A Wild-Type Mouse That Is Heterozygous for Two Immunoglobulin Heavy

Essay

A wild-type mouse that is heterozygous for two immunoglobulin heavy chain alleles (IgH^a/b) generates the population of B cells shown on the left of Figure . A mouse strain, also IgH^a/b, carries an inactivating mutation in the VpreB gene. In addition to producing fewer mature B cells than the wild-type mice, the VpreB-deficient mice generate B cells as shown on the right.
What is the explanation of the difference seen between the wild-type and the VpreB-mutant B cells?