In rabbits, the dominant allele B causes black fur and the recessive allele b causes brown fur; for an independently assorting gene, the dominant allele R causes long fur and the recessive allele r (for rex) causes short fur. A homozygous rabbit with long, black fur is crossed with a rabbit with short, brown fur, and the offspring are intercrossed. In the F₂, what proportion of the rabbits with long, black fur will be homozygous for both genes?