Feb. x6,1924 
493 
Inheritance of Petal Spot in Pima Cotton 
The unit upon which the frequency distributions and statistical con-* 
stants are based is the average of the grades of several flowers on each 
individual plant. On a large majority of the plants in each generation 
10 flowers were graded, and in no case is the average based Nipon as few 
as 2 flowers unless these had differed by not more than one grade. 4 
PARENTAGE OF THE CROSSES AND DESIGNATIONS OF THE 
POPULATIONS 
The two plants selected in 1917 which became the progenitors of the 
“spotless” families were numbered 1 and 3, respectively. The lines of 
descent of the successive progenies of these individuals were as follows: 
1917. Selections 
1918. Progenies 
1919. Progenies 
1920. Progenies 
1921. Progenies 
1922. Progenies 
1923. Progenies 
1 
I 
1 
I 
V 
I-^~I2 
I-3-I2-I4 
I-3-I2-14-2 
1— ! -1 
1-3-12-14-2-23 I-3-I2-14-2-25 
t 
3 
3-2-4-1 
3-2-4-1-22 
| -, 
3-2-4-1-22-1 3-2-4-1-22-27 
The family which furnished the normal or full-spotted parents of the 
crosses was descended from plant PI of 1914, as shown in the following 
pedigree: 
PI 
I 
PI 
PI A 
PI A-55 
I 
PIA-55-38 
PI A-5^-38-17 
1914. Selection 
1915. Progeny 
1916. Progeny 
1917. Progeny 
1919. Progeny 
1920. Progeny 
1921. Progenies 
1922. Progenies 
1923. Progenies 
12 
I 
12-13 
12-13-2 12-13-23 
13 
I 
! ' "“i 
13-8-4 . 13-8-17 
In 1920 plant No. 14 in spotless progeny 1-3-12 was crossed with 
plant No. 12 in normal progeny PI A-55-38-17 and plant No. 1 in 
spotless progeny 3-2-4 was crossed with plant No. 13 in the same 
normal progeny. The spotless parents were of the second and the 
normal parents were of the fifth strictly inbred generation. The F x 
progenies, grown in 1921, were designated 1-3-12-14 X 12 and 3-2-4-1 
X 13. Two individuals were selfed in each of the F x progenies, and 
gave rise to the following F 2 progenies, which were grown in 1922 : 
(1-3-12-14 X 12)—20 (3-2-4-1 X 13)— 21 
(1-3-12-14 X 12)—28 (3-2-4-1 X 13)—24 
* As a test of the reliability of an average based upon a small number of flowers, comparison was made of 
the averages for the first 3 and for all 10 of the flowers graded on 100 plants in the Fa hybrid progenies. 
The mean departure of the 3-flower average from the io-flower average was found to be 0.32 ±0.017 or one- 
third of a grade. The maximum difference, which occurred in a of the 100 plants, was 1.1 grade. It is 
concluded that averages based upon only 3 flowers would have been sufficiently accurate for most of the 
purposes of this investigation. 
