Review all twelve Core monohybrid inheritance requirements with linked evidence
Use the full evidence chain for all twelve Core outcomes without implying wider completion.
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
Which term describes two different alleles of one gene?
What you will learn
- Combine definitions gamete records squares and pedigree evidence
- State model assumptions and separate expected from observed outcomes
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Definitions | Inheritance, genotype, phenotype, homozygous and heterozygous |
| Breeding and expression | Pure-breeding condition; dominant and recessive key |
| Genetic diagrams | Gametes, square cells, 3:1 and 1:1 phenotype predictions |
| Pedigrees and limits | Trait key, family evidence, ambiguity and remaining Supplement work |
Combine definitions gamete records squares and pedigree evidence
Inheritance transmits genetic information between generations. Genotype describes genetic make-up, including the alleles at the gene being followed; phenotype describes observable features. Identical alleles at one gene are homozygous and different alleles are heterozygous. Identical homozygous parents are pure-breeding for the stated model characteristic, whereas a heterozygote is not pure-breeding.
In the original complete-dominance key, T is expressed whenever present, so TT and Tt show pigment. The recessive absence feature is expressed as tt, without T. Tt still carries and can transmit t. Normal gamete labels contain one allele of the tracked autosomal gene: T from TT, t from tt, or T and t alternatives from Tt. Dominance does not define frequency or remove the recessive form from transmission.
State model assumptions and separate expected from observed outcomes
Construct a Punnett square using one contribution from each parent. Tt × Tt gives TT, Tt, Tt and tt: genotype TT:Tt:tt is 1:2:1 and phenotype pigment present:absent is 3:1. Tt × tt gives Tt and tt in equal positions, with phenotype 1:1. Expected counts use these probabilities and the supplied total, while actual sample counts can depart from an exact predicted ratio.
Pedigree interpretation requires the current trait key and family links. In a declared recessive-key example, pigment-producing parents with a tt offspring must both be Tt; a pigment-producing sibling can remain TT or Tt. The Core sequence supplies teaching and self-checked practice, not a mastery certificate. Test crosses, codominance, ABO blood groups and sex linkage are Supplement requirements needing further study beyond these Core lessons
Sources for this lesson
Pause and explain
What distinguishes the phenotype ratios of Tt × Tt and Tt × tt under this rule?
Worked example
Connect a stated Tt × Tt cross from parental allele information to genotype and phenotype predictions.
Show the worked solution
- Derive T/t gametes from each parent and combine the four positions TT, Tt, Tt and tt.
- Apply the dominant-pigment key to group them as three present and one absent. State the 1:2:1 genotype ratio separately from the 3:1 phenotype ratio and retain finite-sample limits.
Answer Connect the chain: parental genotypes → one-allele gametes → square combinations → phenotype rule → predicted ratio. Use pedigree evidence to identify or retain ambiguous parental genotypes.
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.
For Tt × Tt, list the possible square positions, genotype ratio and phenotype ratio using the pigment key.
Give me a hint
Fill the square before grouping phenotypes.
Compare my reasoning
- List TT, Tt, Tt and tt.
- Count genotype categories.
- Combine pigment-producing TT and Tt.
Positions TT, Tt, Tt, tt; genotype TT:Tt:tt = 1:2:1; phenotype pigment present:absent = 3:1.
Look for these in your work
- Correct cells and genotype ratio
- Correct labelled phenotype ratio
A stipulated Tt × tt cross has an expected total of 48 offspring. Give the expected genotype counts and pigment phenotype counts.
Give me a hint
Each outcome category has probability one half.
Compare my reasoning
- Calculate 48 × 1/2.
- Assign the result to Tt and tt.
- Convert to pigment present and absent.
Expected 24 Tt and 24 tt; expected pigment present 24 and absent 24. Actual observations need not be exactly balanced.
Look for these in your work
- Correct expected genotype counts
- Correct phenotype counts with sample limit
In the declared recessive pedigree, both parents show pigment and have one tt offspring and one pigment-producing offspring. Infer parental genotypes, retain the second offspring’s possibilities, and name the remaining scope limits.
Give me a hint
Use each t contribution and the pigment rule, without assuming a sibling genotype.
Compare my reasoning
- Assign Tt to both parents.
- Keep TT or Tt for the pigment-producing sibling.
- Separate Core teaching from remaining Supplement and assessment work.
Both parents are Tt; the pigment-producing sibling can be TT or Tt. These Core lessons alone do not cover the Supplement test-cross, codominance, ABO and sex-linkage requirements or establish human teacher approval, supervised practical competence or cumulative assessment completion.
Look for these in your work
- Correct parents and sibling ambiguity
- Names remaining Supplement and assessment work
Common mistakes
- Giving a phenotype ratio without the allele rule
- Confusing 1:2:1 genotype with 3:1 phenotype
- Claiming Core drafts complete the Supplement or wider curriculum
What can you explain now?
What distinguishes the phenotype ratios of Tt × Tt and Tt × tt under this rule?
Compare with the explanation
The first predicts 3:1 and the second 1:1
Their different gamete contributions give different genotype groupings.
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Make it stick
Without your notes, combine definitions gamete records squares and pedigree evidence and state model assumptions and separate expected from observed outcomes. 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.