Each human gene can come in one or more varieties, called alleles. For each gene type, we inherit one allele from each parent. Each corresponding pair of alleles defines a unique physical or other trait. Either allele in a pair can be dominant or recessive. Two recessive alleles for one trait can actually mask the expression of alleles located at certain other sites on the chromosome and defining different but closely related traits.
Which of the following groups of traits makes most sense as an example of this?

hair color, eye color, and skin color


The third choice makes the most sense. A pair of recessive alleles for skin color can mask the expression of alleles governing hair and eye color located at other sites on the chromosome. Albinos (animals with albinism) are characterized by a lack of pigment, which provides color, in the hair and eyes, and skin, even though each of these three features is governed by alleles at different sites on the chromosome. A lack of pigment is the result of a congenital, or inherited, absence of melanin.

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