Incomplete dominance is a non-Mendelian inheritance pattern in which neither allele is fully dominant, so heterozygotes show an intermediate phenotype that blends both alleles (like a red and white flower producing pink).
Incomplete dominance happens when neither allele in a heterozygote completely masks the other. Instead of one trait winning, you get a blend, a third phenotype that sits between the two homozygous versions. The classic example is snapdragons: cross a red-flowered plant (RR) with a white-flowered plant (WW) and every F1 offspring comes out pink (RW). Neither red nor white is fully expressed, so the pink is the visible middle ground.
This is a deviation from Mendel's model, which is exactly why it lives in topic 5.4 Non-Mendelian Genetics. Mendel's classic crosses predict a 3:1 phenotypic ratio in the F2 generation. With incomplete dominance, the phenotype ratio matches the genotype ratio, so when those pink F1 plants self-pollinate, the F2 shows a 1:2:1 ratio: 1 red, 2 pink, 1 white. Each genotype produces its own visible phenotype, so the numbers shift away from Mendel's prediction.
Incomplete dominance is part of Unit 5: Heredity, specifically topic 5.4. It supports learning objective AP Bio 5.4.A, "Explain deviations from Mendel's model of the inheritance of traits," and connects to EK 5.4.A.1, which says many traits don't follow Mendel's predicted ratios. The big skill here is quantitative: you need to recognize when an observed phenotypic ratio (like 1:2:1) statistically differs from the Mendelian prediction (3:1), and then explain why. That ties into the larger Unit 5 theme that inheritance is more layered than simple dominant-recessive crosses suggest.
Keep studying AP Biology Unit 5
Codominance (Unit 5)
Both are non-Mendelian and both give a 1:2:1 F2 ratio, but they differ in how the heterozygote looks. Codominance shows BOTH alleles fully and separately (think AB blood type, where both A and B antigens appear). Incomplete dominance blends them into one new intermediate trait.
Dominant and Recessive Alleles (Unit 5)
In standard Mendelian crosses, the dominant allele fully hides the recessive one, giving a 3:1 phenotype ratio. Incomplete dominance breaks that rule because no allele is fully dominant, so the heterozygote becomes visibly different from either homozygote.
Phenotype (Unit 5)
Incomplete dominance is really a lesson about how genotype maps to phenotype. Here, three genotypes (RR, RW, WW) each produce a distinct phenotype, so counting phenotypes is the same as counting genotypes, which is why the ratio is 1:2:1 instead of 3:1.
Expect this in MCQs that hand you a cross and ask you to interpret the offspring. A common stem describes red-flowered snapdragons crossed with white, all F1 coming out pink, and then asks for the F2 ratio (the answer is 1 red : 2 pink : 1 white). Your job is to spot the blended heterozygote, recognize it as incomplete dominance rather than simple Mendelian inheritance, and use the genotype ratio to predict phenotypes. While no released FRQ uses the term verbatim, it fits perfectly into questions asking you to explain why an observed ratio deviates from Mendel's prediction, which is exactly the skill in learning objective AP Bio 5.4.A.
This is the most-tested mix-up. In incomplete dominance the heterozygote is a BLEND, like pink from red and white. In codominance the heterozygote shows BOTH alleles fully and at the same time, like AB blood type showing both A and B antigens, or a roan cow with separate red and white hairs. Quick test: if you see a new in-between color, it's incomplete dominance. If you see both original traits side by side, it's codominance.
Incomplete dominance means neither allele is fully dominant, so the heterozygote shows an intermediate, blended phenotype.
The snapdragon cross is the go-to example: red x white gives pink F1 offspring, and self-pollinating them gives a 1:2:1 F2 ratio.
The F2 phenotype ratio (1:2:1) matches the genotype ratio because each genotype produces its own distinct phenotype, unlike Mendel's 3:1.
Incomplete dominance is a deviation from Mendel's model, which is why it sits in topic 5.4 Non-Mendelian Genetics and supports learning objective AP Bio 5.4.A.
Don't confuse it with codominance: incomplete dominance blends the traits, codominance shows both traits fully and separately.
Incomplete dominance is a non-Mendelian inheritance pattern where neither allele fully masks the other, so the heterozygote shows an intermediate phenotype. The classic example is a red snapdragon crossed with a white one producing pink offspring.
No. Incomplete dominance blends the two alleles into one new intermediate trait (red plus white equals pink). Codominance shows both alleles fully and separately at the same time, like AB blood type displaying both A and B antigens.
Crossing two heterozygotes (like two pink snapdragons) gives a 1:2:1 phenotypic ratio. For example, 1 red : 2 pink : 1 white. This differs from Mendel's classic 3:1 ratio because every genotype produces its own visible phenotype.
Because no allele is fully dominant, the heterozygote looks different from both homozygotes. That means each genotype maps to a unique phenotype, so the phenotype ratio (1:2:1) matches the genotype ratio instead of collapsing into 3:1.
Look at the heterozygote. If the offspring show a new in-between trait, like pink from red and white parents, it's incomplete dominance. If they show both parental traits at once, it's codominance, and if one trait disappears entirely, it's standard Mendelian dominance.
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