Answer:
A. They have small variations in their sequences of nitrogenous bases.
Step-by-step explanation:
Two alleles for the same gene are different because they have small variations in their sequences of nitrogenous bases. An allele is a variant form of a gene, and different alleles can encode slightly different versions of the same protein. The sequence of nitrogenous bases (adenine, thymine, guanine, and cytosine) in the DNA molecule determines the genetic information that is encoded in a gene, so differences in the sequence of alleles can result in different versions of the encoded protein. These small variations in the sequence of alleles are caused by mutations, which can arise spontaneously or be induced by various environmental factors. The other answer choices do not accurately describe the differences between two alleles for the same gene.