Assuming classical Mendelian inheritance, how can one differentiate between a homozygous dominant individual and one who is heterozygous for the dominant trait?

Answer: Crossing each one with a homozygous recessive individual

Explanation: Crossing each one with a homozygous recessive individual. Let us call this example in this example: A = dominant trait allele, a = recessive trait allele. If Mendelian classical inheritance is assumed, those individuals with homozygous "AA" and heterozygous "Aa" genotype will express the dominant trait, but ONLY the individual with "aa" (recessive homozygous) genotype will express the recessive trait phenotype, so the latter It is the only case where we can know what the genotype of the individual is just by knowing the phenotype. Thus, when crossing individuals that express the dominant trait but unknown genotype, only two types of results can be obtained, as appropriate: 1) the crossing "AA" (unknown) and "aa" (known) will only be obtained offspring of heterozygous individuals "Aa", where everyone will show the dominant phenotype. 2) Crossing "Aa" (unknown) and "aa" (known) will give a 50% offspring Aa (which express the dominant and 50% aa (express recessive trait), thus recognizing the genotype of the parents in both cases.