Dominance and codominance describe distinct modes of gene expression and allelic interaction that are standard concepts in genetics, requiring no formal citation.
The claim states that dominance and codominance occur at different relative frequencies in genetic traits. This is a fundamental, definitional concept of Mendelian and population genetics (i.e., different genetic loci or traits exhibit different forms of gene interaction, such as complete dominance, incomplete dominance, or codominance). Because this is a matter of elementary scientific observation and textbook knowledge, it falls under COMMON_KNOWLEDGE, and no citations are necessary.