Two pink snapdragon plants (each with genotype C^R C^W, produced by crossing red and white parents) are crossed together. If incomplete dominance were actually blending inheritance — meaning alleles genuinely mix — what would you expect, and what actually happens?
ABlending predicts all pink offspring; actual result is also all pink — the predictions agree
BBlending predicts all pink offspring (since both parents are pink and the blend is fixed); actual result is 1 red : 2 pink : 1 white — the alleles segregated intact and the originals reappear
CBlending predicts 3:1 pink:white; actual result is 1:2:1 red:pink:white
DBoth models predict 1:2:1 ratios but disagree on which phenotypes appear
This is the definitive test distinguishing incomplete dominance from blending inheritance. If alleles truly blended, the pink phenotype would be permanent — you could never recover red or white offspring from two pink parents. But in incomplete dominance, the alleles remain discrete and segregate through meiosis just as Mendel described. Crossing C^R C^W × C^R C^W produces 1/4 C^R C^R (red), 1/2 C^R C^W (pink), and 1/4 C^W C^W (white). The reappearance of red and white in the F2 proves the alleles never mixed — they coexisted in the pink heterozygotes and were sorted out by meiosis.
Question 2 Multiple Choice
A person with type AB blood has genotype I^A I^B. Which correctly describes their red blood cell surface, and why?
AOnly A antigens are displayed, because I^A is dominant over I^B in most contexts
BA blended intermediate antigen that is neither A nor B is produced
CBoth A and B antigens are displayed simultaneously — I^A and I^B are codominant, each directing synthesis of a different surface antigen independently
DNo antigens are displayed because the two alleles cancel each other's enzymatic activity
Codominance means both alleles are fully and independently expressed in the heterozygote — not a blend and not one masking the other. The I^A allele encodes an enzyme that adds N-acetylgalactosamine to a cell-surface glycoprotein (producing the A antigen); the I^B allele encodes a different enzyme that adds galactose (producing the B antigen). Both enzymes function independently in the same cell, so both antigens appear simultaneously. This is why type AB individuals can receive blood from any ABO type (universal recipients) but can only donate to other AB individuals.
Question 3 True / False
Epistasis modifies the phenotypic ratios expected from a dihybrid cross, but the underlying alleles at each locus still segregate according to standard Mendelian rules during meiosis.
TTrue
FFalse
Answer: True
Epistasis operates at the level of phenotype expression, not allele transmission. When the E locus in Labrador retrievers masks the B locus, the 9:3:3:1 ratio becomes 9:3:4, but the alleles at both loci still assort independently according to Mendel's law of independent assortment (assuming the loci are on different chromosomes). Epistasis is a gene-gene interaction at the biochemical pathway level: one gene's product controls whether another gene's product is ever deployed. The mechanism of allele transmission — meiosis, segregation, independent assortment — is unchanged.
Question 4 True / False
Incomplete dominance is a form of blending inheritance because the intermediate phenotype in heterozygotes proves the two alleles have chemically mixed with each other.
TTrue
FFalse
Answer: False
This is the most common misconception about incomplete dominance. The intermediate phenotype arises from gene dosage, not allele mixing. A C^R C^W snapdragon is pink because one copy of C^R produces only half the red pigment that two copies produce — the white allele doesn't contribute pigment, it simply means fewer pigment-producing alleles are present. The alleles themselves remain completely separate and intact. The proof: cross two pink plants and you recover red and white offspring — impossible if the alleles had truly blended. Blending inheritance predicts traits merge permanently; incomplete dominance predicts they recover.
Question 5 Short Answer
How does incomplete dominance differ from true blending inheritance, and what experimental result demonstrates the difference?
Think about your answer, then reveal below.
Model answer: In blending inheritance, the alleles of the two parents permanently merge in the offspring — the original phenotypes can never be recovered. In incomplete dominance, the alleles remain discrete but the heterozygote's phenotype is intermediate (due to gene dosage: fewer allele copies produce less gene product). The definitive experiment is crossing two F1 heterozygotes: if blending occurred, both parents are identical (pink × pink) and all offspring should be pink. Instead, the F2 generation shows a 1:2:1 ratio of red:pink:white — the original red and white phenotypes reappear because the alleles segregated intact through meiosis.
Mendel's actual discovery was that discrete factors (alleles) are transmitted unchanged through generations. Incomplete dominance can superficially resemble blending because F1 heterozygotes look intermediate, but the F2 ratio is the giveaway. This experiment was historically important because pre-Mendelian biologists widely believed in blending inheritance, which would predict that traits average out over generations and that variation should decrease toward the mean — a theory that could not explain the persistence of variation in populations. Mendelian discrete inheritance solved this problem.