Final answer:
In a scenario with pure additive genetic variance, genotypes 'AA' would show the dominant phenotype, 'aa' would show the recessive phenotype, and 'aA' would show either an intermediate or the dominant phenotype. In a cross between two 'Aa' individuals, the genotypic ratio would be 1 AA: 2 Aa: 1 aa, and the phenotypic ratio would be 3:1 for the dominant to recessive phenotype.
Step-by-step explanation:
If a trait is determined purely by additive genetic variance, the phenotypes that we would expect for the following genotypes aa, aA, AA would likely reflect the contributions of the alleles in a simple additive way. In such a case, if 'A' represents a dominant allele and 'a' represents a recessive allele, then the genotype 'AA' would express the phenotype associated with the dominant allele, 'aa' would express the phenotype associated with the recessive allele, and 'aA' or 'Aa' would express an intermediate phenotype or the dominant phenotype if the dominance is complete.
For instance, when considering an Aa x Aa mating, the resulting genotypic ratio of the offspring would be 1:2:1, which corresponds to AA: Aa (and aA): aa. The phenotypic ratio, assuming complete dominance, would be 3:1 as both AA and Aa exhibit the dominant phenotype while only aa exhibits the recessive phenotype. The numbers reflect the proportions you'd expect - for AA, 25% (since both parents provide an A allele), for Aa, 50% (since one parent provides an A allele and the other an a allele), and for aa, again 25% because both parents provide a recessive a allele.