IB Biology HLClassifying Living DiversityPaper 1 & 2~12 min read
Karyograms
Humans have 46 chromosomes. Chimps, gorillas and bonobos all have 48. If we share a recent common ancestor with them, a whole pair had to go somewhere. Karyograms let scientists test where it went — and the answer is sitting in human chromosome 2.
📚 What you need to know
A chromosome is a strand of DNA. A gene is a section of it that codes for one polypeptide.
After replication a chromosome looks like an X, with the two strands joined at the centromere.
Metacentric chromosomes have a central centromere and arms of equal length; acrocentric ones have the centromere near the end.
Stained chromosomes show a banding pattern.
A karyogram is an image of all the chromosomes in a cell, arranged by size, shape and banding and placed in homologous pairs.
It shows the karyotype: the appearance of a complete set of chromosomes, including number, size, shape and banding.
Made by staining cells, photographing the nucleus at metaphase, then cutting up and arranging the images.
Karyograms can test the hypothesis that two ancestral chromosomes fused to form human chromosome 2.
A testable hypothesis needs access to evidence and a statement that can be accepted or rejected.
Chromosome shapes
Where the centromere sits gives a chromosome its shape, and shape is one of the things used to sort them.
The X shape only appears after DNA replication, when each chromosome is two identical strands held together at the centromere.
Karyograms and karyotypes
A karyogram is an image showing all the chromosomes from one cell, arranged by size, shape and banding pattern and lined up with their homologous partners. What it displays is the karyotype:
Karyotype
The appearance of a complete set of an individual’s chromosomes, including their number, size, shape and banding
The pairs are numbered roughly in order of decreasing size. Pair 23 often breaks that order, because it holds the sex chromosomes and the X chromosome is very large.
How one is made
Cells are stained and viewed under a light microscope.
Photographs are taken of the contents of the nucleus during metaphase of cell division, when the chromosomes are condensed and clearly visible.
The photographed chromosomes are cut up and arranged by size, shape and banding pattern — with paper and scissors, or on a computer.
Metaphase is not a random choice. It is the only stage where chromosomes are condensed enough to see as separate objects with clear banding. Photograph a nucleus at any other point and you get a tangle. If a question asks why metaphase, that is the answer.
The chromosome 2 question
Humans have 46 chromosomes. Chimpanzees, gorillas and bonobos all have 48. Evidence shows we share a recent common ancestor with these species, so a whole pair — taking 24 pairs down to 23 — has to be accounted for. Two things could have happened:
A pair of chromosomes disappeared from the genome.
A pair fused with another pair to form a single pair.
Losing an entire pair would have removed a huge number of genes and probably put survival at risk, so the first is unlikely. The second gives a hypothesis that karyograms can actually test:
The hypothesis
Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2
Telomeric DNA in the middle is the most striking piece of evidence. Chromosome ends are the one place that DNA belongs — finding it in the centre suggests two ends were once joined there.
Evidence for the hypothesis
Evidence against
Chimp chromosomes 12 and 13 placed end to end match the length of human chromosome 2
The combined length is not a perfect match; there is a slight overlap
The centromere position of chimp chromosome 12 matches that of human chromosome 2
The centromere position of chimp chromosome 13 does not match anything in human chromosome 2
Human chromosome 2 has satellite DNA where chimp 13’s centromere sits, possibly a remnant centromere
The banding of the long arms of chimp 12 and 13 corresponds to the banding of human chromosome 2
Human chromosome 2 contains telomeric DNA in the middle of the chromosome
Because of this, chimp chromosomes 12 and 13 are sometimes called 2A and 2B, acknowledging the fusion event.
Say it the right way round. None of this means humans evolved from chimps. Both species are living today. The evidence tells us about how humans and chimps may have diverged from their common ancestor, which is a different claim and the one examiners want to see.
Testable and non-testable hypotheses
The fusion hypothesis is a good example of a testable one. Two things make a hypothesis testable.
Access to evidence that supports or refutes it. Chromosome evidence from humans and modern chimps can be examined. Compare that with “the common ancestor of humans and chimps enjoyed singing” — no evidence exists either way, so it is not testable.
A statement that can be accepted or rejected. “Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2” is a clear claim you can decide about.
A hypothesis should not…
Weak version
Better version
be vague
Surface area to volume ratio could affect heat loss
Organisms with a large surface area to volume ratio lose heat more quickly
make a prediction
Surface area to volume ratio will affect heat loss
Organisms with a large surface area to volume ratio lose heat more quickly
draw a causal conclusion
A larger surface area to volume ratio causes increased heat loss
Organisms with a large surface area to volume ratio lose heat more quickly
You are not expected to memorise every detail of the chromosome 2 evidence. What is being assessed is the skill of evaluating evidence — can you look at a set of observations, sort them into support and challenge, and reach a measured conclusion.
Worked examples
WE 1
Describe how a karyogram is produced
Outline how a karyogram is produced from a sample of cells. (3 marks)
Step 1: prepare and view
Cells are stained and examined under a light microscope.
Step 2: photograph
Photographs are taken of the nucleus during metaphase, when chromosomes are condensed and visible.
Step 3: arrange
The images are cut up and arranged in homologous pairs by size, shape and banding pattern.
Stain, photograph at metaphase, arrange into pairsmetaphase is nearly always worth a mark on its own
WE 2
Evaluate the fusion hypothesis
Evaluate the evidence for the hypothesis that human chromosome 2 was formed by the fusion of two ancestral chromosomes. (4 marks)
Support 1
Chimp chromosomes 12 and 13 end to end match the length of human chromosome 2, and their banding corresponds.
Support 2
Human chromosome 2 contains telomeric DNA in the middle, and satellite DNA where chimp 13’s centromere would be.
Against
The combined length is not a perfect match, and the centromere of chimp 13 does not correspond to anything in human chromosome 2.
Judgement
The evidence is strong but not complete, so the hypothesis is well supported rather than proven.
Several independent lines of support, with a couple of mismatches“evaluate” needs both sides plus a judgement — do not just list the support
WE 3
Judge whether a hypothesis is testable
State whether the following is a testable hypothesis and justify your answer: “Early humans may have had better hearing than modern humans.” (2 marks)
Point 1: the judgement
It is not a testable hypothesis as written.
Point 2: the reasons
The word “may” makes it vague, so it cannot be clearly accepted or rejected, and there is no direct evidence of hearing ability from early humans to test it against.
Too vague, and no evidence available either wayname which rule is broken — vagueness, prediction or causal claim
💡 Exam tips
Define a karyotype as the appearance of a complete set: number, size, shape and banding.
A karyogram is the image; the karyotype is what it shows.
Say chromosomes are photographed at metaphase.
Metacentric means central centromere; acrocentric means near the end.
For evaluation questions, split the evidence into for and against, then judge.
Always say humans and chimps diverged from a common ancestor.
⚠ Common mistakes
Saying humans evolved from chimpanzees. Both are living species with a shared ancestor.
Using karyogram and karyotype as synonyms. One is the picture, the other is the appearance it shows.
Only giving supporting evidence in an evaluate question.
Writing that a gene codes for a protein. The syllabus wording is one polypeptide.
Forgetting that pair 23 breaks the size order because of the sex chromosomes.
Writing hypotheses with “may” or “could”. They become impossible to accept or reject.
Up next: Genomes — from single chromosomes to all the genetic information an organism carries, and what the differences between two genomes actually are.
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