Final answer:
The majority of genetic variation among individuals in a population is due to differences in the nucleotide sequence of genes, with multiple alleles arising through mutations contributing to this diversity.
Step-by-step explanation:
The majority of the genetic variation among individuals within a population is due to differences in the nucleotide sequence of genes. Each gene can have multiple forms, known as alleles, which are different versions of a gene that arise through mutations.
These changes in the DNA base pairs within alleles contribute to the genetic diversity observed in a population. For instance, human blood types are determined by three possible gene sequences on a chromosome; the particular combination of these sequences inherited from our parents results in our individual blood type, illustrating how even a small change in nucleotide sequence can lead to different traits within a population.