Chapter 17, Problem 5P

As shown in the following diagram, a single nucleotide difference in a hair follicle enhancer of a human gene called KITLG contributes to the trait of hair color. People with an A–T base pair in the enhancer tend to have dark hair, while people with a G–C base pair at the same position tend to have blond hair. The base pair difference affects the level of KITLG transcription: The blond-associated allele is transcribed only 80% as frequently as the dark hair-associated allele. Explain how a single base pair difference in an enhancer sequence can have this effect.