Final answer:
The mating of true-breeding black and white mice producing all gray offspring demonstrates incomplete dominance, where the phenotype of the heterozygotes is an intermediate of both parents' phenotypes.
Step-by-step explanation:
If true-breeding black and white mice are mated and produce all gray offspring, the inheritance pattern displayed is indicative of incomplete dominance. This genetic phenomenon occurs when the heterozygous phenotype is an intermediate of the two homozygous phenotypes. In the case provided, neither the allele for black nor the allele for white is completely dominant; instead, their combined effect in the heterozygous state results in gray offspring, which is a blend of the two parents' coloring.
Codominance is not the correct answer because, in codominance, both alleles would be fully and separately expressed, resulting in offspring with a phenotype that shows both parental traits distinctly, not a blend. An example of codominance can be seen in human blood type AB, where both A and B antigens are fully expressed.
The inheritance pattern of incomplete dominance is also exemplified in other organisms, such as the classic case of snapdragon flowers, where crossing red and white flowers results in pink offspring. Similarly, in sickle cell anemia, a single normal allele and a single sickle-cell allele results in individuals with both normal and abnormal hemoglobin, which is an intermediate condition.