Incomplete Dominance, Codominance & Multiple Alleles
Learn how incomplete dominance, codominance, multiple alleles like ABO blood type, and polygenic traits break simple Mendelian dominance — with Punnett square practice.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Incomplete Dominance, Codominance & Multiple Alleles, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Mendel's peas were tidy: one allele hid the other, and his monohybrid crosses gave clean 3:1 and 1:2:1 ratios. Real organisms are messier. A red snapdragon crossed with a white one gives pink flowers. A cow can be white and red at the same time. Humans have four blood types produced by three alleles, and human skin color spans a smooth range instead of two categories.
None of this breaks Mendel's laws — alleles still segregate, and Punnett squares still work perfectly. What changes is the relationship between genotype and phenotype: how the protein products of two different alleles interact inside the organism. In this lesson you will learn to recognize each inheritance pattern from the data, use the right notation for it, and predict offspring phenotypes and their probabilities.
None of this breaks Mendel's laws — alleles still segregate, and Punnett squares still work perfectly. What changes is the relationship between genotype and phenotype: how the protein products of two different alleles interact inside the organism. In this lesson you will learn to recognize each inheritance pattern from the data, use the right notation for it, and predict offspring phenotypes and their probabilities.
Incomplete Dominance: A Blended Intermediate
In incomplete dominance, neither allele in a heterozygote is fully expressed, so the heterozygote shows a phenotype between the two homozygotes. The classic case is the snapdragon: a homozygous red plant () crossed with a homozygous white plant () produces all pink offspring ().
The mechanism is dosage. The allele codes for a functional enzyme that makes red pigment; makes none. A heterozygote has only one working copy, so it produces roughly half the normal pigment — spread thin across the petals, that reads as pink. Nothing is literally mixing in the DNA; the alleles are still separate and still segregate cleanly into gametes.
This is why the pink phenotype reappears in predictable ratios instead of vanishing. Cross two pinks () and you get red : pink : white — a 1:2:1 phenotype ratio, because every genotype now has its own look. That 1:2:1 is your signature clue: whenever the phenotype ratio matches the genotype ratio, dominance is not complete.
Notation matters. Do not write for a pink snapdragon, because lowercase implies recessive. Use a shared capital letter with superscripts (, ) so the two alleles look equal in rank.
Where students go wrong: assuming that because the parents blend, the trait can never separate again. It can, and it does in the F2 generation. Another frequent error is expecting a ratio out of habit. If you see three phenotypes among offspring from a two-allele gene, complete dominance is ruled out immediately.
The mechanism is dosage. The allele codes for a functional enzyme that makes red pigment; makes none. A heterozygote has only one working copy, so it produces roughly half the normal pigment — spread thin across the petals, that reads as pink. Nothing is literally mixing in the DNA; the alleles are still separate and still segregate cleanly into gametes.
This is why the pink phenotype reappears in predictable ratios instead of vanishing. Cross two pinks () and you get red : pink : white — a 1:2:1 phenotype ratio, because every genotype now has its own look. That 1:2:1 is your signature clue: whenever the phenotype ratio matches the genotype ratio, dominance is not complete.
Notation matters. Do not write for a pink snapdragon, because lowercase implies recessive. Use a shared capital letter with superscripts (, ) so the two alleles look equal in rank.
Where students go wrong: assuming that because the parents blend, the trait can never separate again. It can, and it does in the F2 generation. Another frequent error is expecting a ratio out of habit. If you see three phenotypes among offspring from a two-allele gene, complete dominance is ruled out immediately.
Codominance: Both Alleles Fully Visible
Codominance also produces a distinctive heterozygote, but instead of an intermediate blend, both alleles are expressed fully and separately. Roan cattle carry : their coats show patches of red hairs and patches of white hairs side by side. Up close you can see both colors; there is no pink hair. Similarly, in some chickens a black-feathered bird crossed with a white-feathered bird gives checkered offspring with distinct black and white feathers.
The difference between codominance and incomplete dominance is often the single hardest distinction in this lesson, because both give a 1:2:1 ratio and both use superscript notation. The test is the heterozygote itself, not the numbers.
Ask yourself: could I point to the two original traits in the hybrid? If yes, it is codominance. If the hybrid looks like something new that is halfway in between, it is incomplete dominance.
One more caution: a spotted animal is not automatically codominant. Many patterning traits involve multiple genes or regulatory effects. Codominance requires evidence that the two alleles of one gene are both being expressed.
The difference between codominance and incomplete dominance is often the single hardest distinction in this lesson, because both give a 1:2:1 ratio and both use superscript notation. The test is the heterozygote itself, not the numbers.
| Feature | Incomplete dominance | Codominance |
|---|---|---|
| Heterozygote phenotype | New intermediate (pink) | Both traits shown at once (red and white patches) |
| Mechanism | Reduced dosage of one product | Both alleles make a detectable product |
| Example | Snapdragon flower color | Roan cattle, ABO blood type AB |
| Phenotype ratio, |
One more caution: a spotted animal is not automatically codominant. Many patterning traits involve multiple genes or regulatory effects. Codominance requires evidence that the two alleles of one gene are both being expressed.
Multiple Alleles and the ABO Blood Group
An individual diploid organism carries at most two alleles for a gene, but a population can carry many. When a gene has three or more versions circulating, biologists call it a multiple-allele system. Human ABO blood type is the standard example, with three alleles: , , and .
The allele codes for an enzyme that places the A sugar antigen on red blood cell surfaces; places the B antigen; makes no functional enzyme, so no antigen. and are codominant with each other — a person with both displays both antigens and has type AB blood. Both are completely dominant over .
Three alleles combine into six genotypes and four phenotypes. Notice that type O is the only phenotype with a single possible genotype, which makes it extremely useful in problems: an O child proves that each parent carried at least one .
Students commonly assume a person can be "triple" something, or that AB means one A allele plus one O allele. Also remember that the Rh factor (the plus or minus) is a separate gene entirely — do not fold it into the ABO Punnett square unless the problem asks for a two-gene cross.
The allele codes for an enzyme that places the A sugar antigen on red blood cell surfaces; places the B antigen; makes no functional enzyme, so no antigen. and are codominant with each other — a person with both displays both antigens and has type AB blood. Both are completely dominant over .
| Phenotype (blood type) | Possible genotypes | Antigens present |
|---|---|---|
| A | or | A |
| B | or | B |
| AB | A and B | |
| O | none |
Students commonly assume a person can be "triple" something, or that AB means one A allele plus one O allele. Also remember that the Rh factor (the plus or minus) is a separate gene entirely — do not fold it into the ABO Punnett square unless the problem asks for a two-gene cross.
Polygenic Traits and Continuous Variation
Human height, skin color, eye color, and kernel color in wheat do not sort into two or three neat boxes. They form a smooth range, or continuous variation, usually shaped like a bell curve in a large population. These are polygenic traits: many genes, each contributing a small additive effect, control one characteristic.
Imagine skin pigmentation governed by just three genes, each with a dark-contributing allele and a light-contributing allele. A person could carry anywhere from zero to six dark alleles, producing seven shades. Add more genes and the categories blur into a continuum. Real human pigmentation involves well over a dozen genes.
Two consequences follow. First, most individuals fall near the middle, because there are far more ways to get an intermediate number of contributing alleles than to get all or none — the same reason rolling several dice usually gives a middling total. Second, environment often layers on top: nutrition affects height, sun exposure affects skin tone. Traits shaped by both genotype and environment are called multifactorial.
The biggest misconception here is confusing polygenic inheritance with incomplete dominance. Incomplete dominance is one gene with a blended heterozygote and discrete categories (red, pink, white — nothing else). Polygenic inheritance is many genes producing a graded spectrum. Also note that polygenic traits are impractical to solve with Punnett squares; a three-gene cross needs a 64-box square. Instead, biologists describe them statistically, which is why family resemblance for height is a tendency rather than a prediction.
Imagine skin pigmentation governed by just three genes, each with a dark-contributing allele and a light-contributing allele. A person could carry anywhere from zero to six dark alleles, producing seven shades. Add more genes and the categories blur into a continuum. Real human pigmentation involves well over a dozen genes.
Two consequences follow. First, most individuals fall near the middle, because there are far more ways to get an intermediate number of contributing alleles than to get all or none — the same reason rolling several dice usually gives a middling total. Second, environment often layers on top: nutrition affects height, sun exposure affects skin tone. Traits shaped by both genotype and environment are called multifactorial.
The biggest misconception here is confusing polygenic inheritance with incomplete dominance. Incomplete dominance is one gene with a blended heterozygote and discrete categories (red, pink, white — nothing else). Polygenic inheritance is many genes producing a graded spectrum. Also note that polygenic traits are impractical to solve with Punnett squares; a three-gene cross needs a 64-box square. Instead, biologists describe them statistically, which is why family resemblance for height is a tendency rather than a prediction.
Key terms
- Incomplete dominance.
- An inheritance pattern in which the heterozygote shows an intermediate phenotype between the two homozygotes, because one allele produces a reduced amount of functional product.
- Codominance.
- An inheritance pattern in which both alleles in a heterozygote are fully and separately expressed, so both traits appear simultaneously in the phenotype.
- Multiple alleles.
- A situation in which a gene exists in three or more allelic forms within a population, though any one diploid individual still carries only two.
- ABO blood group.
- A human multiple-allele system with alleles , , and ; and are codominant and both are dominant over , giving four phenotypes.
- Antigen.
- A surface molecule, such as the A or B sugar on red blood cells, that the immune system can recognize as self or foreign.
- Polygenic trait.
- A characteristic controlled by two or more genes whose effects add together, producing continuous variation rather than discrete categories.
- Continuous variation.
- A phenotype distribution in which individuals span a smooth range of values, typically forming a bell-shaped curve in a large population.
- Multifactorial trait.
- A trait whose phenotype results from the combined influence of multiple genes and environmental factors.
Worked example
A woman with type A blood whose mother had type O blood marries a man with type AB blood. Determine the possible blood types of their children and the probability of each. Then state whether any child could have type O blood.
Step 1 — Assign the mother's genotype. Type A means or . Her own mother was type O (), so she must have inherited an allele from her. Therefore the woman is .
Step 2 — Assign the father's genotype. Type AB has only one possibility: .
Step 3 — Set up the cross . The mother's gametes carry or ; the father's carry or .
Step 4 — Convert genotypes to phenotypes. is type A. is type AB, because and are codominant. is type A, since is dominant over . is type B.
Step 5 — Tally probabilities. Type A: 2 of 4, or (50 percent). Type AB: 1 of 4, or 25 percent. Type B: 1 of 4, or 25 percent.
Step 6 — Answer the type O question. Type O requires the genotype , meaning an allele from each parent. The father is and carries no allele, so no child from this couple can be type O. Notice how one piece of family history (the grandmother) locked in the mother's genotype — that move is the key to most blood type problems.
Step 2 — Assign the father's genotype. Type AB has only one possibility: .
Step 3 — Set up the cross . The mother's gametes carry or ; the father's carry or .
| (father) | (father) | |
|---|---|---|
| (mother) | ||
| (mother) |
Step 5 — Tally probabilities. Type A: 2 of 4, or (50 percent). Type AB: 1 of 4, or 25 percent. Type B: 1 of 4, or 25 percent.
Step 6 — Answer the type O question. Type O requires the genotype , meaning an allele from each parent. The father is and carries no allele, so no child from this couple can be type O. Notice how one piece of family history (the grandmother) locked in the mother's genotype — that move is the key to most blood type problems.
Practice questions
In snapdragons, plants are red, plants are white, and plants are pink. If two pink snapdragons are crossed, what phenotype ratio is expected in the offspring?
- 3 red : 1 white
- 1 red : 2 pink : 1 white
- All pink
- 2 red : 1 pink : 1 white
Answer: 1 red : 2 pink : 1 white
Crossing gives genotypes . Because dominance is incomplete, each genotype has its own appearance, so the phenotype ratio matches the genotype ratio: 1 red : 2 pink : 1 white. The 3:1 option assumes complete dominance, which would require the heterozygote to look identical to a homozygote. "All pink" describes the F1 generation from a red by white cross, not the F2 from two pinks.
A breeder crosses a red-coated bull with a white-coated cow and gets calves whose coats show distinct patches of red hairs and white hairs, with no blended coloring anywhere. Identify the inheritance pattern, explain the evidence for your choice, and predict the offspring phenotypes if two of these patched calves are later bred together.
Answer: This is codominance. Crossing two roan (heterozygous) cattle gives an expected 1 red : 2 roan : 1 white ratio.
The deciding evidence is the appearance of the heterozygote at close range. Both original traits are visible as separate red hairs and white hairs, meaning both alleles are producing a detectable product simultaneously — that is codominance. If the hairs had all been an intermediate pink shade, the pattern would be incomplete dominance instead. For the second cross, write the roan animals as . The Punnett square yields (red) : (roan) : (white), so about 25 percent red, 50 percent roan, and 25 percent white.
Explain why human height forms a smooth bell-shaped range in a large population, while snapdragon flower color falls into only three discrete categories, even though both traits involve heterozygotes with intermediate appearances.
Answer: Height is polygenic, so many genes each add a small increment and produce many possible totals; snapdragon color is one gene with three genotypes, so only three phenotypes exist.
Flower color is controlled by a single gene with two alleles, giving exactly three genotypes and therefore exactly three phenotypes — red, pink, white, with nothing in between. Height is influenced by many genes, each contributing a small additive effect, plus environmental factors such as nutrition. With dozens of contributing alleles, the number of possible totals is large and the categories overlap into a continuum. Intermediate totals are also far more common than extremes, because many different allele combinations add up to a middling value, which is what produces the bell shape.
FAQ
- What is the fastest way to tell incomplete dominance from codominance?
- Look only at the heterozygote. If you can still point to both parental traits in it — red patches and white patches, black feathers and white feathers, A antigens and B antigens — it is codominance. If the heterozygote looks like a single new intermediate shade that is halfway between the parents, such as pink, it is incomplete dominance. Both patterns give a 1:2:1 ratio, so the ratio alone cannot distinguish them.
- Can a child have a blood type that neither parent has?
- Yes, and it happens often. Two type A parents who are both can have a type O child (). An parent and an parent can produce children of all four types: AB, A, B, and O, each at 25 percent. What is not possible is a type O child from a parent with type AB blood, since an AB parent has no allele to pass on.
- If a gene has three alleles, can one person inherit all three?
- No. A diploid organism has two copies of each autosomal gene — one from each parent — so any individual carries at most two alleles. "Multiple alleles" describes variation across the whole population, not within one person. Three alleles in a population produce six possible genotypes, and each individual has exactly one of those six.
- Do these patterns mean Mendel was wrong?
- No. Segregation and independent assortment still hold: alleles still separate during meiosis, and Punnett squares still predict genotype ratios correctly. What incomplete dominance, codominance, and polygenic inheritance change is how genotypes translate into visible phenotypes. Mendel happened to choose pea traits with complete dominance, which made his patterns easy to see, but the underlying rules of inheritance he discovered apply to all of these cases.
Learn this with a teacher, not a page
The Crimsora tutor teaches Incomplete Dominance, Codominance & Multiple Alleles live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.