THE MANNER OF OCCURRENCE OF MUTATION 445 
POR U 
A. Manner of origin, and time of occurrence of the mutation 
This mutant gene causes the eye to be of a very light yellow 
color, perhaps most aptly characterized as ‘écru.’ This color 
may easily be mistaken for white if white-eyed flies are not 
present for comparison, but comparison readily shows that the 
‘éerw’ eyes are distinctly yellower than the white. On the other 
hand, they are very slightly lighter than ‘tinge’ and ‘buff,’ 
which were heretofore the nearest to white in the series. Ecru 
males and females are alike in color. 
Eeru Paeey first among the descendants of a cross of a 
red-ey ed ! a by red-eyed ee o (j—jaunty wings, S’—star eye, 
ee: all these factors are in chromosome II). In a cross 
of this sort certain of the offspring have the same composition 
as their parents, and these may be used to make another cross 
of a type exactly like that by which they themselves were pro- 
duced. Thus, flies of the same heterogyzous composition may 
be maintained and crossed generation after generation. In the 
present instance this process had been carried on for about five 
months (in order to maintain a stock containing the lethal), 
and during this time no other eye color than red appeared; but 
suddenly, in about the tenth generation, a single male fly was 
found with écru-colored eyes. That this male was a product of 
the cross, and not due to contamination from outside, was 
proved by crossing it to a dons when it gave the count to be 
4; 71 
expected of a male of composition 3 *. The mutant itself had 
star eyes, owing to the dominant factor 8’. The écru color in 
this cross proved to be recessive, as all of the offspring were red- 
eyed except one sterile écru-eyed son, produced by primary 
non-disjunction. In the second generation, produced by breed- 
ing together the offspring, half of the males and none of the 
females were écru, and the rest were red; this proved that écru 
was due to a single sex-linked factor. 
