The big idea: Every body cell carries a full set of chromosomes. The number and appearance of those chromosomes is called the cell's karyotype.
To study them, scientists make a karyogram — a photograph in which the chromosomes are cut out and arranged in matching homologous pairs, lined up from largest to smallest.
Ploidy is a separate idea: it counts how many complete sets of chromosomes a cell has. Body cells are diploid (2n) — two sets — while gametes are haploid (n) — one set.
- Chromosome
- A single, very long molecule of DNA wound around proteins; humans have 46 in each body cell.
- Homologous chromosomes
- A matching pair of chromosomes — the same size, with the centromere in the same place, carrying the same genes (one inherited from each parent).
- Karyotype
- The number and appearance (size and shape) of all the chromosomes in a cell.
- Karyogram
- A processed photograph of a cell's chromosomes, cut out and arranged in homologous pairs by size and centromere position.
- Ploidy
- The number of complete chromosome sets in a cell — haploid (n), diploid (2n) or polyploid (3n, 4n…).
- Centromere
- The point where two sister chromatids are joined; its position (central or near one end) helps classify a chromosome.
| Term | What it is | Think of it as |
|---|---|---|
| Karyotype | The number and appearance of all the chromosomes in a cell | the CHROMOSOME PROFILE itself (the facts about the chromosomes) |
| Karyogram | A processed photograph with the chromosomes arranged in homologous pairs by size and centromere position | the PICTURE you arrange to display that profile |
| Ploidy | The number of chromosome sets in a cell (n, 2n, 3n…) | how many COMPLETE SETS the cell carries |
Karyotype vs karyogram — don't mix them up: The karyotype is the information (how many chromosomes there are and what they look like).
The karyogram is the arranged picture you make to display that information.
A quick check: a karyogram is a diagram — the image.
A karyogram is built by hand from a single cell that has been caught in division, when its chromosomes are short and visible.
Once the chromosomes are arranged, you can count the ploidy, classify each chromosome, and spot any error in the number of chromosomes.
How a karyogram is built
- Stain the chromosomes in a dividing cell so they show up under a microscope.
- Photograph the spread of chromosomes.
- Cut out each chromosome from the image (or do this digitally).
- Pair up the homologues — match each chromosome with its partner of the same size and centromere position.
- Arrange the pairs in order, largest to smallest, with the sex chromosomes placed last.
Classifying chromosomes — three criteria: To decide which chromosomes are partners, you compare them using three features:
size (length), centromere position, and banding pattern.
Homologous chromosomes match on all three — that is how you know they belong in the same pair. A chromosome with its centromere very near one end is described as acrocentric.
| Criterion | What you compare | Why it helps pair them up |
|---|---|---|
| Size (length) | How long each chromosome is | Homologous chromosomes are the same length, so length sorts them into pairs |
| Centromere position | Whether the centromere is central or near one end | Partners share the same centromere position (e.g. both acrocentric) |
| Banding pattern | The stripes produced by staining | Homologous partners show the same banding pattern, confirming the match |
Counting ploidy — haploid vs diploid: Look at whether the chromosomes come in pairs.
If every chromosome has a homologous partner, the cell has two sets — it is diploid (2n), a body (somatic) cell. In humans 2n = 46.
If the chromosomes are single (no partners), the cell has one set — it is haploid (n), a gamete. In humans n = 23.
So a chromosome count tells you the cell type: a haploid count points to a gamete; a diploid count points to a somatic cell.
| Term | Chromosome sets | Where you find it | Example (human) |
|---|---|---|---|
| Haploid (n) | One set of chromosomes | Gametes (egg and sperm) | n = 23 |
| Diploid (2n) | Two sets — homologous pairs | Body (somatic) cells | 2n = 46 |
| Polyploid (3n, 4n…) | Three or more sets | Many plants and some crops | wheat is hexaploid (6n) |
Diploid (2n) — body cell
- Two sets of chromosomes
- Chromosomes appear in homologous pairs
- Found in somatic (body) cells
- Human example: 2n = 46
Haploid (n) — gamete
- One set of chromosomes
- Chromosomes are single (no partner)
- Found in gametes (egg, sperm)
- Human example: n = 23
Reading an error — non-disjunction: A karyogram can reveal a chromosome number error: an extra or a missing chromosome.
These arise from non-disjunction — when a chromosome pair (or sister chromatids) fails to separate during meiosis, so a gamete ends up with one chromosome too many or too few.
Three copies of one chromosome is trisomy — for example trisomy 21 (Down syndrome) or trisomy 18 (Edward's syndrome). The karyogram shows the tell-tale three chromosomes in one numbered row instead of a pair.
A memory hook: Karyogram = the arranged picture; ploidy = how many sets.
Pairs → diploid (body cell); singles → haploid (gamete).
An extra chromosome in one row = trisomy, caused by non-disjunction (a pair that did not separate).
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How this is tested: Karyograms are a Paper 1B data favourite: you are shown a karyogram or a table of chromosome data and asked to read it.
Common single-mark tasks: suggest (with a reason) whether data come from a gamete or a somatic cell; state the total diploid number shown; state a criterion used to classify chromosomes; identify the event (non-disjunction) behind an abnormal count.
Longer tasks ask you to describe the steps used to build a karyogram, or to classify a cell by ploidy and function.
IB-style question — gamete or somatic cell?
A microscope image of a fox cell shows 38 chromosomes, every one with a homologous partner, so they form 19 matching pairs. Suggest, with a reason, whether this cell is a gamete or a somatic cell. [2]
How to score both marks
- Make the decision. The cell is a somatic (body) cell.
- Give the reason from the data. The chromosomes are present in homologous pairs (two complete sets), so the cell is diploid (2n = 38). Gametes are haploid and would show single chromosomes (n = 19), not pairs. (Mark 1: somatic cell. Mark 2: because it is diploid / has chromosomes in homologous pairs, whereas a gamete would be haploid.)
Final answer
A somatic (body) cell — its chromosomes are in homologous pairs, so it is diploid (2n = 38); a gamete would be haploid (n = 19) with single chromosomes.
✓ Why this scores full marks: It does two things: states the cell type and justifies it using the data (pairs → diploid → somatic).
A 'suggest with a reason' mark is lost if you only name the cell type without explaining why the chromosome count points to it.
| Term | Chromosome sets | Where you find it | Example (human) |
|---|---|---|---|
| Haploid (n) | One set of chromosomes | Gametes (egg and sperm) | n = 23 |
| Diploid (2n) | Two sets — homologous pairs | Body (somatic) cells | 2n = 46 |
| Polyploid (3n, 4n…) | Three or more sets | Many plants and some crops | wheat is hexaploid (6n) |