Feb. 16,1924 
Inheritance of Petal Spot in Pima Cotton 
allelomorph, full-spotted, is almost completely dominant. Consequently 
the first-generation heterozygotes resulting from accidental cross-pollina¬ 
tion with a normal stock of the variety could be recognized as soon as the 
first flowers open. 
Since spotless petal is a simple recessive, there should be no great diffi¬ 
culty in transferring this character to any other strain of the Pima va¬ 
riety. Complications would ensue were spotless associated with marked 
inferiority in characters of practical importance. Fortunately, the spot¬ 
less families described in this paper are satisfactory in fruitfulness and in 
the length, strength, and abundance of the fiber, hence there is no 
reason to expect that the offspring of such a cross would be inferior, either 
agriculturally or commercially. 
SUMMARY 
A conspicuous red spot near the base of the petal characterizes Sea 
Island and Egyptian cottons, while the petals of most varieties of upland 
cotton are spotless. In a hybrid between the Holdon variety of upland 
cotton and the Pima variety of Egyptian cotton, this character showed 
unifactorial segregation, the ratio of spotted to spotless plants in F 3 hav¬ 
ing been approximately 3:1. 
Two plants, discovered in 1917 in a field of Pima cotton, gave rise to 
families breeding true for a spotless or very faintly spotted petal. 
Crosses between these families and an inbred family in which the spot is 
fully developed gave opportunity for study of the inheritance of this char¬ 
acter within the Pima variety. 
In the first generation of the hybrid the spot was almost but not quite 
dominant, the hybrid means having been significantly lower than those of 
the spotted parental populations. The variability of the hybrid was of 
the same order of magnitude as that of the parental families. 
The second generation of the crosses showed very definite segregation 
into a spotted and a spotless (strictly speaking a very faintly spotted) 
class, the percentages of spotless individuals not departing significantly 
from the 25 per cent expected with a single factor difference. The varia¬ 
bility within the spotted class was, however, greater than in the popula¬ 
tions representing the spotted grandparents, and in two of the four F 2 
progenies the frequency distributions of the spotted segregates were 
bimodal. These facts indicate that the heterozygotes are partly distin¬ 
guishable from the pure dominants. 
In the third generation 24 progenies were grown from plants in each F 2 
progeny representing, respectively, the most spotted, least spotted and 
most nearly intermediate condition. The behavior in F 3 was in complete 
accord with the expectation. The eight progenies of fully spotted F 2 plants 
bred true, as did the eight progenies of spotless F 2 plants, while there was 
sharp segregation in each of the eight progenies of F 2 plants which were 
most nearly intermediate. The percentages of spotless individuals in the 
segregating F 3 progenies in no case departed significantly from the ex¬ 
pected 25 per cent. When all of the segregating progenies are taken as 
one array the percentage of spotless individuals is 26.5 + 1.6. The proof 
is conclusive that the spotless or very faintly spotted condition is a simple 
recessive. 
The spotted class in seven of the eight segregating F 3 progenies gave a 
bimodal distribution, this fact confirming the evidence from F x and F } 
that full-spotted is not completely dominant and that the heterozygotes 
are partly distinguishable. 
