Comparing dominance patterns, and their trade-offs for a species

Complete dominance, incomplete dominance, co-dominance and multiple alleles are often asked TOGETHER — as a comparison, with an advantage and disadvantage for the species.

Why the SAME genotype ratio can give DIFFERENT phenotype ratios

COMPLETE DOMINANCE: the heterozygote looks IDENTICAL to the homozygous dominant — so a 1:2:1 genotype ratio collapses to a 3:1 phenotype ratio, since two of the three genotypes look the same.
INCOMPLETE DOMINANCE: the heterozygote shows a BLENDED, intermediate phenotype — so the SAME 1:2:1 genotype ratio now gives a 1:2:1 phenotype ratio too, since all three genotypes look different.
MULTIPLE ALLELES: more than two alleles exist in the population's gene pool (though any individual still carries only two) — increasing the number of POSSIBLE genotypes and phenotypes beyond what a two-allele gene could ever produce.
TRADE-OFF FOR THE SPECIES: more visible variation (incomplete dominance, co-dominance, multiple alleles) gives natural selection more distinct phenotypes to act on, which can be an ADVANTAGE if conditions change — but it can also mean a harmful recessive allele is HARDER to hide/carry silently, which can be a DISADVANTAGE if the intermediate or recessive phenotype is less fit.

Worked example Explaining why two crosses give the same genotype ratio but different phenotype ratios A cross between two rabbits heterozygous for black/chinchilla fur gives the same genotype ratio as a cross between two heterozygous for chinchilla/albino, but different phenotype ratios. Explain why. GENOTYPE RATIO: both crosses are heterozygote × heterozygote, so BOTH produce the same underlying 1:2:1 genotype ratio, regardless of which alleles are involved. BLACK/CHINCHILLA CROSS: black shows COMPLETE dominance over chinchilla, so the heterozygote looks black — collapsing the 1:2:1 genotype ratio to a 3:1 phenotype ratio (black : chinchilla). CHINCHILLA/ALBINO CROSS: chinchilla shows INCOMPLETE dominance over albino, so the heterozygote shows its OWN distinct intermediate phenotype — keeping the full 1:2:1 ratio visible in the phenotypes too. CONCLUSION: the genotype ratio depends only on which alleles pair up at fertilisation; the phenotype ratio additionally depends on the DOMINANCE RELATIONSHIP between those specific alleles. ✓ The key move is separating "what genotypes result" (always the same 1:2:1 for two heterozygotes) from "what those genotypes look like" (depends entirely on the dominance pattern between the specific alleles involved). ✕

Treating "genotype ratio" and "phenotype ratio" as always the same

A 1:2:1 genotype ratio only produces a MATCHING 1:2:1 phenotype ratio when every genotype looks visibly different (incomplete dominance/co-dominance). Under complete dominance, two of the three genotypes look identical, collapsing the ratio to 3:1 — always check which dominance pattern applies before assuming the ratios match.

Dominance pattern	Heterozygote phenotype	1:2:1 genotype → phenotype ratio
Complete dominance	Identical to homozygous dominant	Collapses to 3:1
Incomplete dominance	Distinct, blended intermediate	Stays 1:2:1
Co-dominance	Both alleles fully, separately visible	Stays 1:2:1, but as a new combined phenotype

Practice for this standard covers the whole topic — flashcards and quiz.