Explain heterozygosity and why a heterozygote is not pure-breeding
A heterozygote can transmit different alleles; possible outcomes need not all appear in a small sample.
Section B16.3 · Monohybrid inheritance
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
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 is heterozygous at the gene?
What you will learn
- Identify two different alleles at the model gene
- Explain non-pure-breeding without promising exact small-family ratios
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Parent genotype | Tt: two different allele forms |
| Possible gametes | T or t at the tracked gene |
| Tt × Tt | Possible offspring TT, Tt and tt |
| Small sample | No guarantee that every possible outcome appears |
Identify two different alleles at the model gene
Heterozygous means having two different alleles of a particular gene. Tt is heterozygous in the supplied model because T and t are different forms at that gene. It does not mean two unrelated genes or two chromosomes of different species. Like homozygous, the term applies to the specified gene rather than automatically describing the entire genome.
A heterozygous individual is not pure-breeding. Its normal gametes can carry either allele at the gene. In the stated model, Tt crossed with Tt can produce TT, Tt or tt offspring, so the parental heterozygous genotype is not the only possible outcome. If T is dominant, the possible phenotypes include the recessive feature even though both parents show the dominant one.
Explain non-pure-breeding without promising exact small-family ratios
Not pure-breeding does not require every small observed family to contain every possible genotype or phenotype. A model probability describes possibilities and their expected proportions under the assumptions. Four actual offspring are not guaranteed to be one TT, two Tt and one tt. A family showing only the dominant phenotype cannot by itself turn a known Tt genotype into a homozygous one.
The partner’s genotype also affects the possible outcomes. Tt crossed with TT gives TT or Tt; both can show the dominant phenotype under the rule. That shared observable outcome does not make Tt pure-breeding, because the genotype is still different-allele and can transmit either form. Explain the allele pair and transmission rather than relying only on one observed feature.
Sources for this lesson
Pause and explain
Why is Tt not pure-breeding in this model?
Worked example
Why does a Tt × Tt cross show that the heterozygotes are not pure-breeding?
Show the worked solution
- Each parent can contribute T or t, so set out both alternatives from both parents.
- Combine the alternatives into TT, Tt, Tt and tt. More than the original Tt genotype is possible; finite samples need not show every possibility.
Answer Tt carries two different alleles. Its possible gametes are T and t; Tt × Tt can give TT, Tt and tt, so it does not reproduce one fixed heterozygous genotype.
Try a topic test
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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.
Define heterozygous using Tt and name the possible gamete alleles at this gene.
Give me a hint
Compare the two forms and separate them into gamete contributions.
Compare my reasoning
- Identify T and t as different alleles.
- Give one of them per normal gamete.
Tt is heterozygous because it has two different alleles of the gene. Normal gametes can carry T or t at that gene.
Look for these in your work
- Correct definition
- T or t gamete alleles
A Tt × Tt model permits TT, Tt and tt. Explain why this is not pure-breeding without claiming all three must occur in four observed offspring.
Give me a hint
Possible outcomes differ from guaranteed sample outcomes.
Compare my reasoning
- Identify the different possible genotypes.
- Compare them with the parental Tt genotype.
- Keep finite observations separate.
The offspring can have genotypes other than Tt, so the parents are not pure-breeding. A small observed family need not contain every possible genotype; the model does not guarantee all three in four offspring.
Look for these in your work
- Non-pure-breeding linked to different possibilities
- Avoids guaranteed small-family composition
Tt crossed with TT can give TT and Tt, both showing pigment under the supplied rule. Does the shared pigment phenotype make the known Tt parent pure-breeding?
Give me a hint
Use the known allele pair rather than one cross’s visible outcomes.
Compare my reasoning
- Retain the Tt genotype.
- Recognise two possible offspring genotypes.
- Distinguish shared phenotype from pure-breeding genotype.
No. The parent remains heterozygous and can contribute T or t; the offspring can be TT or Tt. A shared pigment phenotype does not make the parent an identical homozygote or pure-breeding.
Look for these in your work
- Keeps Tt heterozygous
- Separates phenotype similarity from pure breeding
Common mistakes
- Defining heterozygous as two unrelated genes
- Assuming a small family must contain every possible phenotype
- Calling Tt pure-breeding because one mating gives only dominant phenotypes
What can you explain now?
Why is Tt not pure-breeding in this model?
Compare with the explanation
It can transmit either allele and does not guarantee offspring of one fixed genotype
The different-allele pair permits different outcomes when the contributions combine.
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Make it stick
Without your notes, identify two different alleles at the model gene and explain non-pure-breeding without promising exact small-family ratios. 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.