how should we measure species dominance?

By definition, the terms dominant and recessive refer to the genotypic interaction of alleles in producing the phenotype of the heterozygote. The key concept is genetic: Which of the two alleles present in the heterozygote is expressed, such that the organism is phenotypically identical to one of the two homozygotes. It is sometimes convenient to talk about the trait corresponding to the dominant allele as the dominant trait, and the trait corresponding to the hidden allele as the recessive trait. However, this can easily lead to confusion in understanding the concept as phenotypic. For example, to say that "green peas" dominate "yellow peas" confuses inherited genotypes and expressed phenotypes, and will subsequently confuse discussion of the molecular basis of the phenotypic difference.

Dominance is not inherent. One allele can be dominant to a second allele, recessive to a third allele, and codominant to a fourth.

Dominance is unrelated to the nature of the phenotype itself, that is, whether it is regarded as "normal" or "abnormal," "standard" or "nonstandard," "healthy" or "diseased," "stronger" or "weaker," or more or less extreme. A dominant allele may account for any of these trait types. Other distinctions are made between the gene locus (e.g. "A gene influencing seed shape"), the alleles at that locus (e.g. the "round" or "wrinkled alleles"), and the phenotype the alleles produce (e.g. "round" or "wrinkled").