Distinguish dominance, codominance and multiple alleles

Mechanism before a label

Dominance describes a heterozygote phenotype for a specified trait, not which allele is stronger or more common. In incomplete dominance the heterozygote has an intermediate phenotype; in codominance both distinguishable products or traits are expressed.

A population can contain more than two alleles at a locus even though a diploid individual typically has two copies. Multiple genes can contribute to a quantitative trait, and one gene can influence several traits. These relationships are not captured by a universal single-locus dominant/recessive chart.

Genotype probabilities follow segregation, while mapping genotype to phenotype requires a biological model. Changing the scoring category can change the phenotype ratio without changing the cross. State penetrance and environmental assumptions rather than promising one phenotype from every genotype.

Distinguish one gene with several effects from several genes

Pleiotropy means one gene influences multiple traits. For example, a fictional enzyme used in two tissues could affect both pigment production and a metabolic output. Those effects can co-segregate because they share a genetic cause; their correlation is not proof of two independently assorting loci. OpenStax trait relationships, checked 31 October 2026.

Changed evidence: A supplied one-gene model predicts that the same allele changes both measured outputs. Another model places separate causal variants at two nearby linked loci. Does observing both outputs together in one family distinguish the models?

Check pleiotropy versus linkage

No. Both models can yield co-inheritance. Independent recombination and functional evidence could distinguish a single causal gene from linked causes; a correlation alone cannot. Pleiotropy also differs from a polygenic trait, where multiple genes contribute to one measured outcome.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Genotype class	Value (expected offspring)
RR	25
Rr	50
rr	25

Rr × Rr with complete detection and equal viability.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Distinguish dominance, codominance and multiple alleles

Supplied synthetic evidence: Rr × Rr with complete detection and equal viability.

Genotype class: reference condition

Quantity	Value	Unit
RR	25	expected offspring
Rr	50	expected offspring
rr	25	expected offspring

Task: A common allele is recessive for a trait. Does its frequency contradict recessiveness? Explain.

The letters do not establish any actual pigment mechanism.

Compare with the conditional interpretation

Genotypes are 1:2:1; incomplete dominance exposes three phenotype classes.

If complete dominance collapses RR and Rr into one category, the phenotype counts become 75:25. With incomplete dominance or codominance, the 25:50:25 genotype categories can remain visibly distinct.

Case index for this investigation

0. Track homologs and chromatids through meiosis
1. Explain crossing over and assortment without mixing mechanisms
2. Derive cross probabilities and distinguish expectation from outcome
3. Use a testcross to estimate recombination and retain limits
4. Test an inheritance null with chi-square reasoning
5. Distinguish dominance, codominance and multiple alleles
6. Interpret chromosome-linked and cytoplasmic inheritance
7. Separate genotype, environment and their interaction

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Worked reasoning

If complete dominance collapses RR and Rr into one category, the phenotype counts become 75:25. With incomplete dominance or codominance, the 25:50:25 genotype categories can remain visibly distinct.

Genotypes are 1:2:1; incomplete dominance exposes three phenotype classes.

Apply to a new case

A common allele is recessive for a trait. Does its frequency contradict recessiveness? Explain.

Compare your reasoning with the worked response

No. Dominance describes the heterozygote phenotype; frequency reflects population history and processes. Neither rarity nor frequency determines dominance.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

The letters do not establish any actual pigment mechanism. Expression intensity and dominance relationships require empirical definition.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 5.4. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.