Lesson 2 of 9 / Inheritance and phenotype
Read the heterozygote first
Are both effects visible, or is the phenotype intermediate?
I can distinguish complete dominance, codominance and incomplete dominance as preparation for dihybrid problems.
I can use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.Syllabus
Syllabus 8876, Core 2(l). Use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.
I can solve dihybrid crosses including codominance, incomplete dominance, multiple alleles and sex linkage.Syllabus
Syllabus 8876, Core 2(o). Solve dihybrid crosses including codominance, incomplete dominance, multiple alleles and sex linkage.
About 5 min
Make a guess. You are not marked.
Red and white snapdragons produce pink offspring. Have the alleles blended into a new pink allele?
Show the answer
The alleles stay separate.
Pink is incomplete dominance. Crossing two pink plants gives red, pink and white again.
The key idea
The heterozygote reveals the dominance relationship. Codominance expresses both allele-associated effects; incomplete dominance gives an intermediate phenotype.
Explore the idea
Compare the heterozygote
The genotype RW remains the same across these hypothetical models. What changes is how the allele combination is expressed, not whether R and W segregate.
Explanation
Complete dominance gives the heterozygote the same scored phenotype as one homozygote. In incomplete dominance, the heterozygote is intermediate between the two homozygous phenotypes. A red-by-white flower model can therefore produce pink heterozygotes.
Codominance means both allele-associated effects are detectable in the heterozygote. In an original coat-colour model, a heterozygote with both red and white hairs differs from a uniformly intermediate colour. The observation determines the category, not the letters chosen for the alleles.
An R W by R W cross still gives genotypes RR, RW and WW in a 1:2:1 expectation. With incomplete dominance or codominance, those three genotypes can give three distinguishable phenotypes. Mendelian segregation continues even when a 3:1 phenotypic ratio does not apply.
Use paired capital symbols or superscript-style notation when helpful, but define them. Alleles do not permanently blend: a heterozygote still produces distinct R and W gametes, allowing the homozygous phenotypes to reappear in later generations.
Step by step
- 1
Inspect heterozygote evidence
One parental phenotype, intermediate phenotype or both effects distinguish the models.
- 2
Keep alleles separate
The heterozygote still produces separate allele-bearing gametes.
- 3
Translate genotypes to phenotypes
Apply the stated relationship after constructing the genetic combinations.
Worked example
Both colours are present
RW animals have separate red and white hairs. What phenotype relationship is shown, and what fraction of RW x RW offspring are expected to have both?
One way to explain it
Codominance is shown. The genotype expectation is 1 RR : 2 RW : 1 WW, so 1/2 are RW and express both hair colours.
Why this answer works
- Both visible effects indicate codominance rather than an intermediate blend.
- Use allele segregation to calculate the fraction.
Watch out for this
A student says: "An intermediate phenotype means the original alleles blend into a new allele." What is wrong with this?
Show the answer
The alleles remain distinct and segregate into gametes. The intermediate phenotype describes their combined expression.
Check your understanding
Which observation best supports incomplete dominance?
Show the answer
The heterozygote has a uniform intermediate phenotype.
It is intermediate between the two homozygotes.
Quick recall
Are both effects visible, or is the phenotype intermediate?