Suppose in a species of petunia, locus A determines petal color and locus B determines whether the petals can express any pigment. At locus A, purple petal color (A) is dominant over yellow petal color (a). At locus B, pigmentation (B) is dominant over lack of pigmentation (b).

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many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.