HSC Exam Practice

Sample response. A chromosomal mutation is a large-scale change to the structure of a chromosome segment — and sometimes to chromosome number — rather than a change to a single base in a single gene. It often affects multiple genes simultaneously.

Marking notes. 1 mark for identifying the change as structural (and/or number-level) and large-scale relative to a point mutation; 1 mark for noting that multiple genes can be affected at once.

Sample response. Deletion is the loss of a chromosome segment, removing the genes in that region. Duplication is the repetition of a chromosome segment, increasing the copy number of the genes in that region. Inversion is the reversal of a segment's orientation within the same chromosome, changing gene order. Translocation is the movement of a chromosome segment to a different chromosome location, often onto a non-homologous chromosome.

Marking notes. 1 mark each for an accurate distinction of deletion, duplication, inversion and translocation — maximum 4.

Sample response. Gene dosage refers to the number of copies of a particular gene present in a cell, which influences how much of that gene's product is made. Duplication (or chromosome number changes such as trisomy) typically alters gene dosage by increasing copy number.

Marking notes. 1 mark for defining gene dosage in terms of gene copy number and amount of product; 1 mark for naming duplication (or trisomy / chromosome number change) as a category that alters it.

Sample response. A point mutation alters one base (or a few adjacent bases) within a single gene, so its effect is restricted to that one gene's product. A chromosomal mutation changes a much larger region of DNA — a multi-gene segment or even a whole chromosome — so multiple genes can be removed, duplicated, reversed or relocated together. As a result, multiple gene products and regulatory regions may be affected simultaneously, and the resulting phenotype is often broader and less predictable than that of a point mutation.

Marking notes. 1 mark for the scale distinction (single base / one gene vs multi-gene chromosomal segment); 1 mark for noting that multiple genes and/or regulatory regions may be affected at once; 1 mark for linking this to broader phenotypic consequences.

Sample response. A chromosome number disorder example is Down syndrome — trisomy of chromosome 21 (three copies of chr 21). A structural chromosomal mutation example is cri-du-chat syndrome — deletion of part of the short arm of chromosome 5 (5p−). Alternatives accepted: chronic myeloid leukaemia (Philadelphia chromosome — translocation t(9;22)); Turner syndrome (45,X — monosomy X); Klinefelter syndrome (47,XXY — trisomy of sex chromosomes).

Marking notes. 1 mark for a valid number example with chromosome specified; 1 mark for a valid structural example with chromosome specified. Both must be correctly classified to earn full marks.

Sample response. An inversion breaks the chromosome at two breakpoints, reverses the segment between them, and rejoins it. If a breakpoint falls inside a gene's coding sequence, the gene is physically cut in two and can no longer produce a functional protein. If a breakpoint falls inside a regulatory region (e.g. a promoter or enhancer), the gene's transcription may be lost or wrongly activated. Even without breakpoint disruption, repositioning a gene next to different neighbouring regulatory DNA can change its expression. Therefore an inversion can disrupt gene function despite no DNA being gained or lost.

Marking notes. 1 mark for naming breakpoints as the physical site of disruption; 1 mark for a mechanism (gene split / regulatory region disrupted / new regulatory neighbour); 1 mark for connecting this to functional loss without net base gain or loss.

Sample response (a). Translocation affects the largest mean number of gene products per case, with a mean of 80 affected gene products.

Sample response (b). Inversion typically only affects the small number of genes that sit at or near the inversion breakpoints; the rest of the chromosome remains intact and most genes continue to function normally. Translocation, by contrast, moves an entire multi-gene segment to a new chromosomal location and can also create fusion genes at the breakpoints — so many more gene products are altered per event, both because more genes are physically relocated and because some experience changed regulatory environments at their new location.

Sample response (c). The figure supports the lesson's claim. Inversion affects on average only 4 gene products per case — consistent with a localised change at the breakpoints. Translocation affects on average 80 gene products and deletion 35 — these involve much larger DNA regions (multi-gene segments) and therefore disrupt many more genes per event. The trend across the categories (inversion < duplication ≈ deletion < translocation) tracks the size of the genomic region typically involved.

Marking notes. (a) 1 mark for identifying translocation and quoting 80. (b) 1 mark for noting inversion is a localised breakpoint-and-reverse change; 1 mark for noting translocation moves a multi-gene segment and may create fusion / new regulatory contexts. (c) 1 mark for citing at least two specific category figures from the chart; 1 mark for connecting region size to number of gene products affected; 1 mark for explicit link back to the lesson's "larger region → broader consequences" principle.

Sample response. Translocation is a particularly important example for demonstrating that chromosome-level mutations can have major phenotypic consequences. Unlike a point mutation, which changes only one codon in one gene, a translocation moves an entire multi-gene segment from one chromosome to another at specific breakpoints. This can produce two distinct phenotypic effects that a point mutation cannot. First, the relocated genes are placed next to new regulatory DNA, so their expression can change even though their coding sequence is unaltered. Second, a breakpoint inside one gene combined with a breakpoint inside another gene can fuse two genes into a single new gene that does not exist in a typical genome. The Philadelphia chromosome in chronic myeloid leukaemia is the standard example: a reciprocal translocation t(9;22) fuses parts of ABL1 (chromosome 9) and BCR (chromosome 22) to produce the BCR–ABL1 fusion gene, encoding a constitutively active tyrosine kinase that drives uncontrolled white-blood-cell division. The clinical phenotype — a major blood cancer — could not be produced by any single missense substitution in either parent gene, because no substitution can create an entirely new fusion protein with new regulation. The example justifies the lesson's central claim that the larger the genomic region affected, the broader the possible biological consequences. The judgement is therefore that translocation is well-justified as a representative example: it makes the abstract scale logic of chromosomal mutation concrete by linking breakpoints and gene fusion to a recognisable disease phenotype.

Marking notes. 1 mark — defines translocation correctly (movement of a chromosome segment, often to a non-homologous chromosome). 1 mark — explains the role of breakpoints (precise sites of breakage and rejoining; can fall inside genes or regulatory regions). 1 mark — names a valid translocation example (Philadelphia chromosome / CML / BCR–ABL1; alternatives such as Burkitt lymphoma MYC–IgH t(8;14) accepted). 1 mark — identifies at least one of the two mechanisms by which translocation has phenotypic consequences (new regulatory context, fusion gene). 1 mark — contrasts with point mutation (single codon / single protein; cannot create a new fusion gene) using the scale logic of the lesson. 1 mark — reaches an explicit evaluative judgement that translocation is well-justified as a key example, integrating definition, mechanism and named example.