Describe a diploid nucleus using two chromosome sets
Diploid means two chromosome sets. In an n = 3 model this gives three corresponding pairs and six individuals; paired chromosomes need not carry identical allele forms.
Section 17.1 · Chromosomes, genes and proteins
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
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
What defines a diploid nucleus?
What you will learn
- Connect two supplied sets with corresponding pairs
- Distinguish paired chromosomes from identical DNA forms
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Definition | Two chromosome sets |
| One supplied set | Types 1, 2 and 3 |
| Diploid model | Three pairs; six individuals |
| Information limit | Corresponding does not mean identical alleles |
Connect two supplied sets with corresponding pairs
A diploid nucleus contains two sets of chromosomes. In a simple three-type model, set one has types 1, 2 and 3 and set two also has types 1, 2 and 3. Pairing corresponding types gives three pairs and a total of six individual chromosomes. The definition is two sets, not two chromosomes in every organism.
Each chromosome in a corresponding pair is a separate chromosome. The pair can carry the same gene identities at corresponding positions while carrying the same or different allele forms. A pair is therefore not proof of two DNA molecules with identical information at every position. The paired-position key supplied earlier distinguishes identity, form and copy count.
Distinguish paired chromosomes from identical DNA forms
In the usual fertilisation model, one chromosome set comes from each gamete and the resulting nucleus contains two sets. This combines information contributions rather than turning the two gamete chromosomes into a single object. In the three-type model, each gamete contributes three individual chromosomes and the combined nucleus has six arranged as three corresponding pairs.
The original paired-set diagram uses two colours to label the stipulated contributions, not to imply one colour is dominant or better. It represents unreplicated chromosome individuals and leaves cell-division stages to later sections. Human sex-chromosome pairing also has a specific XX/XY distinction; a general corresponding-pair model should not claim X and Y have identical information.
Pause and explain
An n = 3 diploid model has which pair and individual counts?
Worked example
Why does a diploid n = 3 model have six chromosomes rather than two?
Show the worked solution
- Identify two sets of three.
- Multiply set number by set size.
Answer It contains two sets, each with three chromosome types, so the total is six individuals arranged as three pairs.
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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.
Define a diploid nucleus and apply the definition to n = 3.
Give me a hint
Use two sets of the supplied size.
Compare my reasoning
- Give the two-set definition.
- Calculate two times three.
A diploid nucleus contains two chromosome sets; the n = 3 model has six individual chromosomes.
Look for these in your work
- Define two sets
- Give the total
Count pairs and individuals when each of three supplied chromosome types occurs twice.
Give me a hint
Pair the corresponding type labels.
Compare my reasoning
- Identify three corresponding pairs.
- Count two individuals per pair.
There are three pairs and six individual chromosomes in the stipulated model.
Look for these in your work
- Give pair count
- Give individual count
A paired DNA model has A and a at the same supplied gene position. Does pairing prove identical information?
Give me a hint
Corresponding type and allele identity differ.
Compare my reasoning
- Retain the same gene position.
- Identify different allele forms.
No. The chromosomes correspond at the supplied gene position but carry different A and a forms, so pairing does not prove identical DNA information.
Look for these in your work
- Use the corresponding position
- Distinguish forms
Common mistakes
- Diploid means two sets, not always two chromosomes.
- Corresponding pairs need not have identical allele forms.
What can you explain now?
An n = 3 diploid model has which pair and individual counts?
Compare with the explanation
Three pairs and six individuals
Each of three types is represented twice.
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Make it stick
Explain connect two supplied sets with corresponding pairs and distinguish paired chromosomes from identical DNA forms. 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.