Read allele notation without confusing genes alleles and gametes
Read the key, then separate the diploid allele pair from one-allele gamete records.
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
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 do T and t represent under the supplied key?
What you will learn
- Interpret a two-allele genotype at one model gene
- Keep gene identity allele identity and gamete state separate
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Same gene | T and t are different forms under this key |
| Identical pair | TT or tt is homozygous |
| Different pair | Tt or tT is heterozygous |
| Normal model gamete | Carries T or t at this gene, not the two together |
Interpret a two-allele genotype at one model gene
In the protein-coding examples used here, a gene is a length of DNA that codes for a protein; an allele is a version of that gene. A genotype record such as Tt follows two allele forms at the same gene in a diploid model. T and t are not two unrelated genes, and a lower-case letter does not mean the allele is missing. The key defines the two forms and their expression rule.
Use matching symbols for the same gene. In a simple dominance model, a capital and lower-case version of one letter often distinguish the dominant and recessive alleles. This is a notation convention, not evidence by itself: read the supplied key. Tt and tT contain the same allele combination and are usually written in the standard order Tt. Changing the written order does not change the genotype.
Keep gene identity allele identity and gamete state separate
A homozygous genotype has two identical alleles of a particular gene, such as TT or tt. A heterozygous genotype has two different alleles of that gene, such as Tt. The terms apply at the specified gene; an organism can be homozygous at one gene and heterozygous at another. Neither term by itself describes all of the organism’s genetic information.
In the supplied normal gamete model, a gamete carries one allele of this autosomal gene rather than both together. A Tt individual can therefore contribute T or t at that gene during gamete production. A gamete labelled Tt would incorrectly retain both gene copies under this model. This one-allele statement concerns the gene being followed; a gamete still carries many other genes on its complete haploid chromosome set.
Pause and explain
Which statement about Tt and tT is correct?
Worked example
Does writing tT instead of Tt change the genotype, and what can enter its normal gametes?
Show the worked solution
- Both labels contain one T and one t; writing order does not create a third allele or a different genotype.
- Separate the two allele alternatives during gamete formation: a gamete carries T or t at this gene, not the two-allele pair.
Answer Tt and tT name the same two-allele genotype. Normal gametes from the stipulated Tt individual can carry T or t at the gene, not Tt together.
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.
Classify TT, tt, Tt and tT as homozygous or heterozygous at the model gene.
Give me a hint
Compare whether the two allele symbols are identical.
Compare my reasoning
- Identify matching pairs in TT and tt.
- Identify different pairs in Tt and tT.
TT and tt are homozygous; Tt and tT are heterozygous and represent the same allele combination.
Look for these in your work
- Both homozygous genotypes identified
- Both heterozygous records identified as the same combination
A student writes Tt inside every normal gamete of a Tt parent. Correct the possible gamete labels at this autosomal gene.
Give me a hint
A normal haploid gamete receives one member of the allele pair.
Compare my reasoning
- Separate T from t.
- Write one allele per gamete record.
The possible labels are T or t, one allele at the model gene per normal gamete, not Tt in each gamete.
Look for these in your work
- Labels T or t
- One allele per gamete at the specified gene
An organism is TT at one gene and Aa at a separate gene. Explain why calling its entire genome homozygous is too broad.
Give me a hint
Apply each term at the gene being described.
Compare my reasoning
- Classify TT at the first gene.
- Classify Aa at the second.
- Limit each label to its gene.
It is homozygous at the T gene and heterozygous at the A gene. Homozygous and heterozygous describe the particular allele pair, so TT at one gene does not establish homozygosity across the whole genome.
Look for these in your work
- Classifies each pair correctly
- Limits the conclusion to each specified gene
Common mistakes
- Reading T and t as two different genes
- Treating the lower-case allele as absent
- Writing both alleles in one normal model gamete
What can you explain now?
Which statement about Tt and tT is correct?
Compare with the explanation
They contain the same allele combination
Written order does not alter the alleles present.
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Make it stick
Without your notes, interpret a two-allele genotype at one model gene and keep gene identity allele identity and gamete state separate. 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.