Difficulty: Introductory | Prerequisites: Basic Mendelian genetics, Punnett squares, understanding of dominant and recessive alleles
Big picture: Standard Mendelian genetics assumes one allele is fully dominant over the other, but that is not always how inheritance works. This unit introduces two patterns where both alleles influence the phenotype of a heterozygote. Incomplete dominance and codominance sit at the heart of non-Mendelian genetics and show up repeatedly in genetics exams. You should already be comfortable drawing Punnett squares and interpreting genotype-to-phenotype relationships before working through this material.
Some traits do not follow simple dominant/recessive rules. In incomplete dominance, the heterozygote shows a blended, intermediate phenotype (black + white feathers = bluish-grey). In codominance, both alleles are fully expressed side by side in the heterozygote (red fur + white fur = roan, with patches of both colours). Both patterns produce a 1:2:1 phenotypic ratio when two heterozygotes are crossed, unlike the 3:1 ratio of standard Mendelian dominance.
Incomplete dominance
A pattern of inheritance where the heterozygote displays an intermediate phenotype between the two homozygous phenotypes. Neither allele is fully dominant. In simple terms, the two alleles "blend" to produce something in between.
Codominance
A pattern of inheritance where both alleles in a heterozygote are fully and simultaneously expressed in the phenotype. Think of it as both alleles "showing up" rather than blending. The individual displays both traits at once.
Homozygous
Having two identical alleles for a given gene (e.g. C^B C^B or C^W C^W). In simple terms, both copies of the gene match.
Heterozygous
Having two different alleles for a given gene (e.g. C^B C^W). In simple terms, the two copies of the gene are mismatched, and this is where incomplete dominance and codominance become visible.
Phenotype
The observable physical trait (e.g. feather colour, fur colour). Think of it as "what you can see."
Genotype
The combination of alleles an organism carries for a trait (e.g. C^B C^W). Think of it as the genetic "recipe" behind the phenotype.
Punnett square
A grid used to predict the genotypes and phenotypes of offspring from a particular cross. Each parent's possible gametes line the top and side, and each box shows one possible offspring genotype.
In Andalusian fowls, feather colour is controlled by a single gene with two alleles:
C^B = black allele
C^W = white allele
Because neither allele is dominant, the heterozygote (C^B C^W) produces a third, intermediate phenotype: bluish-grey feathers.
Genotype | Phenotype |
|---|---|
C^B C^B | Black (homozygous) |
C^W C^W | White (homozygous) |
C^B C^W or C^W C^B | Bluish-grey (heterozygous) |
Cross 1: Homozygous black x homozygous white (C^B C^B x C^W C^W)
All offspring are C^B C^W. Result: 0% black, 0% white, 100% bluish-grey.
Cross 2: Black x bluish-grey (C^B C^B x C^B C^W)
Offspring: 50% C^B C^B (black), 50% C^B C^W (bluish-grey), 0% white.
Cross 3: Bluish-grey x bluish-grey (C^B C^W x C^B C^W)
Offspring: 25% C^B C^B (black), 50% C^B C^W (bluish-grey), 25% C^W C^W (white). This is the classic 1:2:1 ratio.
Cross 4: Black x black (C^B C^B x C^B C^B)
All offspring are C^B C^B. Result: 100% black, 0% white, 0% bluish-grey.
In incomplete dominance, the phenotypic ratio matches the genotypic ratio. When two heterozygotes cross, you get 1:2:1 for both genotype and phenotype, not the 3:1 you see in simple dominance.
In cattle, fur colour is controlled by a gene with two codominant alleles:
R = red fur allele
W = white fur allele
Unlike incomplete dominance, the heterozygote does not blend the two colours. Instead, both alleles are fully expressed: the animal grows patches of red fur and patches of white fur. This mixed coat is called roan.
Genotype | Phenotype |
|---|---|
RR | Red (homozygous) |
WW | White (homozygous) |
RW or WR | Roan (heterozygous, both colours expressed) |
Cross 1: Roan bull x roan cow (RW x RW)
Offspring: 25% RR (red), 50% RW (roan), 25% WW (white).
Cross 2: Red bull x white cow (RR x WW)
All offspring are RW. Result: 0% red, 0% white, 100% roan.
Cross 3: White bull x white cow (WW x WW)
All offspring are WW. Result: 0% red, 100% white, 0% roan.
Cross 4: Red bull x roan cow (RR x RW)
Offspring: 50% RR (red), 50% RW (roan), 0% white.
The heterozygote is the tell. If it looks like a blend of the two homozygous phenotypes (a new, intermediate colour), that is incomplete dominance. If it shows both phenotypes at the same time (patches of each colour, both proteins present), that is codominance. The genotypic ratios are the same in both cases; the phenotypic expression is what differs.
Students often think incomplete dominance and codominance are the same thing. They are not. Incomplete dominance produces a blended intermediate; codominance produces an organism that shows both traits simultaneously.
Students sometimes assume that incomplete dominance means the trait will "stay blended" forever. It will not. Cross two bluish-grey birds, and you can get pure black and pure white offspring again (the 1:2:1 ratio). The alleles do not mix permanently.
A common error is thinking codominance means one allele is slightly more dominant than the other. In codominance, both alleles are fully and equally expressed. There is no hierarchy.
Students frequently confuse the allele notation. In incomplete dominance, superscripts (C^B, C^W) are used rather than uppercase/lowercase, because neither allele is dominant. This is a deliberate notational choice, not just a stylistic preference.
⚠️ You will almost certainly be asked to distinguish between incomplete dominance and codominance. Have a one-sentence answer ready: "Incomplete dominance blends; codominance shows both."
⚠️ Punnett squares for these crosses work exactly the same way as in Mendelian genetics. The only difference is how you interpret the heterozygote's phenotype.
⚠️ Watch for questions that ask you to identify the inheritance pattern from a description. If the heterozygote looks "in between," it is incomplete dominance. If the heterozygote shows "both," it is codominance.
⚠️ The 1:2:1 phenotypic ratio (from a heterozygote x heterozygote cross) is a signature of both incomplete dominance and codominance. If an exam question gives you a 1:2:1 ratio, you are dealing with one of these patterns, not simple dominance.
True or false: In incomplete dominance, the heterozygote shows both parental phenotypes at the same time.
False. That describes codominance. In incomplete dominance, the heterozygote shows an intermediate (blended) phenotype.
Fill in the blank: When two bluish-grey Andalusian fowls are crossed, the expected phenotypic ratio is ____.
1 black : 2 bluish-grey : 1 white (1:2:1).
True or false: A roan cow has the genotype RR.
False. Roan cattle are heterozygous (RW). RR cattle are red.
Fill in the blank: In codominance, both alleles are fully ____ in the heterozygote.
Expressed.
True or false: Crossing a homozygous black bird with a homozygous white bird produces 50% black and 50% white offspring.
False. All offspring are heterozygous and therefore bluish-grey (incomplete dominance).
Q: What is the phenotypic ratio when two heterozygous organisms are crossed in an incomplete dominance scenario?
A: 1:2:1. One homozygous dominant phenotype, two intermediate (heterozygous) phenotypes, one homozygous recessive phenotype.
Q: A roan bull is crossed with a white cow. What are the expected offspring phenotypes and their probabilities?
A: RW x WW produces 50% RW (roan) and 50% WW (white). No red offspring are possible because neither parent carries two R alleles.
Q: How can you tell from a cross result whether a trait follows incomplete dominance or codominance?
A: Look at the heterozygote. If it shows an intermediate, blended phenotype (e.g. bluish-grey from black and white parents), it is incomplete dominance. If it shows both parental phenotypes simultaneously (e.g. red and white patches), it is codominance.
Q: Two bluish-grey Andalusian fowls are crossed. What percentage of offspring will be white?
A: 25%. The cross is C^B C^W x C^B C^W, producing 1 C^B C^B : 2 C^B C^W : 1 C^W C^W.
Q: Can two roan cattle produce a pure red calf?
A: Yes. RW x RW gives a 25% chance of RR (red) offspring.
Incomplete dominance and codominance connect directly to blood typing (covered in the next set of notes), where the ABO blood group system is a textbook example of codominance: the I^A and I^B alleles are codominant with each other, while both are dominant over i. These patterns also lay the groundwork for understanding polygenic inheritance, where multiple genes contribute to a single trait and blending-like effects become even more pronounced.
If you are comfortable with these two inheritance patterns, you are well placed to tackle sex-linked genetics next, which adds another layer by placing genes on the X chromosome.
Related Terms / Search Tags
non-Mendelian genetics, incomplete dominance, codominance, blending inheritance, Andalusian fowl, roan cattle, intermediate phenotype, 1:2:1 ratio, heterozygote phenotype, Punnett square, genotypic ratio, phenotypic ratio, allele expression, superscript allele notation, C^B C^W, codominant alleles, both alleles expressed, non-Mendelian inheritance patterns, CP biology genetics