From the way genetics is introduced in Bio 101, it is easy to become convinced that genes can have only two possible alleles, but, in fact, for many genes, there are many possible alleles. The collection of possible alleles for a gene is referred to as a multiple allelic series. Blood type alleles are an example of a multiple allelic series, where there are three possible alleles (A, B, and O).
Multiple allelic series are often expressed in this form: A1 > A > a > a' . . .etc. The series shows the dominance relationships. In other words, in multiple allelic series, an allele may be either dominant or recessive depending on what it is paired with. 'A' would be dominant when paired with 'a', but recessive when paired against 'A1'.
In studying genetics for the MCAT, pay special attention to the wrinkles and complexities such as multiple alleles that take you one step further than the basic introductory genetics. Not advanced knowledge, but a step past the basics.