Answer
The gene for colorblindness is carried only on the X chromosome. Males have only one X chromosome. If a male's single X chromosome contains the gene for colorblindness, then he will be colorblind. On the other hand, females have two X chromosomes. Since the allele for colorblindness is recessive, females must have two copies of the allele for colorblindness to be colorblind. If a woman only has one allele for colorblindness, she is only a carrier because the allele for normal color vision will mask the effects of the defective allele (the allele for colorblindness).
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