Among the traits listed, blood type is the only one determined solely by genetics. It is classified genetically based on inherited alleles, while other traits, such as skin color and athletic ability, are influenced by both genetics and environment. Thus, blood type stands out as genetically pure. ;
The trait determined solely by genetics is blood type, as it is completely influenced by inherited alleles. In contrast, skin color, athletic ability, and big muscles involve both genetic and environmental factors. Therefore, the correct option is C: blood type.
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