TYPES OF FACTOR INTERACTIONS 169 
The important point involved in this case, however, is the ingenious 
way in which the investigators made use of the linkage relations and the 
known fact that crossing-over does not occur in the male in order to study 
these factors, particularly with reference to their constancy, since they 
are variable in phenotypic expression. They took a truncate male which 
9 (bT) (pT) X 
oi (67) (bT) (pT s)XX 
(bT) (pT3)X black pink @ 
long 
(bT) (bT) (Pts) (pT s) XX 
(DT) (Pi3)X black red @Q 
long 
(Bt) (6T) (Pts) (pT 3) XX 
(Bt) (pT s)X Gray pink @ 
long or truncate 
(Bt) (6T) (Pts) (pT 3)XX 
(Bt) (Pts) X Red gray @ 
long or truncate 
(bT)(bT) (pT's) (pT s)XY 
(bT) (pT3) Y Black pink 7 
long 
(bT) (bT) (Pts) (pT'3)X Y 
(bT) (Pts) Y Black red & 
long 
(BT) (bT) (pT 2) (pT s)XY 
(Bt) (pT s)¥ Gray pink 7 
long or truncate 
: (BY (bT)(PT 3) (pT) XY 
(Bt) (Pts) ¥ Gray red J 
long or truncate 
Fia. 80.—Checkerboard analysis of F2 generation obtained by mating an Ff; male Droso- 
phila of the constitution (b7) (Bt) (p7'3)(Pt3)X Y with a pink black long female. 
contained the truncate factor and also the truncate intensifier of the 
third chromosome and mated it to a long-winged black-bodied female with 
pink eyes. The genetic constitution of the truncate male with respect 
to the factors involved was (Bt)(Bt)(Pts) (Pts) X Y, and the contrasted black 
female was (bT)(bT)(pT's)(pTs)XX. A male from such a cross is of the 
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