Apply dominant and recessive allele expression in a one-gene model
Dominance is an expression rule, not a statement of frequency, value or allele deletion.
Section B16.3 · Monohybrid inheritance
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does Tt contain?
What you will learn
- Use the supplied dominant expression rule
- Identify when the recessive phenotype is expressed
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| TT | Dominant allele present; pigment present |
| Tt | Dominant phenotype; t still carried |
| tt | No dominant allele; recessive phenotype |
| Population frequency | A separate question from dominance |
Use the supplied dominant expression rule
In the simple complete-dominance model used here, a dominant allele is expressed in the phenotype if it is present in the genotype. The original key states that T produces pigment when present. TT and Tt therefore both show pigment. Dominant describes the expression relationship at this gene, not whether the allele is common, beneficial, stronger or more desirable.
A recessive allele is expressed in the phenotype only when no dominant allele of that gene is present in this model. The key uses t for pigment absence, so tt lacks pigment. Tt still carries and can transmit t even though the phenotype shows the T-associated feature. The recessive allele has not disappeared from the genotype and is not prevented from entering a gamete.
Identify when the recessive phenotype is expressed
Read the question’s key before converting genotypes to phenotypes. Letter case is a convenient notation in the simple model; it does not create the biological rule. Other inheritance patterns require their own rules, including codominance in the Supplement sequence. Do not describe every heterozygote in biology as showing only one allele’s contribution.
Apply the two rules in order: check the allele pair, find whether a dominant allele is present, then give the stated phenotype. For a supplied set TT, Tt, tt and tt, two show pigment and two do not. That record gives a one-to-one observed count, but it does not establish the parents’ genotypes or a universal frequency of either allele. Expression and population frequency are different questions.
Sources for this lesson
Pause and explain
When is the recessive phenotype expressed under the supplied rule?
Worked example
With T dominant for pigment and t recessive for absence, classify TT, Tt and tt.
Show the worked solution
- Look for T in each allele pair: TT and Tt both contain it, whereas tt does not.
- Use the expression key to assign pigment or absence. Retain the t in Tt even though its recessive feature is not expressed.
Answer Given T produces pigment whenever present: TT and Tt show pigment; tt does not. Tt still contains and can transmit t.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Assign pigment phenotypes to TT, Tt and tt using the supplied rule. Explain why Tt does not show pigment absence.
Give me a hint
First check whether T is present.
Compare my reasoning
- Identify T in TT and Tt.
- Identify its absence in tt.
- Apply the stated expression rule.
TT and Tt show pigment; tt lacks pigment. Tt has the dominant allele T, so the recessive absence phenotype is not expressed under this rule.
Look for these in your work
- Three correct phenotypes
- Uses presence of T to explain Tt
A population record says t is more frequent than T. Does this make t dominant? Explain using the definition.
Give me a hint
Frequency and phenotype expression describe different properties.
Compare my reasoning
- Identify the frequency statement.
- Recall the supplied expression rule.
No. Dominance concerns expression when the allele is present with its alternative in the genotype. The relative frequency of t does not change the stated T/t expression rule.
Look for these in your work
- Rejects frequency as definition
- Explains genotype-expression relationship
In a supplied record TT, Tt, tt, tt, calculate pigment present to absent and explain why this record alone does not identify a parental cross.
Give me a hint
Assign each phenotype before counting.
Compare my reasoning
- Count two pigment-present and two pigment-absent records.
- Simplify two to two.
- Distinguish observed records from known parent genotypes.
The observed count ratio is 2:2, simplified to 1:1. It does not by itself establish a parental cross; parental genotypes and the transmission model would need to be supplied or investigated.
Look for these in your work
- Correct 1:1 observed ratio
- Avoids identifying parents from the record alone
Common mistakes
- Calling dominant common or biologically better
- Saying t is absent from Tt because its feature is not expressed
- Applying complete dominance when a different expression rule is supplied
What can you explain now?
When is the recessive phenotype expressed under the supplied rule?
Compare with the explanation
When no dominant allele of this gene is present
tt has no T and expresses the recessive feature in this model.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Without your notes, use the supplied dominant expression rule and identify when the recessive phenotype is expressed. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.