Final answer:
Two alleles of the same gene might have a nucleotide sequence difference, contributing to genetic diversity and leading to different traits such as blood type, which can be homozygous or heterozygous.
Step-by-step explanation:
Two alleles of the same gene might have a nucleotide sequence difference between them at the same place in the DNA sequence. Such allelic variation is the basis for genetic diversity within species. For instance, in humans, blood type is determined by three possible gene sequences, known as the A, B, and O alleles. An individual's blood type is a result of the specific combination of these alleles they inherit from their parents. A person could have two identical alleles (homozygous) such as AA, BB, or OO, or two different alleles (heterozygous) like AB, AO, or BO. This variability contributes to the distinctive traits observed within a species.