True-breeding black mice and true-breeding white mice are mated and all the offspring are grey. Which pattern does that suggest?

1. Each true-breeding parent is homozygous, so every offspring is heterozygous. 2. Under simple dominance the heterozygotes would look exactly like one parent, either black or white. 3. Instead they are grey, a colour intermediate between the two parental colours. 4. An intermediate heterozygote phenotype is the signature of incomplete dominance. 5. Codominance would show both colours at once rather than a single blended shade. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 12 "Mendel's Experiments and Heredity", review questions_