Final answer:
If mating true-breeding black and white mice results in all gray offspring, the inheritance pattern is indicative of incomplete dominance, where the offspring's phenotype is a blend of the parents'.
Step-by-step explanation:
When black and white true-breeding mice are mated and produce all gray offspring, the inheritance pattern indicative of this result is most likely incomplete dominance. This is because the phenotype of the offspring (gray color) is intermediate between the phenotypes of the parents (black and white). In incomplete dominance, heterozygous individuals show a blend of the two parental phenotypes, rather than expressing just one dominant trait.
This differs from codominance, where both alleles would be fully and separately expressed in the phenotype of the heterozygotes, such as in the case of the AB blood type in humans, where A and B alleles are both expressed.