Another semantic objection is the visibility objection which refutes the claim that there are US racial groups in human population structures. Philosophers such as Joshua Glasgow and Naomi Zack believe that US racial groups cannot be defined by visible traits, such as skin colour and physical attributes: "The ancestral genetic tracking material has no effect on phenotypes, or biological traits of organisms, which would include the traits deemed racial, because the ancestral tracking genetic material plays no role in the production of proteins it is not the kind of material that 'codes' for protein production." Spencer contends that certain racial discourses require visible groups, but disagrees that this is a requirement in all US racial discourse.