Abstract1. An attempt was made to investigate the inheritance of degree of dominance, number of genes involved, estimated contribution per pair of genes, nature of gene action, and heritability of staple length in Upland cotton, and its interrelationships with several important fiber characters, namely perimeter, wall thickness, and weight fineness. 2. Varieties Florida 1377 and D.P.L. 45-867, which represent the extremes in the length range in Upland cotton, were selected for this study. These noncommercial varieties also differ widely with regard to perimeter and wall thickness. Crosses were made and F1 and F2 populations were grown in the same year with the parents. 3. Staple length was determined by hand for each of the 1,388 F2, 24 F1, 26 Florida 1377, and 41 D.P.L. 45-867 plants. 4. Florida 1377 had a mean fiber length of 0.61 inch as compared with 1.37 inches for the D.P.L. 45-867. The mean parental difference was 0.76 inch. The F1 and F2 populations had a mean fiber length of 1.06 inches. The F2 segregated as a typical quantitative character. Its frequency distribution comprised a large number of classes (19), indicating that a large number of genes were segregating for this character. There was a significant difference between the mean of the parents and those of the F1 and F2 populations, indicating partial dominance of long staple over short. This was also apparent from the F2 distribution. This condition may handicap selection of long fiber in the F2. The minimum number of genes, as estimated by the Castle-Wright formula, was 9.38. Based on this estimate, the average contribution per pair of genes was estimated as 0.08 inch. The data were not adequate to determine whether arithmetic or geometric gene action was involved. Staple length gave a high heritability, 85 percent in F2. 5. The relationships of perimeter, wall thickness, and weight fineness with fiber length were studied. A significant negative correlation of —0.319 was found between length and perimeter of fiber. It is evident that some difficulty could be encountered in attempting to combine long fiber with large perimeter.
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