NIH Genetics Home Reference genetic factors Multiple genetic traits eye color https://ghr.nlm.nih.gov/primer/traits/eyecolor https://ghr.nlm.nih.gov/primer/traits/eyecolor eye color contribute https://ghr.nlm.nih.gov/primer/traits/eyecolor genetically bit more complex eye color https://ghr.nlm.nih.gov/primer/traits/eyecolor NIH Genetics Home Reference https://ghr.nlm.nih.gov/primer/traits/eyecolor genetics of eye color