Karyograms at Higher Level: The same statement as SL. HL papers set karyograms of apes in Paper 1A and ask you to evaluate the fusion evidence, so practise reading the number and the sex fast.
Practise this as you read
- Identify the species and sex of a karyogram.
- Evaluate the chromosome 2 fusion hypothesis.
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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: the great apes: Chimpanzees, bonobos, gorillas and orangutans all have 48 chromosomes; humans have 46. Comparing their karyograms band by band is how the human chromosome 2 puzzle was found.
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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.
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.
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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 questionEvaluate[3 marks]
Evaluate the hypothesis that human chromosome 2 arose by the fusion of two chromosomes of an ancestral primate.
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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.