168 GENETICS IN RELATION TO AGRICULTURE 



glabrous, those whites which tinge on fading are hairy and those which 

 show no sign of coloration on fading are glabrous. The apparent diffi- 

 culty is therefore merely due to the fact that some plants which possess 

 C and R are still white on account of the action of other factors. 



Altenburg and Muller's Truncate -winged Drosophila. An even 

 more complicated case of factor interaction is that concerned in the 

 production of truncate wings in Drosophila (Fig. 79). The factors 

 here involved appear to be the following: 



I a factor for truncate wings. It is a recessive factor located in the 

 second chromosome, and without this factor the truncate 

 wing character cannot appear. 



ti a factor which intensifies the expression of the 

 truncate wing character, but which is not absolutely 

 essential. This factor is located in the first chromosome. 

 3 another factor which intensifies the expression 

 of the truncate wing, but is not absolutely essential to it. 

 B' the dominant factor for bar eyes which in ad- 

 dition acts as an intensifier of truncate. This is a first 

 FT 70 o chromosome factor, 

 line drawing of a b a factor for black body color located in the second 

 truncate- winged chromosome. This factor has such an influence that 

 terMffrgan.) flies of the constitution (bT)(bt} or even (Bt)(bT) may 



display the truncate wing character. 



The truncate wing character was particularly baffling on account of 

 the extraordinary relations which it displayed both in hybridization and 

 in selected strains. In hybridization instead of a 3 : 1 ratio of long to 

 truncate wing the ratio was about 7 : 1 and in selected strains even after 

 100 generations of selection there were still about 5 per cent, of long winged 

 flies. That these long winged flies were different genetically from 

 the truncate winged flies was shown by breeding tests for in such tests 

 they did not produce as high a percentage of truncate winged flies as 

 did those which had truncate wings. By means of linkage relations, 

 however, it was possible to determine the factors concerned, and their 

 specific effects. Particularly noteworthy is the fact that the factor B' 

 for bar eyes acts as an intensifier for truncate, thus providing an analo- 

 gous case to that in stocks where the color factors are necessary for the 

 action of the factor for hairiness. No less interesting is the affect of b, 

 for it was found that this factor, whether homozygous or heterozygous, 

 changed the dominance relations in the allelomorphic pair Tt, so that 

 the truncate wing character is expressed in such individuals when hetero- 

 zygous for t. Furthermore since truncate appears more readily in the 

 female than in the male it would appear that the sex factors also act as 

 intensifiers. 



