LIMITATION OF THE LINKAGE GROUPS 135 



number of linkage groups approaches so near the number 

 of the chromosome pairs. In the snapdragon, Baur has 

 described two linked groups. He states that there are 16 

 pairs of chromosomes. In wheat one linked group has 

 been described. There are 8 pairs of chromosomes (Fig. 

 53, b). In Indian com there appear to be a few linkage 

 groups, and probably 10 pairs of chromosomes. In oats, 

 Surface finds two linked genes. In Primula there is one 

 group composed of several linked genes, and 12 pairs of 

 chromosomes (Fig. 53, c). 



In the silkworm moth one linked group of genes has 

 been found by Tanaka, and Yatsu records (Fig. 55) 20 

 pairs of chromosomes. In DrosopJiila virilis three linked 

 groups of genes have been found by Metz, who has also 



^ 



1** 'A 



Fig. 53. — Chromosome group of pea, a, wheat, 6, and primula, c. 



described six pairs of chromosomes for this fly. In Droso- 

 phila buscJcii there is one group of linked genes and four 

 pairs of chromosomes. In D. repleta one group, and six 

 pairs of chromosomes. The groups of chromosomes in 

 some of the different species of Drosophila, as described 

 by Metz, are shown in Fig. 54. As indicated by the 

 arrangement of the figures (that correspond fairly closely 

 with the actual arrangement of the chromosome in the 

 cells themselves) it appears that one pair of chromosomes 

 in one species is at times represented by two pairs in 

 related species, and this view is borne out by the attach- 

 ment of the spindle fibre to the middle of the chromosomes 

 in the bent pairs, but to the inner ends of the two that 

 supposedly correspond to its halves in other species. 



In the mouse one group of linked genes has been 

 reported. There are 20 pairs of chromosomes (Fig. 55, fc) . 

 In man no linked genes are known, if we do not count sex- 



