The big idea: A karyogram shows a cell's chromosomes sorted into pairs. Making one is called karyotyping.
The points to remember
- Cells are stopped in metaphase, when chromosomes are most condensed and easy to see.
- The chromosomes are stained and photographed under a microscope.
- Each chromosome is matched with its homologous partner.
- Pairs are arranged by length, from the longest to the shortest, and numbered.
- The sex chromosomes go last: XX in a female, XY in a male.
Remember it as: Photograph, pair, line up by size.
Real example: a hospital genetics lab: Hospital labs grow white blood cells from a blood sample, stop them at metaphase and photograph the chromosomes. A karyogram can show a missing or extra chromosome, such as the third chromosome 21 in Down syndrome.
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The big idea: Chromosomes are classified by three features: length, the position of the centromere and the banding pattern.
The points to remember
- Length: pair 1 is the longest, the smallest come last.
- Centromere position: in the middle, off-centre, or near one end (acrocentric).
- Banding pattern: stains give each chromosome its own stripes, the same on both partners.
- Count every chromosome for the number: 46 in humans, 48 in chimpanzees.
- Look at the last pair for the sex: two matching X, female; a large X and a small Y, male.
Size, not shape: Say length, centromere position or banding. 'Shape' alone is too vague, and 'no Y' is not a reason: say the pair matches in size.
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The big idea: Humans have one pair fewer than other great apes. The hypothesis: human chromosome 2 formed when two ape chromosomes fused.
The points to remember
- The bands of human 2 match those of chimpanzee 12 and 13 laid end to end.
- Telomere DNA, normally only at chromosome ends, is found in the middle of human 2.
- Human 2 has a second, inactive centromere where the second chimpanzee chromosome's would be.
- So the evidence supports fusion in our ancestors after the split from chimpanzees.
- The idea is a testable hypothesis: evidence can support or reject it. 'Humans are special' is not testable.
Evaluate it: Three independent lines of evidence point the same way, which makes the hypothesis strong. It is still a hypothesis: new data could test it further.
How it is tested: Paper 1B gives a karyogram: count the chromosomes, find a row by centromere position, deduce the sex and describe how it was made. Paper 1A asks which species or sex a karyogram is from.
IB-style questionDescribe[2 marks]
Describe how a karyogram is constructed.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Deduce the sex with a reason: Female: the last pair matches in size. Male: one large X and one small Y.