Chromosomal Mutation — Source Critique

Q1 — Sample Band 6 response (7 marks), annotated

The quote contains three distinct scientific errors.

Error 1 — wrong mutation class. Down syndrome is not a point mutation. It is a chromosome number change (trisomy 21) — affected individuals carry three whole copies of chromosome 21 rather than two. A point mutation alters a single base or a few bases within one gene's sequence; trisomy 21 leaves the DNA sequence of every gene intact but changes their copy number across an entire chromosome. [1 — identifies error and correct biology]

Error 2 — wrong scale of effect. The quote claims the effect is "restricted to one protein" because a "single base of DNA is altered". Even if it were a point mutation that would be defensible; but because the actual cause is an extra whole chromosome, hundreds of genes on chromosome 21 are present in three copies, so the dosage (and therefore the amount of gene product) of many genes is altered simultaneously. This is precisely the lesson's scale logic — large DNA regions affected → broad phenotypic consequences. [1 — scale logic + gene dosage]

Error 3 — wrong detection / wrong "fix". Down syndrome cannot be "corrected" by editing a single DNA base because no single base is wrong. The underlying change is detected by karyotyping (or modern FISH / chromosomal microarray), which images the chromosomes in metaphase and reveals three copies of chromosome 21. Pre-implantation genetic screening uses these techniques to identify embryos with trisomy 21, not to "correct" them at the base level. [1 — detection technique named correctly + "correction" framing rejected]

Defensible reformulation. "Down syndrome is caused by trisomy 21 — an extra whole copy of chromosome 21, not a point mutation — which raises the gene dosage of hundreds of genes on that chromosome and is detected cytogenetically by karyotyping rather than by single-base sequencing." [1 — single defensible sentence integrating all three corrections]

Marking criteria.

• 1 mark — Identifies Error 1 (it is not a point mutation; it is a chromosome number change / trisomy 21).
• 1 mark — States the correct biology for Error 1 (extra whole chromosome 21, all genes on it in three copies).
• 1 mark — Identifies Error 2 (effect is not restricted to one protein) and applies the lesson's scale logic / gene dosage to explain why many gene products are affected.
• 1 mark — Identifies Error 3 (cannot be "corrected" by editing a single base) and explains why.
• 1 mark — Names karyotyping (or FISH / chromosomal microarray) as the technique that detects the chromosome number change.
• 1 mark — Uses precise lesson terminology throughout (chromosomal mutation vs point mutation; chromosome number; gene dosage; aneuploidy / trisomy).
• 1 mark — Reformulates into a single defensible sentence that integrates all three corrections.

Q2 — Sample Band 6 response (7 marks), annotated

The student's paragraph contains three distinct scientific errors.

Error 1 — wrong scale. A translocation is not "essentially a point mutation occurring on a chromosome". A point mutation alters a single base or a few adjacent bases within one gene; a translocation moves a multi-gene segment of DNA from one chromosome to another. The two categories operate at fundamentally different scales, which is exactly why their consequences differ. [1 — scale distinction]

Error 2 — wrong mutation classification. The student then calls the same change "an inversion mutation". An inversion reverses a segment's orientation within the same chromosome; a translocation moves a segment to a different chromosome (in t(9;22), between non-homologous chromosomes 9 and 22). These are two separate categories from the four listed in the lesson. [1 — translocation vs inversion correctly distinguished]

Error 3 — wrong mechanism. The claim that "every translocation simply rearranges genes within the same chromosome and never creates new ones" is incorrect. The Philadelphia chromosome demonstrates that a translocation can fuse two normally separate genes (BCR and ABL1) across breakpoints, producing an entirely new fusion gene — BCR–ABL1 — that does not exist in a typical genome. The resulting fusion protein is a constitutively active tyrosine kinase that drives uncontrolled cell division. This is far more than a single amino-acid change. [1 — fusion gene mechanism + breakpoint terminology]

Why this matters. The Philadelphia chromosome shows precisely the lesson's point: a translocation can (a) place genes next to new regulatory regions and/or (b) join two genes into a single new gene with novel behaviour. A single base substitution can at most change one amino acid in one existing protein — it cannot create a new fusion gene or move a gene into a new regulatory context. That is why a chromosome-level change can have phenotypic consequences that a point mutation cannot. [1 — mechanism comparison with the lesson's scale logic]

Defensible reformulation. "In chronic myeloid leukaemia, a reciprocal translocation between chromosomes 9 and 22 — t(9;22) — joins part of the ABL1 gene (chromosome 9) with part of the BCR gene (chromosome 22) at specific breakpoints, producing the Philadelphia chromosome and a novel BCR–ABL1 fusion gene. The resulting fusion protein is a constitutively active tyrosine kinase that drives uncontrolled cell division — a phenotype that no single missense substitution in ABL1 could reproduce, because translocation can both relocate genes into new regulatory contexts and physically fuse separate genes into a single new gene." [1 — clean rewrite; 1 — precise lesson terminology applied throughout]

Marking criteria.

• 1 mark — Identifies Error 1 (translocation is not a point mutation; rejects the scale equivalence).
• 1 mark — Identifies Error 2 (translocation is not the same as inversion; defines each correctly — translocation between chromosomes, inversion within a chromosome).
• 1 mark — Identifies Error 3 (translocation can create new genes; rejects "never creates new ones") and names the Philadelphia chromosome / BCR–ABL1 fusion as evidence.
• 1 mark — Uses the term breakpoint correctly to explain how the fusion gene is generated.
• 1 mark — Explains why the Philadelphia chromosome demonstrates that translocation can have effects a point mutation cannot (relocation into new regulatory context AND/OR fusion of two genes).
• 1 mark — Reaches an integrated reformulated paragraph using precise terminology (translocation, breakpoint, fusion gene, gene regulation, t(9;22)).
• 1 mark — Maintains the lesson's central scale logic throughout (chromosomal mutation can affect many genes / gene contexts at once; point mutation usually cannot).