One gene can affect more than one trait.

Children with extreme forms of PKU often

have blond hair and reduced body pigment.

This is because the absence of PAH is a

metabolic block that prevents conversion of

phenylalanine into tyrosine, which is the

precursor of the pigment melanin. The relationship

between severe mental retardation

and decreased pigmentation in PKU makes

sense only if one knows the metabolic connections

among phenylalanine, tyrosine,

and melanin. If these connections were not

known, the traits would seem completely

unrelated. PKU is not unusual in this regard.

Many mutant genes affect multiple

traits through their secondary or indirect

effects.