The big idea: Two pictures of an organism's DNA turn up again and again in the exam — and they answer different questions.
A karyogram shows all of an organism's chromosomes, cut out and arranged in matching pairs by size. It tells you the chromosome number, the sex, and whether any chromosome is extra or missing.
A DNA profile (DNA fingerprint) shows a pattern of bands, unique to an individual. It is used to match samples — most often to work out who a child's parents are.
Your job in the exam is to read these images, not to make them.
- Karyotype
- The number and appearance of all the chromosomes in an organism's cells.
- Karyogram
- A photograph or chart in which the chromosomes of one cell are arranged in homologous pairs by size and banding pattern.
- Autosomes
- The chromosomes that are not the sex chromosomes — pairs 1 to 22 in humans.
- Sex chromosomes
- The 23rd pair in humans: XX in a female, XY in a male.
- DNA profile (DNA fingerprint)
- A pattern of bands produced from an individual's DNA that is (almost) unique to them; used to identify people and their relatives.
Don't mix the two up: A karyogram = a picture of whole chromosomes in pairs → tells you number and sex.
A DNA profile = a pattern of bands → tells you identity and parentage.
Same DNA, two very different images — read what the question actually shows you.
Both images are read in a fixed, learnable way. Practise the steps and the marks are easy.
A karyogram is built by photographing a cell's chromosomes, cutting them out, and arranging the matching pairs by size:
| Step | What you do | What you read off |
|---|---|---|
| 1. Stain & photograph | A cell is stopped in mitosis (chromosomes condensed) and photographed | All the chromosomes of one cell, scattered |
| 2. Cut out & pair | Each chromosome is cut out and matched to its partner (a homologous pair) | 23 pairs in a human = 46 chromosomes |
| 3. Arrange by size | Pairs are lined up from largest (pair 1) to smallest (pair 22), then the sex pair | A standard ordered chart — the karyogram |
| 4. Read the result | Count the chromosomes; check pairs 1–22 (autosomes) and the last pair (sex chromosomes) | Number, any extra/missing chromosome, and the sex |
Finding the sex from a karyogram: Look only at the last pair (the sex chromosomes).
Two matching chromosomes (XX) → female.
One large and one much smaller chromosome (XY) → male — the short partner is the Y chromosome.
If a question asks you to justify the sex, say which chromosomes you looked at and what they looked like (e.g. 'the 23rd pair is one large X and one small Y, so male').
| Sex chromosomes (last pair) | Sex | How you can tell |
|---|---|---|
| XX — two chromosomes the same size | Female | The 23rd pair is two matching X chromosomes (both large) |
| XY — one large (X) and one much smaller (Y) | Male | The 23rd pair is mismatched: a large X next to a short Y |
Comparing two species' karyograms: A favourite question gives you another animal's karyogram (a finch, an orangutan, a gorilla…) and asks you to compare and contrast it with a human one.
Compare these features: the total chromosome number, the relative sizes of the chromosomes, and the banding pattern. A 'compare AND contrast' answer needs both a similarity (both show homologous pairs) and a difference (a different chromosome number).
| Feature to compare | Human | Another species (e.g. an ape or bird) |
|---|---|---|
| Total chromosome number | 46 (23 pairs) | Usually a different number — e.g. a chimpanzee has 48, a chicken has 78 |
| Are they in homologous pairs? | Yes — each has a matching partner | Yes — diploid karyograms also show pairs |
| Relative sizes / lengths | Largest pair 1 → smallest pair 22 | The size pattern differs — some species' chromosomes are larger or smaller |
| Banding pattern (after staining) | A characteristic pattern for each pair | The banding pattern of matching pairs differs between species |
| Sex chromosomes | XX female / XY male | May differ (e.g. in birds the sexes are ZW/ZZ), or be present as in mammals |
A DNA profile is read completely differently — you match bands.
Every band a child has must match a band in one of its parents. Half of a child's bands come from the mother and half from the father, so to find a parent you check that all the child's bands are accounted for by that adult and the known other parent.
| Band position (size) | Mother | Father (true) | Child |
|---|---|---|---|
| Largest band (top) | ● | — | ● (matches mother) |
| Upper-middle band | — | ● | ● (matches father) |
| Lower-middle band | ● | — | — |
| Smallest band (bottom) | — | ● | ● (matches father) |
The golden rule of DNA profiling: Every band in the child must appear in one of its two parents.
To identify a father: take the bands the child did not get from the mother — the true father must have all of those.
A candidate who is missing even one of the child's bands (and the mother doesn't have it either) cannot be the parent.
Why we even have variation: sexual reproduction: Karyograms and profiles differ between individuals because sexual reproduction generates genetic variation. Three sources do this:
Crossing over in meiosis — homologous chromosomes swap sections, making new combinations of alleles.
Independent assortment in meiosis — the pairs line up and separate randomly, so gametes get a random mix of maternal and paternal chromosomes.
Random fertilisation — any one of millions of genetically different sperm can fuse with any egg.
Together these make every offspring (except identical twins) genetically unique — which is exactly why a DNA profile can tell people apart.
Three sources of variation — in order
- Crossing over (meiosis I) — homologous chromosomes exchange sections → new allele combinations on a chromosome.
- Independent assortment (meiosis I) — the random way the pairs face → a random mix of maternal/paternal chromosomes in each gamete.
- Random fertilisation — any sperm can fertilise any egg → a new, unique combination of two parents' genes.
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How this is tested: On Paper 1 a 1-mark item gives a paternity-test DNA profile and asks you to identify the child's biological father (or which couple are a child's parents). Use the golden rule: every child band must match a parent.
On Paper 2 a 1–2 mark item gives another species' karyogram and asks you to compare and contrast / distinguish it from a human one, or to identify the sex and justify it from the sex chromosomes.
A 2-mark Explain asks how sexual reproduction generates genetic variation — name and describe two of: crossing over, independent assortment, random fertilisation.
IB-style question — identify the biological father from a DNA profile
A DNA profile is run for a child, its mother, and two men (Mr Adesina and Mr Brandt) who could be the father. The child has four bands. Two of the child's bands match the mother. Of the remaining two bands, Mr Adesina has both, and Mr Brandt has only one. Identify the child's biological father and explain your reasoning. [2]
How to score both marks
- Account for the mother's bands first. Two of the child's four bands match the mother — those came from her, so set them aside. The other two bands must have come from the father.
- Apply the golden rule. The biological father must have every band the child did not get from the mother. Mr Adesina has both of those bands; Mr Brandt is missing one, so Brandt cannot be the father.
- State the answer. The biological father is Mr Adesina — all of the child's non-maternal bands appear in his profile. (Mark 1: identifies Mr Adesina. Mark 2: every child band traced to a parent / Brandt missing a band → ruled out.)
Final answer
Mr Adesina is the biological father: after removing the two bands the child shares with the mother, the remaining two bands are both present in Mr Adesina's profile, whereas Mr Brandt is missing one of them, so Brandt is ruled out.
✓ Why this scores full marks: The answer does two things: it names the father AND it explains the rule ('every child band must match a parent; the other man is missing one band').
A bare name with no reasoning usually drops the explain mark.
Exam Tips:
- Always subtract the mother's bands first — what's left must all come from the true father.
- One missing band is enough to rule a candidate out — parentage needs a FULL match of the child's bands.
- DNA profiles match bands; karyograms count and pair chromosomes. Don't confuse the two images.