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 RB 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: 
t—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. 
t;—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. 
ts—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 
chromosome factor. 
Fig. 79.—Out- a 
line drawing of a b—a factor for black body color located in the second 
truncate-winged ¢hromosome. This factor has such an influence that 
Drosophila. (Af- 3 5 : 
ter Morgan.) 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. 
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