Major histocompatibility complex class II B allele polymorphism and its association with resistance/susceptibility to Aeromonas hydrophila in the “whole white” color patterns of Cyprinus carpio var. color: Major histocompatibility complex class II B allele polymorphism and its association with resistance/susceptibility to Aeromonas hydrophila in the “whole white” color patterns of Cyprinus carpio var. color