Biologists once assumed that the phenotype of a dominant allele would always be the most common phenotype in a population. Which finding led biologists to a different conclusion?

A dominant allele can be common or rare; dominance describes how an allele affects the phenotype in a heterozygote, not how well it helps an organism survive. Finding populations where a recessive phenotype improves survival showed that a recessive phenotype can be favored and become common. Therefore, the correct choice is C, the discovery that recessive-allele phenotypes can help a population survive.

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