In humans, the Xbr gene is located on chromosome 12 and is responsible for the production of a protein involved in liver function. There are 3 different alleles of the Xbr gene that exist in the human population: Xbr1, Xbr2, and Xbr3. What is the maximum number of different alleles of the Xbr gene that a genetically normal person can have in the genome of one of their skin cells?

a. 3
b. 2
c. 6
d. 4
e. none