Lesson 2 of 9 / Mutation, cell division and cancer
Change chromosome structure or number
How does losing a chromosome differ from deleting part of one?
In this lesson: Explain aneuploidy, trisomy 21 and structural chromosome aberrations.
About 7 min
The key ideaChromosome aberrations change dosage or arrangement at a larger scale than a single gene sequence.
Explore the idea
Compare chromosome amount and order
Use this chromosome-segment order as the reference.
Explanation
Aneuploidy is an abnormal number of particular chromosomes rather than a complete extra chromosome set. Nondisjunction during division can produce cells with an extra or missing chromosome. Trisomy 21 means three copies of chromosome 21 and is a common cause of Down syndrome; it affects dosage of many genes.
A deletion removes a chromosome segment; duplication repeats a segment. Both can alter gene dosage. An inversion reverses the orientation of a segment within a chromosome. A translocation moves a segment to a different chromosome, commonly a non-homologous one.
A balanced rearrangement may preserve the total amount of DNA yet still matter if a breakpoint disrupts a gene or changes its regulatory surroundings. It can also affect meiotic pairing and the balance of DNA passed to gametes. Therefore no net DNA loss does not guarantee no biological consequence.
Describe evidence at the appropriate scale. A karyotype can reveal chromosome number and large changes, but not every point mutation. A short DNA sequence can reveal a substitution without showing the complete chromosome complement. Choose the method to match the question.
Step by step
- 1
Count chromosomes
Ask whether an entire chromosome is extra or missing.
- 2
Compare segment order
Track labels through deletion, duplication, inversion or transfer.
- 3
Predict dosage or position effects
A balanced total can still disrupt function.
Worked example
Work through the evidence
A chromosome labelled A-B-C-D-E becomes A-B-D-C-E. Another becomes A-B-C-D-D-E. Name the changes.
One way to explain it
The first has an inversion of the C-D segment. The second has a duplication of D.
Why this answer works
- Track order separately from copy number.
- Neither diagram alone describes a whole extra chromosome.
Is this true? "Trisomy 21 means one base has been inserted into gene 21."
It means three copies of chromosome 21, a chromosome-number change affecting many genes.