180 BOTANICAL GAZETTE [SEPTEMBER 
MULLER and ALTENBURG,S who have conducted a critical examination of 
the fruit-fly for mutations occurring on the first and second chromosomes, 
state that the vast majority of mutations have a lethal or semilethal effect 
when present in the homozygous (recessive) condition. It is obvious, there- 
fore, that a critical search for mutations must involve a very special technique. 
MULLER is in possession of this technique through his intimate knowledge of 
the linkage groups on the chromosomes in question, and his ability to detect 
the absence of certain expected classes. On the sex chromosome he uncovered 
the startling fact that 50 per cent of the mutations were located in a restricted 
region at one end of the chromosome, which amounted to about 2 per cent of 
its length as charted from cross-over values. It is an open question whether 
this indicates a highly mutable region of the chromosome, or whether cross- 
over values are an inaccurate index of length. 
The most promising phase of MULLER’s work arises from this critical 
study of the rate of mutation. Considering the whole length of the sex chro- 
mosome, one mutation occurs in 106 gametes. For the second chromosome 
the corresponding value is one in 175 gametes. ZELENY states that mutation 
is as frequent in one sex as in the other. Having established these constants, 
MULLER is now investigating the possibility of modifying the normal rate of 
mutation. Already he has been successful in depressing the rate one-half by 
means of low temperatures. Eventually such work may be of great practical 
value. A knowledge of the conditions necessary for the maximum rate of 
mutation should enable the pedigree culturalist to achieve much more rapid 
results than otherwise. 
2. DEFIcreNcy.—A rare phenomenon is described by BripcEs,® working 
on the fruit-fly. This is more extensive than a simple locus change, being 4 
“regional mutation,” a loss or “inactivation” of a portion of a chromosome. 
LIC B 
resulted in the appearance of an extra piece of chromosome which duplicates 
in content a known region of the sex chromosome. 
4. NON-DISJUNCTION.—This phenomenon, made famous through the 
classic work of BRIDGES on the sex chromosome of the fruit-fly, may prove to 
' be a fairly common occurrence. In an irregular reduction division one of the 
chromosomes fails to ‘‘disjoin”’ seu from its mate. As a result, one or 
two gametes are formed with an extra chromosome, and others which lack 
this chromosome. The mating of one of the former with a normal gamete 
would produce a zygote with an extra chromosome. BLAKESLEE, BELLING, 
5 Mutter, H. J., and Atrensure, E., A study of the character and mode of 
origin of eighteen mutations in the X-chromosome of Drosophila. Anat. Rec. 20: 
213. 1921. 
6 Bripces, CaALvin B., Vermilion-deficiency. Jour. Gen. Physiol. 1:645-656- 
1919. : 
