Darwin did not work on dominant and recessive characters. It was Mendel who carried out experiments on heredity and he did so exclusively on pea plants i.e. Pisum sativum.
While carrying out crossing experiments, Mendel saw that only certain characters gopt expressed in the F1 generation. Again when the F1 generation plants were self crossed, this same character was expressed in most plants in F2 generation. This showed that certain characters were dominant.
The other character which was recessive was expressed only when both factors for this charcater was present. Thus Mendel concluded that certain characters which were expressed in homozygous and heterozygous conditions were dominant and the other character which was expressed only in homozygous condition in the absence of the dominant trait were recessive.
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