Linkage Data. 21 



LINKAGE IN GROUP I, AND CONSTRUCTION OF 

 X-CHROMOSOME MAP. 



(Figure 7.) 



In a previous paper (Metz, 1918) the linkage of the 8 characters 

 yellow, frayed, vesiculated, hairy, magenta, forked, rugose, and 

 glazed was considered, and a "chromosome map" constructed with 

 the genes placed in the order given. Two of the genes were ' ' located ' ' 

 only provisionally — frayed because the stock was lost, and hairy 

 because of insufficient data. The position of frayed (with reference 

 to yellow) remains uncertain; but that of hairy is now known to be 

 to the "left" (i. e., above) instead of to the right of magenta (Wein- 

 stein, 1920, and present data). Weinstein (1. c.) has "located" 

 the gene for the character crossveinless at 17.6, or about 2 units 

 to the left of vesiculated. 



In the present paper the genes for the new characters are "located" 

 with reference to those already known, and additional data are given 

 on several of the latter. Unless otherwise specified the new data in 

 the tables include only the male offspring from the various matings. 



In this connection we would emphasize the fact that we have made 

 no attempt to carry out a detailed and exhaustive study of linkage, 

 even in the sex-linked group of characters. This would represent a 

 separate study, aside from the purpose of the present work, and one 

 which can be carried out much better after more mutant characters 

 are obtained. It will be noted, therefore, that numerous experiments 

 which would be of value for a critical study have not yet been made. 



Since chromosome "maps" are now usually placed vertically, 

 instead of horizontally, on the page, the terms "above" and "below" 

 are here used in place of "left" and "right," respectively. 



The data from linkage experiments on sex-linked characters are 

 given in full on pages 78 to 88 (experiments 1 to 53) and are summar- 

 ized in table 2. These are arranged according to the number of genes 

 concerned and the order of the loci on the chromosome map, beginning 

 with the "zero end." Thus the first experiments are those involving 

 2 pairs of genes, then those involving 3 pairs, and so on; and in each 

 series the uppermost loci are considered first. The summary in 

 table 2 includes the calculated cross-over percentages for successive 

 regions in each experiment. 



The present section deals separately with the linkage data on 

 each successive gene, and indicates the method and data used in 

 "locating" the genes and constructing the map. The data are 

 included, in summary form, in table 3, although not all of the data 

 in this table are used in constructing the map. For the latter 

 purpose, an effort has been made to use the experiments considered 

 most reliable, and particularly those involving adjacent loci. By 



