52S 



THE AMERICAN NATURALIST [Vol. XLVIII 



and of 12 units. The data from the coupling experiments 

 (which even though less extensive then those from the 

 repulsion experiments are probably more accurate) give 

 a value of about 18 units. Since the repulsion data give 

 10 units, 18 is probably too high, and an intermediate 

 position correct. The higher (12) of the two possible 

 values is then the correct value. The position at 10 is not 

 excluded by these data, but is far less probable. In a case 

 in which one of the two first values is very small, as here, 

 the accuracy demanded of the remaining or third value is 

 much greater than in cases where neither of the values 

 are small, and one has only to decide between two very 

 different values by aid of the third. There are other ways 

 of arriving at this order of genes which are independent 

 of the size of the values. One of those methods, such for 

 example, as that of double crossing over, would definitely 

 settle the order of these three genes, but unfortunately 

 such data have not yet been published. 



If hooded lies beyond red at 12, the complete first chro- 

 mosome diagram will be as follows : 



R R H 



In the above diagram R indicates the locus of round 

 (and also of long). The symbols in the diagrams are 

 used to designate loci which may be occupied by either 

 allelomorph of the pair. 



It has been observed that hooded flowers have always 

 a uniform color in standard and wings, instead of having 

 these two regions colored differently as in the normal or 

 bicolor type. Bateson assumed that this unicolorism was 

 only another somatic effect of the hooded gene. However, 

 an alternative explanation is that the unicolor is caused by 

 a specific gene which is very closely linked to hooded. If 

 this should be found to be the case, then this fourth gene 

 also will be located at about 12 units from round. 



