Use paired gene-position models without confusing genes and alleles
A supplied three-position chromosome pair has three gene identities and six position copies. A/a can be two forms of one gene; B/B can be two copies of the same form.
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 the nucleus, DNA base pairing, gametes, fertilisation, enzymes, membrane carriers and chemical synapses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A and a mark the same supplied gene position. What is represented?
What you will learn
- Read corresponding positions in a supplied chromosome key
- Count gene identities separately from allele labels
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Gene identity | Corresponding position from the key |
| A and a | Two different forms of one gene |
| B and B | Two copies of the same named form |
| Three-position pair | Three identities, six copies |
Read corresponding positions in a supplied chromosome key
A supplied pair of corresponding chromosome drawings can carry the same gene at the same marked position. One chromosome may show form A and the other form a. This represents one gene identity with two different allele forms in that pair, not two different genes created by using two letters. The position key establishes which gene the labels refer to.
An original model marks three gene positions on each of two paired chromosome drawings. It therefore shows three gene identities, represented across two copies of each position. If all six labels are counted without distinguishing identity from copy, the result confuses six position copies with six different genes. Counting requires the question’s category, not merely the number of marks.
Count gene identities separately from allele labels
Two corresponding positions can carry the same allele form, such as B and B, or different forms, such as A and a. A form appearing twice in the model has two copies here, but remains one named allele form. The exercise does not ask for dominance, phenotype or segregation predictions, which require further inheritance rules.
The original paired-position diagram is a deliberately small chromosome model rather than a human karyotype. Matching bands represent supplied corresponding gene positions, not measured lengths or complete DNA records. Use the key to state gene identity, allele form and copy count independently; do not infer a whole organism’s features from the bands alone.
Sources for this lesson
Pause and explain
Three corresponding positions on two chromosomes give which counts?
Worked example
A model shows A and a at one corresponding position. How many gene identities and named forms are shown?
Show the worked solution
- Use the supplied same-gene position key.
- Separate identity from form.
Answer One gene identity and two named allele forms are shown at that position.
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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.
Interpret A and a at one corresponding gene position.
Give me a hint
Give identity and forms separately.
Compare my reasoning
- Use one shared gene identity.
- Name two alternative allele forms.
The position represents one gene with two different allele forms, A and a.
Look for these in your work
- Name one gene identity
- Distinguish two forms
Count gene identities and position copies for three marked corresponding positions on two paired chromosomes.
Give me a hint
Each position is represented twice.
Compare my reasoning
- Count three shared position identities.
- Multiply three by two for position copies.
There are three gene identities and six position copies in the supplied model.
Look for these in your work
- Use identity count
- Use copy count
A paired model has B and B at one position. Does the repeated label create two different allele forms?
Give me a hint
Repeat copies and distinct forms differ.
Compare my reasoning
- Identify the same named form on both copies.
- Distinguish two copies from two forms.
No. B is one named allele form represented in two copies at that position; repetition does not create a new alternative form.
Look for these in your work
- Retain the same form
- Distinguish copies
Common mistakes
- A copy count is not a count of distinct gene identities.
- Paired position labels do not establish a phenotype.
What can you explain now?
Three corresponding positions on two chromosomes give which counts?
Compare with the explanation
Three gene identities and six position copies
Each identity is represented twice.
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Make it stick
Explain read corresponding positions in a supplied chromosome key and count gene identities separately from allele labels. Use the stated cell, chromosome sets and information pathway before opening the solutions.
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.