in shape, or in the attachment to the spindle fibers 

 (Fig. 23). When the germ-cells mature these chromo- 

 somes conjugate and then separate. Since they retain 

 their individual differences, they can be traced to the 

 poles. 



Now in these grasshoppers there is, in the male, an un- 

 paired chromosome that is connected with sex determina- 

 tion (Fig. 23). It passes at the maturation division to one 

 pole or to the other. It serves as a land-mark for the 

 other pairs of chromosomes. Miss Carothers, who first 

 made these observations, found that a marked pair (one 

 bent, one straight) separates at random with respect to 

 the sex chromosome. 



Carrying the matter further, other chromosome pairs 

 were found to show at times constant differences in some 

 individuals. A study of these chromosome pairs at reduc- 

 tion has shown, again, a random distribution of the mem- 

 bers of the pairs with respect to one another. Here then 

 we have objective evidence of the independent assort- 

 ment of the pairs of chromosomes. This evidence paral- 

 lels Mendel 's second law, which calls for independent dis- 

 tribution of the members of different linkage groups. 



The Number of the Linkage Groups and the 

 Number of the Chromosome Pairs. 



Genetics has shown that the hereditary elements are 

 linked in groups, and in one case with certainty, and in 

 several other cases with some probability, there is a defi- 

 nite and fixed number of these linkage groups. In Droso- 

 phila there are only four such groups, and there are four 

 pairs of chromosomes. In the sweet pea there are seven 

 chromosome pairs (Fig. 24), and probably seven inde- 

 pendent pairs of Mendelian characters have been found 

 by Punnett. In the edible pea there are also seven pairs 



