Biology · Grades 10–12
Variation beyond simple dominance
Recognize incomplete dominance, codominance and polygenic traits.
Learn the idea
Incomplete dominance
A heterozygote can show an intermediate phenotype in a specified model.
Codominance
Both allele-associated products can be expressed in a heterozygote.
Polygenic
Many traits depend on multiple genes and often environmental conditions.
Evidence
A distribution of observed values does not alone identify the exact inheritance mechanism.
Worked examples
A stated model has red RR, white WW and pink RW flowers. What pattern is described?
The heterozygote has an intermediate phenotype.
Incomplete dominance.
For RW × RW in that flower model, what percentage is expected to be pink?
Two of four equally likely genotype combinations are RW.
50 %
Check your understanding
Which describes codominance?
- One allele is always deleted.
- All heterozygotes have no phenotype.
- Both allele-associated products are expressed.
What does polygenic mean?
- No genetic contribution.
- Influenced by multiple genes.
- Caused by one atom.
Why can genetically similar organisms differ in an observed trait?
Answers and marking notes
Answers and marking notes
- Both allele-associated products are expressed.
Expression of both products differs from a simple intermediate phenotype.
- Influenced by multiple genes.
Several loci contribute to the trait.
- Environmental conditions and developmental effects can influence expression.
A valid example may involve resources or temperature.
Further reading and review references
- openstax.org · reference 1
- www.genome.gov · reference 2
- www.genome.gov · reference 3
- www.genome.gov · reference 4
- www.genome.gov · reference 5
- www.genome.gov · reference 6
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.