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<abstract language="eng">In the present paper the difference in yielding ability of two genotypes which differ by only one single gene is compared : Na Na (bourbon) and Na na (murta). 20 plants of each of 14 murta (Na na) progenies were used for comparison after discarding all na na plants which are dwarfed and practically unfruitful; each progeny was made of 10 Na Na and 10 Na na individuals. The individual yields were recorded during 8 years (1939 to 1946) and it was shown that the na allele, besides its well known effect on the morphology of the leaves, has a remarkable influence on the yielding capacity of the Na na plants, reducing it, in average, to only 55,9% as compared with the Na Na genotype. The Bartlett's test of homogeneity of variance when applied to study the variability between the plants of the bourbon and murta groups has shown that both groups are homogeneous, but a significant difference between their variances was found, the bourbon one being larger. The same test when applied to study the variation between the annual yields of the same groups of plants has led to the same conclusion. The Na na plants however were more suceptible to the die-back of the lateral branches after harvest, which even caused the death of several plants. Considering that fruit size and weight of both varieties (bourbon and murta) are practically identical, it is concluded that the lower fruit number of the murta genotype is responsable for its lower yields. As its flowers are normally fertile, pollen being produced in abundance, it is supposed that a lower number of internodes, or a lower number of flowers in the inflorescences or even a lower total number of flowers in each leaf axil is responsable for the decrease in yield of the murta variety. The results of this study are also of practical importance suggesting the elimination of all murta plants in a coffee plantation and their replacement by bourbon

Genética de Coffea: XI - A influência do gen recessivo na sôbre a produtividade do cafeeiro (Coffea arabica L.)