INHERITANCE OF petal SPOT IN PIMA COTTON 1 
By Thomas H. Kearney 
Physiologist in Charge, Alkali and Drought Resistant Plant Investigations, Bureau of 
Plant Industry, United States Department of Agriculture 
INTRODUCTION 
Egyptian cotton, like the related Sea Island cotton (Gossypium barba- 
dense L.) is characterized by a conspicuous red spot near the base of the 
otherwise yellow petal. Wlien the flower first opens, the red pigment 
appears to be confined to the cells of the epidermis, but soon diffuses into 
the subjacent tissue. The spot is entirely lacking in most of the varie¬ 
ties of upland cotton (G. hirsutum L.). 
It has been found that in a hybrid between the Pima variety of Egyp¬ 
tian cotton and the Holdon variety of upland cotton, the parental fami¬ 
lies having shown, respectively, pronounced development and complete 
absence of the spot, the spotless condition behaved as a simple recessive. 3 
A 3:1 ratio was obtained in F 3 and the behavior of F s progenies of indi¬ 
viduals representing different points on the F 3 frequency curve confirmed 
the unifactorial nature of the segregation. The findings of other investi¬ 
gators as to the inheritance of this character are summarized in the pub¬ 
lication referred to (p. 31). 
In the Pima variety the petal spot is almost invariably well developed, 
although the character is rather sensitive to environmental influences 
and there is often variation on the individual plant in the size, shape 
and intensity of the spot. The normal range was far exceeded, however, 
by two individuals discovered in 1917 in a field of Pima cotton at Sacaton, 
Ariz. 8 The inbred descendants of these plants, closely studied during five 
generations, have shown at most only a very weak development of the 
spot while in many of the flowers the spot has appeared to be completely 
absent. Table I gives the mean grade of petal spot for the successive 
generations of these families and for the normal populations with which 
they were compared, the means having been based upon the averages 
of several flowers graded on each plant. 
Table I. —Mean grade of petal spot in successive inbred generations of the “ spotless” 
Pima families and in populations of this variety showing a normal development of the 
spot 
Year. 
“Spotless 
” families. 
Normal populations. 
Number of 
plants. 
Mean grade 
of petal 
spot. 
Number of 
plants. 
Mean grade 
of petal 
spot. 
1919 . 
38 
t. 0±0. 04 
13 
7 - S ±°-19 
I920. 
140 
. 9±o. 04 
1921. 
29 
. 9±o. 06 
34 
7.8±o. 04 
1922. 
62 
. 7±o. 02 
54 
7. 6±o. 03 
1923 . 
106 
2. o±o. 04 
hi 
8. idbo. 02 
1 Received for publication Dec. 14, 1933. 
* Kearney, Thomas H. segregation and correlation op characters in an upland-egyptian 
cotton hybrid. U. S. Dept. Agr. Bui. 1164, p. 21-26, fig. 37, pi. 7. 1933. 
* Kearney, Thomas H. heritable variations in an apparently uniform variety op cotton. 
In Jour. Agr. Research, v. si, p. 339-241. pi. 54 * i 9 «* 
(491) 
Journal of Agricultural Research, 
Washington, D. C. 
Vol. XXVII, No. 7 
Feb. x6, 1934 
Key No. G -367 
