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Chromosome mutations involve changes to the structure or number of chromosomes, as opposed to gene mutations that affect individual nucleotide bases. These large-scale changes can have profound effects on an organism's phenotype and viability. This lesson covers structural chromosome mutations, changes in chromosome number (aneuploidy and polyploidy), and their biological and medical significance as required by the Edexcel A-Level Biology specification.
Structural mutations alter the arrangement of genes on a chromosome. They typically occur when chromosomes break and rejoin incorrectly during cell division.
| Type | Description | Consequence |
|---|---|---|
| Deletion | A segment of a chromosome is lost | Genes are missing — usually lethal in homozygous state |
| Duplication | A segment is copied and present twice on the same chromosome | Extra copies of genes — may alter gene dosage |
| Inversion | A segment is reversed in orientation | Gene order is changed — may disrupt gene expression if breakpoints fall within genes |
| Translocation | A segment moves from one chromosome to a non-homologous chromosome | Can create fusion genes — e.g. Philadelphia chromosome in chronic myeloid leukaemia |
flowchart TD
A["Structural Chromosome Mutations"] --> B["Deletion"]
A --> C["Duplication"]
A --> D["Inversion"]
A --> E["Translocation"]
B --> B1["Loss of genetic material<br/>e.g. Cri du chat syndrome (5p deletion)"]
C --> C1["Extra copies of genes<br/>Altered gene dosage"]
D --> D1["Reversed gene order<br/>May disrupt regulation"]
E --> E1["Genes move between chromosomes<br/>e.g. Philadelphia chromosome"]
In chronic myeloid leukaemia (CML), a reciprocal translocation occurs between chromosomes 9 and 22. Part of the BCR gene on chromosome 22 fuses with the ABL gene from chromosome 9, creating the BCR-ABL fusion gene. This fusion gene codes for a tyrosine kinase enzyme that is constitutively active, driving uncontrolled cell division. The drug imatinib (Gleevec) specifically inhibits this kinase — a landmark example of targeted cancer therapy based on understanding a chromosome mutation.
Cri du chat ("cry of the cat") syndrome results from a deletion on the short arm of chromosome 5 (5p−). The deleted region includes the TERT gene and other genes critical for development. Affected infants have a distinctive high-pitched cry (resembling a cat), intellectual disability, and characteristic facial features. The severity depends on the size of the deletion — larger deletions produce more severe symptoms.
Aneuploidy is the condition in which an organism has one or more extra or missing chromosomes compared to the normal diploid number. The most common cause is non-disjunction — the failure of homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) to separate properly during cell division.
When non-disjunction occurs, one gamete receives an extra copy of a chromosome and another gamete is missing that chromosome entirely. If the abnormal gamete is involved in fertilisation, the resulting zygote is aneuploid.
| Term | Chromosome complement | Example |
|---|---|---|
| Monosomy | 2n − 1 (one chromosome missing) | Turner syndrome (45, X) |
| Trisomy | 2n + 1 (one extra chromosome) | Down syndrome (trisomy 21) |
| Nullisomy | 2n − 2 (both homologues of one pair missing) | Usually lethal in humans |
flowchart TD
A["Normal Meiosis I"] --> B["Homologous pairs separate correctly"]
B --> C["Normal gametes: n chromosomes each"]
D["Non-Disjunction in Meiosis I"] --> E["Both homologues go to same pole"]
E --> F["Gamete with n+1 chromosomes"]
E --> G["Gamete with n−1 chromosomes"]
F --> H["Trisomy if fertilised (2n+1)"]
G --> I["Monosomy if fertilised (2n−1)"]
Key point for exams: Non-disjunction can occur in either meiosis I or meiosis II. In meiosis I, homologous chromosomes fail to separate. In meiosis II, sister chromatids fail to separate. The outcome is the same — aneuploid gametes — but the genetic consequences differ slightly.
| Feature | Non-disjunction in Meiosis I | Non-disjunction in Meiosis II |
|---|---|---|
| What fails to separate | Homologous chromosomes | Sister chromatids |
| Number of abnormal gametes | All 4 gametes are abnormal (2 with n+1, 2 with n−1) | 2 gametes are abnormal, 2 are normal |
| Genetic consequence | The extra chromosomes are non-identical homologues | The extra chromosomes are identical copies |
| Relative frequency | More common | Less common |
Down syndrome is the most common viable human autosomal trisomy. It results from non-disjunction producing a gamete with two copies of chromosome 21. Key features include:
The risk of trisomy 21 increases with maternal age because oocytes are arrested in meiosis I from before birth and accumulate errors over decades.
| Maternal age | Approximate risk of Down syndrome |
|---|---|
| 20 years | 1 in 1,500 |
| 30 years | 1 in 800 |
| 35 years | 1 in 270 |
| 40 years | 1 in 100 |
| 45 years | 1 in 30 |
| Condition | Karyotype | Key features |
|---|---|---|
| Edward syndrome | Trisomy 18 | Severe developmental abnormalities; most die within first year |
| Patau syndrome | Trisomy 13 | Severe developmental abnormalities; most die within first year |
| Turner syndrome | 45, X (monosomy X) | Female phenotype; short stature; infertility |
| Klinefelter syndrome | 47, XXY | Male phenotype; tall stature; infertility; some breast development |
| Triple X syndrome | 47, XXX | Female phenotype; usually mild or no symptoms |
| XYY syndrome | 47, XYY | Male phenotype; tall stature; usually mild or no symptoms |
Why are sex chromosome aneuploidies more viable? The X-inactivation mechanism (Barr body formation) means that extra X chromosomes are largely silenced. The Y chromosome carries few essential genes. This is why XXX, XXY, and XYY individuals are viable, while most autosomal trisomies are lethal.
Polyploidy is the condition in which an organism has more than two complete sets of chromosomes. It is rare in animals (usually lethal) but extremely common and important in plants.
| Term | Chromosome sets | Example |
|---|---|---|
| Triploid (3n) | Three sets | Banana (seedless) |
| Tetraploid (4n) | Four sets | Potato, some wheat varieties |
| Hexaploid (6n) | Six sets | Bread wheat (Triticum aestivum) |
Bread wheat (Triticum aestivum) is hexaploid (6n = 42) and arose through two rounds of allopolyploidy:
Exam tip: Polyploidy is a mechanism of speciation because a polyploid individual is reproductively isolated from the original diploid population. This is an example of sympatric speciation and is especially important in plants.
Many crop plants are polyploid, and polyploidy often confers advantages:
Colchicine (a chemical extracted from autumn crocus) is used artificially to induce polyploidy by inhibiting spindle formation during mitosis, preventing chromosome separation.
| Feature | Gene mutation | Chromosome mutation |
|---|---|---|
| Scale | One or a few bases | Large sections or whole chromosomes |
| Detection | DNA sequencing | Karyotyping (light microscope level) |
| Examples | Substitution, insertion, deletion | Deletion, duplication, translocation, aneuploidy |
| Frequency | Common (most are silent) | Rarer; often more severe |
| Role in evolution | Provides allelic variation | Can cause speciation (polyploidy) |
Chromosome mutations involve changes in chromosome structure (deletion, duplication, inversion, translocation) or number (aneuploidy, polyploidy). Non-disjunction during meiosis produces aneuploid gametes, leading to conditions such as Down syndrome. Polyploidy — especially allopolyploidy — is a key mechanism of speciation in plants and has been exploited in agriculture. These large-scale genetic changes complement the smaller-scale gene mutations as sources of variation.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology), which expects candidates to distinguish gene mutations (single-base scale, lesson 1) from chromosome mutations (whole-segment or whole-chromosome scale, this lesson), to classify structural rearrangements as deletion, duplication, inversion or translocation, to define aneuploidy (2n ± 1) and contrast it with polyploidy (3n, 4n, …), and to identify non-disjunction in meiosis I (homologues fail to separate) or meiosis II (sister chromatids fail to separate) as the cytological cause of aneuploid gametes. Synoptic links run backwards to lesson 1 (gene mutations) for the contrast between molecular and chromosomal scales of mutation; forwards to lesson 3 (meiosis and genetic variation) for the meiotic mechanism whose failure produces aneuploidy; outwards to Topic 4 (Biodiversity and Natural Resources) for sympatric speciation by polyploidy in plants (bread wheat is the textbook hexaploid); to Topic 6 (Infection, Immunity and Forensics) for chromosomal cancer rearrangements — the Philadelphia chromosome t(9;22) BCR-ABL fusion driving chronic myeloid leukaemia, treated by imatinib; and to Topic 2 (Membranes, Proteins, DNA and Gene Expression — cells) for the cytological technique of karyotyping, by which chromosome number and structural rearrangements are visualised. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) Classify each of the following chromosome mutations and state one named human (or human-relevant) example for each. (4)
(i) A 5–10 megabase segment is lost from the short arm of chromosome 5. (ii) A reciprocal exchange occurs between chromosomes 9 and 22, fusing parts of two genes. (iii) A gamete carries 24 chromosomes instead of 23 because both copies of chromosome 21 went to the same pole at anaphase I. (iv) A plant cell undergoes DNA replication but cytokinesis fails, producing a cell with four complete chromosome sets.
(b) Explain why monosomy X (45, X — Turner syndrome) is the only viable autosomal-or-sex-chromosome monosomy in humans, while trisomy of chromosome 21 (Down syndrome) is viable but trisomies 13 and 18 are usually fatal within the first year. (4)
Solution with mark scheme:
(a) M1 (AO2) — chromosomal deletion. Loss of a multi-megabase segment from one arm of one chromosome is a structural deletion. Many genes are simultaneously absent from one homologue, so even heterozygotes show phenotype because the remaining homologue cannot fully compensate (haploinsufficiency). The named example is cri-du-chat syndrome (5p deletion), characterised by a high-pitched cat-like cry in infancy, intellectual disability and characteristic facial features.
A1 (AO2) — reciprocal translocation. A reciprocal exchange between non-homologous chromosomes that fuses parts of two genes is a reciprocal translocation, here producing a fusion gene. The named example is the Philadelphia chromosome t(9;22), which fuses the BCR gene on chromosome 22 with the ABL1 tyrosine-kinase gene on chromosome 9; the BCR-ABL fusion protein is constitutively active and drives chronic myeloid leukaemia (CML). (BCR-ABL is the molecular target of imatinib, a paradigm of mutation-targeted cancer therapy.)
A1 (AO2) — aneuploidy by meiosis I non-disjunction. A gamete carrying both copies of chromosome 21 reflects non-disjunction at meiosis I, where the homologous pair failed to separate at anaphase I; both homologues moved to the same pole, producing two gametes with n+1 = 24 chromosomes and two with n−1 = 22. Fertilisation of an n+1 gamete by a normal gamete yields a trisomy 21 zygote (47, +21) — Down syndrome — the most common viable human autosomal trisomy.
A1 (AO2) — autopolyploidy. Replication followed by failure of cytokinesis produces a single cell with four complete chromosome sets, i.e. autotetraploidy (4n). In animals this is generally lethal; in plants it is common and often advantageous, producing larger cells, fruits and seeds. Many cultivated potatoes (Solanum tuberosum) are tetraploid. Artificial induction with colchicine (which inhibits spindle formation) is routine in plant breeding.
(b) M1 (AO3.1) — gene-dosage tolerance. Down syndrome is viable because chromosome 21 is the smallest human autosome (~225 genes), so a 50% increase in dose for a small gene set is physiologically tolerable. Chromosomes 13 (~300 genes) and 18 (~270 genes) carry more critical developmental loci, so trisomy produces lethal dose imbalance.
A1 (AO3.1) — X inactivation buffers extra X dose. Monosomy X is viable because females normally inactivate one X in every somatic cell (Lyonisation, Barr body); an XO cell behaves much like a 46,XX cell with one Barr body. Extra X chromosomes (XXX, XXY, XXXY) are likewise tolerated because all but one X are silenced. Sex-chromosome aneuploidies are therefore the only consistently viable aneuploidies in humans.
A1 (AO3.2) — Y carries few essential genes. Loss of a Y (giving XO) is tolerated because the Y carries few essential autosomal-equivalent genes, dominated by SRY (sex determination) and a handful of spermatogenesis loci; these are not required for somatic survival. Loss of an X (giving Y0) is not viable because the X carries hundreds of essential housekeeping genes.
A1 (AO3.2) — biological synthesis. Aneuploidy lethality therefore follows two rules: (i) the larger the chromosome (more genes), the less tolerable the dose imbalance; and (ii) chromosomes subject to dose-compensation mechanisms (X inactivation) tolerate aneuploidy far better than autosomes. These two rules together explain the human survival pattern: 21 trisomic > 18, 13 trisomic; sex-chromosome aneuploidies > autosomal aneuploidies; monosomy autosomal lethal; monosomy X uniquely viable.
Total: 8 marks (M2 A6).
Question (6 marks): A study karyotyped 10,000 spontaneously aborted human conceptuses and 10,000 live births, and recorded the proportion of each carrying the chromosomal abnormalities below.
| Chromosomal abnormality | Spontaneous abortions (%) | Live births (%) |
|---|---|---|
| Trisomy 16 | 7.5 | 0.0 |
| Trisomy 21 | 2.3 | 0.13 |
| Monosomy X (45, X) | 9.0 | 0.04 |
| Triploidy (3n = 69) | 6.5 | 0.0 |
| Reciprocal translocations (balanced) | 0.6 | 0.2 |
Discuss what these data show about the survival consequences of different chromosome mutations, using the data above.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that aneuploidy arises from non-disjunction in meiosis and that polyploidy involves whole extra chromosome sets |
| 2 | AO1.2 | Stating that balanced translocations preserve total gene content, while trisomies and monosomies disrupt gene dosage |
| 3 | AO2.1 | Recognising that trisomy 16 and triploidy are absent from live births despite being common in abortions, indicating prenatal lethality |
| 4 | AO2.7 | Linking trisomy 21 survival (0.13% of live births) to its small chromosome and modest dose imbalance, and monosomy X survival (0.04%) to X-inactivation tolerance |
| 5 | AO3.1 | Concluding that abortion-rate-to-live-birth ratio is a quantitative index of chromosomal lethality (trisomy 16: ∞; monosomy X: ~225×; trisomy 21: ~18×; balanced translocation: ~3×) |
| 6 | AO3.2 | Justifying that balanced reciprocal translocations are mostly tolerated because gene dosage is preserved, but unbalanced gametes from translocation carriers explain the residual elevation in abortion rate |
Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Specimen question modelled on the Edexcel 9BI0 paper format. Edexcel reliably tests chromosome-mutation classification through "given karyotype data, classify and explain survival" prompts; candidates who do not link survival to gene-dosage and X-inactivation lose AO3 marks.
Lesson 1 (gene mutations) — same principle, different scale. Gene mutations alter one or a few bases within a single gene; chromosome mutations alter whole segments (deletion, duplication, inversion, translocation) or whole chromosomes (aneuploidy, polyploidy). Both produce heritable variation and feed into natural selection, but chromosome mutations affect many genes simultaneously, are detectable by karyotyping rather than sequencing, and are generally more severe at the homozygous level. A* candidates state the scale distinction explicitly when answering "describe the types of mutation" prompts.
Lesson 3 (meiosis and genetic variation) — meiosis is where aneuploidy originates. Aneuploidy is almost always the cytological consequence of non-disjunction: failure of homologous chromosome separation at anaphase I (more common; gives all four gametes abnormal) or sister chromatid separation at anaphase II (less common; gives two of four gametes abnormal). The maternal-age effect on Down-syndrome incidence reflects the fact that human oocytes are arrested in meiosis I from before birth and accumulate spindle-assembly errors over decades. A* candidates link aneuploidy mechanistically to the meiotic stage at which separation failed.
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