Final answer:
The percentage of muscle mass differences attributable to genetic variation is based on the diversity of alleles and heritability in a population, which determines how much phenotypic variation is susceptible to evolutionary forces.
Step-by-step explanation:
The percentage of differences in muscle mass between individuals that can be explained by genetic variation varies. This variability is because the majority of human genes have two or more possible versions, known as alleles. The differences in these alleles contribute significantly to the genetic diversity observed among individuals. Furthermore, heritability is an important concept in understanding this variation. It is the fraction of phenotype variation that can be attributed to genetic differences among individuals in a population. A higher heritability means the phenotypic variation is more susceptible to the evolutionary forces that act on heritable variation within a population. Hence, these genetic differences are responsible for variations in traits such as muscle mass.