190 
GENETICS: C. B. BRIDGES 
Proc. N. A. S. 
his father had been homozygous bent. Furthermore, in a cross of 
Diminished to eyeless there appeared a single not-Diminished eyeless 
female. Genetical tests showed that this exceptional female was homo- 
zygous for eyeless, although her father had lacked eyeless entirely. 
These exceptions to normal inheritance invited a comparison with the 
well-known exceptions to sex-linked inheritance which are due to non- 
disjunction of the sex chromosomes; 4 for it is evident that non-disjunction 
of the fourth chromosome would produce such exceptions, and also that it 
would give rise to the whole series of effects that are characteristic of defi- 
ciency. Diminished individuals are deficient not simply for a section of 
the fourth chromosome including the loci for bent and eyeless, but are 
deficient for a whole fourth chromosome; i.e., they are haploid for the 
fourth chromosome. A Diminished strain would arise from the occur- 
rence of "primary' ' non-disjunction in either sex, whenever at maturation 
the pair of fourth chromosomes fails to disjoin and both members pass to 
the same pole, leaving the other pole without a fourth chromosome. Such 
nullo-IV eggs or sperm in union with normal haplo-IV sperm or eggs would 
give rise to individuals with only a single fourth chromosome. Judging 
by the number of independent strains of Diminished and by the two con- 
trolled exceptions, the frequency of primary non-disjunction of the fourth 
chromosome is of the same order as for the first. 
If primary non-disjunction gave rise to nullo-IV gametes and thus to 
haplo-IV strains, the same process may also give ' rise conversely to 
diplo-IV gametes and thus to triplo-IV zygotes. No such individuals 
have as yet been indentified. 8 Aside from possible direct somatic effects of 
triploidy or tetraploidy for the IV chromosome, the presence of an extra chro- 
mosome would be detectable by duplication-phenomena, 9 just as the haplo- 
IV type was detectable by deficiency phenomena, and could be proved 
cytologically. The eyeless exception referred to above was produced by 
a diplo-IV egg, so that the process is well substantiated even here. 
III. Cytological examination of the oogonial cells of Diminished indi- 
viduals showed, as had been predicted, that only a single one of the small 
spherical chromosomes was present, while each of the other chromosomes 
was present in the usual paired condition. Four independent strains of 
Diminished, including the one that arose in the cross, were thus proved to 
be haploid for the fourth chromosome (fig.l). 
IV. The haploid nature of the Diminished individuals made possible a 
back-cross determination of the bent eyeless crossover value without the 
necessity of a double recessive. The situation in the fourth chromosome 
is now comparable with that in the first, with haploidy for the fourth chro- 
mosome giving the Diminished type and pseudo-dominance of fourth- 
chromosome recessives, while haploidy for the first gives the male type and 
pseudo-dominance of sex-linked recessives. Females heterozygous for 
