Genes A and B are on the same chromosome. If a person has one chromosome with the alleles AB and a second chromosome with the alleles ab, how could they produce a gamete with a chromosome containing the alleles Ab or aB?

Genes and alleles assort independently during gamete formation. The possible gametes for the following will be: AB, aB, Ab, ab. The alleles will assort independently for each of the genes, hence there are equal chances for a gamete to carry the AB, aB, Ab or ab genes.

As Mendel's studies explained that alleles assort independently into gametes hence there are equal chances for a gamete to carry all the variability.