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AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
with c c C. In the latter case, if the chromosome carrying C failed to 
divide or if the two halves failed to separate after division, one of the result- 
ing daughter nuclei would heccCorccCC (colored) and the other c c 
(colorless). Thus both somatic mutation and chromosome non-disjunction 
might readily account for the observed cases of aberrant endosperm, and 
neither mutation nor non-disjunction could reasonably be expected to 
cause such an anomaly in genotypes where it has never been observed. 
It was pointed out in the writer's 191 8 paper that crucial evidence in 
support of one or other of these hypotheses might be obtained only from 
crosses in which linked aleurone and endosperm factors are simultaneously 
involved. It was known that the aleurone factor pair C c is thus linked with 
waxy endosperm, Wx wx (Bregger, 1918; Kempton, 1919), but no aberrant 
seeds positively known to involve both these factor pairs were available. 
The writer was not unaware of G. N. Collins's case (1913) involving the 
aleurone factor pair / i with Wx wx, but the linkage relations of these factors 
were not known. It has since been shown by Hutchison (1921) that the 
factor pair / i is closely linked with a factor pair for shrunken endosperm, 
Sh sh, which in turn is linked with C c and Wx wx. The linkage group 
as at present known, therefore, is made up of the pairs C c, I i, Sh sh, and 
Wx wx. Consequently, in accordance with the chromosome hypothesis, 
all these factor pairs are assumed to lie in one pair of homologous chromo- 
somes. 
Assuming, then, that C and Wx lie in the same chromosome, it can 
readily be seen how a crucial test of the somatic-mutation and the chromo- 
some-non-disjunction hypotheses is afforded by appropriate crosses. If 
the female parent oi a cross be colorless and waxy, c wx, and the male 
parent be colored and corneous, C Wx, the resulting endosperm will be 
c c C wx wx Wx, and the three homologous chromosomes carrying these 
genes in the "fecundated" endosperm nucleus will be as follows: 
c 
wx 
c 
wx 
C 
Wx 
Diagram i 
Now if, at any division of an endosperm nucleus, the two halves (a and b) 
resulting from a longitudinal split of the chromosome carrying C and Wx 
(chromosome 3 of diagram i) should fail to separate (non-disjunction) and 
should go together to one pole (A), the resulting daughter nuclei {A and B) 
would be as shown in diagram 2. 
