Final answer:
Susan's paternal grandmother's most likely genotype for earlobe type is Rr, indicating she was heterozygous for the trait, meaning she carried both the dominant allele for unattached earlobes and the recessive allele for attached earlobes.
Step-by-step explanation:
The most likely earlobe genotype of Susan's paternal grandmother is Rr.
Since attached earlobes are a recessive trait and Susan's grandmother has unattached earlobes, which is a dominant trait, her genotype must include at least one dominant allele, R. However, since Susan and all of her immediate family have attached earlobes, represented by rr, it suggests that her grandmother was a carrier of the recessive allele, hence having one dominant allele R and one recessive allele r. This indicates she was heterozygous. If her genotype was RR, Susan's parent (the child of the grandmother) would also likely have unattached earlobes and could not pass two r alleles to Susan.