§36 
GENETICS: A. H. STURTEVANT 
Proc N. A. S. 
this case a section from near the middle of an X- chromosome of D. 
melanogaster appears to have broken loose and attached itself to 
the left-hand end of a normal X-chromosome. Weinstein 7 has shown 
that such an occurrence might lead to a change of sequence of 
identical loci such as is here reported. If we suppose that the simulans 
third chromosome was originally constituted as is that of melanogaster, 
the situation as we now find it may be supposed to have arisen as follows : 
A section, including the peach locus, broke loose and attached itself near 
the right-hand end of a normal third chromosome. After this condition 
had become established the peach locus near the middle of the chromosome 
mutated or became ' ' deficient," so that in effect the peach locus was 
moved to the right end of the chromosome. Such an interpretation will 
account for the observed facts. 8 
There is, however, another possible method whereby the same result 
might be supposed to have been brought about, viz., by the simple in- 
version of a section of a normal chromosome. Such an accident seems 
not unlikely to occur at the stage of crossing over. If we suppose a chro- 
mosome to occasionally have a "buckle" at a crossing over point, it is 
conceivable that crossing over might be followed by fusion of the broken 
ends in such a way as to bring about an inversion of a section of chromosome. 
Either of the two suppositions discussed will account for the observed re- 
sults, but they should lead to different relations for other loci in the same 
chromosome ; it is hoped that further work will lead to the disco veryof ad- 
ditional parallel mutations, so that the maps may be studied in more detail. 
If an inversion of the kind suggested above occurred within a species, 
then individuals bearing one normal chromosome and one chromosome 
with an inverted section would probably show no crossing over in the 
region in question, since it seems probable that synapsis in this region 
would be abnormal or absent. It would also be not surprising if crossing 
over in adjacent regions was decreased. But individuals homozygous 
for the inverted section would be expected to show free crossing over 
again, since there should now be no difficulty at synapsis. 
The relations indicated are those that have actually been found in the 
cases of the two "crossover genes" in melanogaster known as C HI * and 
C//r- 10 These "genes" both cause, in individuals heterozygous for them, 
the disappearance of crossing over in the immediate regions where the 
"genes" themselves lie, and a considerable reduction of crossing over in 
neighboring regions. In individuals homozygous for either of these 
"genes," however, the percentage of crossing over rises to (or beyond) 
that found in "normal" individuals. Experiments are now under way 
in an attempt to determine if these "genes" are really simply inverted 
chromosome sections, but it will probably be a long task to definitely 
settle the matter. 
