636 GENETICS: A. WEINSTEIN Proc. N. A. S. 
TABLE 6 
Coincidence: in D. Virius 
REGIONS 
COIN- 
CIDENCE 
SOURCE OF DATA 
Yellow crossveinless and crossveinless vesicu- 
lated 
0 
This paper, table 1 
Yellow crossveinless and crossveinless magenta . . 
0 
848 
This paper, tables 2, 3B 
Yellow vesiculated and vesiculated magenta. . . . 
0 
977 
Metz, 1918, tables 11, 19, 22 
Yellow crossveinless and magenta forked 
1 
4 
This paper, table 2 
Yellow vesiculated and magenta forked 
0 
989 
Metz, 1918 table 19 
Yellow crossveinless and forked rugose 
0 
795 
This paper, tables 2B, C 
Yellow vesiculated and forked rugose or forked 
glazed 
0 
590 
Metz, 1918 tables 20, 21 
increases and then decreases as the regions considered become separated 
farther. This agrees very closely with the conditions shov^n to exist in 
the X chromosome of D. melanogaster by Muller (1916)^^ and Wein- 
stein (1918).^ A similar rise and fall of coincidence have been demon- 
strated for the second chromosome of D. melanogaster by Bridges'^ and 
for the third chromosome by Gowen (1919).^^ 
The Homology oj Apparently Similar Factors. — Although the evidence 
so far obtained favors the homology of the yellow crossveialess forked 
series in D. virilis and D. melanogaster, it should be kept in mind that 
the data are by no means sufficient to make a homology certain for 
it is known that similar somatic effects may be produced by non- 
homologous genes. Thus in melanogaster there are mutant characters 
that resemble each other in appearance but are due to mutant genes in 
different loci; for example, the various eye colors resembling pink (ruby, 
garnet, purple, maroon, claret, etc.) and the various body colors resembling 
black (sable, ebony). In some cases two such "mimic" genes have been 
found to be very close together in the same chromosome, as in the case 
of miniature and dusky, which are not more than 1.8 units apart. It is 
obvious that a miniature mutation in virilis, even if it showed closely 
similar linkage relations, could not be definitely homologized with either 
of these two genes. A yellow virilis male with miniature wings did, indeed, 
occur in a cross of a yellow female by a wild- type male; and this miniature 
variation, if a mutation and recessive, must have been sex-linked. Un- 
fortunately it proved sterile and its linkage relations could not be tested. 
Since mutations that have similar somatic effects may occur in homologous 
chromosomes of the same species and occupy non-homologous loci, it is 
certainly hazardous to take it for granted that mutations of similar effect 
