Final answer:
The d) Fisher 344 strain is a common inbred strain of rats, which is valuable for research due to their genetic uniformity. When true-breeding mice of different colours are mated and produce offspring with a blended colour, this indicates a codominance inheritance pattern.
Step-by-step explanation:
The common inbred strain of rats mentioned in the options is the Fisher 344. Inbred strains are genetically uniform groups of animals produced by brother-sister mating for 20 or more generations. In the context of genetics, they are essentially homozygous at virtually all loci because of their breeding history. This contrasts with heterozygous organisms which carry two different alleles at a locus. This uniformity makes inbred strains valuable for scientific and medical research, as it eliminates genetic variability as a factor in experimental results. Regarding the additional information provided, if black and white true-breeding mice are mated and the result is all grey offspring, the inheritance pattern suggested is codominance. This is because neither the black nor white colour is completely dominant; instead, both alleles contribute to the phenotype, resulting in a blend of the two in the offspring. This is different from complete dominance where one allele completely masks the expression of the other, and the offspring would be of the dominant colour only.