Referate. 373 
heterozygous state. In the first paper mentioned above Emerson tenta- 
tively accepts Shull’s hypothesis and expresses the view that the two types 
of mottling in beans are entirely unrelated to each other, being due to the 
existence of two distinct and independent mottling units, which he calls 
M and X. Upon this assumption he works out the various possible hybrid 
combinations among beans possessing these factors and shows that certain 
crosses of mottled with self-colored (e. g. PPMMxx > PPmmXX) should 
produce in the F,, mottled and self-colored beans in the ratio 7:1, or 
Ir: 5, and that certain crosses between mottled and white (e. g. PPMMxx 
>< ppmmXX and PPmmXX > ppMMxx) should yield mottled, self-colored 
and white in the ratio 21: 3:8 or 33: 15: 16, the first ratio in each pair 
being produced if M is epistatic to X and the second if M is hypostatic 
to X. The critical crosses necessary to demonstrate these ratios have not 
yet been made. 
In the second paper the author reviews this discussion of the relation 
of the two types of mottling, and presents a third explanation which has 
advantages over the hypotheses of Shull and Emerson. This third hypo- 
thesis, proposed in correspondence by Dr. Spillman, assumes that there 
are not two distinct and independent genes determining the two types of 
mottling, but that there are two factors which act together to produce 
the mottling, and that these factors are coupled in the permanently 
mottled races. Spillman considers these two factors to be distinct functions, 
Y and Z, of the same morphological organ in the cell, presumably a chromo- 
some, or a part of achromosome. Any homozygous plant having a chromo- 
some with both of these functions, Y and Z, will be a permanently mottled 
race. Either one or both of these functions may be lost, thus giving rise to 
three different non-mottled races, one having a pair of chromosomes with 
the function Y, another having the corresponding pair of chromosomes with 
the function Z, and the third having the corresponding chromosomes with 
neither the function Y nor Z. Whenever two of these defect-races are 
crossed together, one of which has the function Y the other the function 
Z, the result will be a mottled F, hybrid, and as these two functions, Y 
and Z, belong to corresponding chromosomes, they will display “spurious 
allelomorphism”, being separated into different germcells, so that only the 
heterozygote of the second and succeeding generations will again have 
associated together the two factors necessary for mottling. Both the Shull- 
Emerson hypothesis and the Spillman hypothesis fit all the facts known, 
and it will doubtless be entirely impossible to demonstrate the truth of 
one and the falsity of the other, but Spillman’s hypothesis has the ad- 
vantage that it does not require a character to be visible when inherited 
from only one parent, but invisible when inherited from both parents, this 
being the fundamental difficulty with Shull’s hypothesis. 
In the second paper, Emerson gives evidence indicating that hypostatic 
colors which are entirely invisible in self-colored beans, may become visible 
_as underlying colors in mottled beans, and consequently that crosses in 
which both mottled and self-colored beans are produced, there may be a 
larger number of mottled phenotypes than of self-colored phenotypes, tho 
the number of genotypes of the mottled and self-colored classes are equal. 
This is due to the fact that the hypostatic colors in the mottled beans do 
not occupy the same areas as the epistatic colors, thus allowing the former 
to be partially uncovered by the latter. Each hypostatic color as well as 
each epistatic color may thus determine a class of mottled beans, while 
only the epistatic colors can be used in grouping the self-colored beans. 
