Traits included physical features such as flower color. Today, these factors are called alleles. Â Mendel developed the hypothesis that some factors could be dominant, while others were recessive. According to his theory, a dominant factor is expressed when only one factor is present in the offspring. On the other hand, a recessive factor expresses its phenotype when both factors are present in the offspring. Today, the term genotype refers to the combination of factors possessed by an organism.