9477 / 2027

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?

I can explain aneuploidy, trisomy 21 and structural chromosome aberrations.

  • I can explain gene mutations and numerical/structural chromosome aberrations.Syllabus

    Syllabus 9477, Core 2(k). Explain gene mutations and numerical/structural chromosome aberrations.

About 7 min

Make a guess. You are not marked.

A chromosome segment is reversed (an inversion). No DNA is lost or gained. Can this still affect the person?

  1. It can if a break falls in a gene.
  2. The same genes are still present.
  3. Losing or gaining DNA is what has effects.
Show the answer

It can if a break falls in a gene.

Breakpoints can cut through a gene or move it near different regulatory DNA. Pairing in meiosis can also be upset.

The key idea

Chromosome aberrations change dosage or arrangement at a larger scale than a single gene sequence.

Explore the idea

Compare chromosome amount and order

Reference: A-B-C-D-EABCDE

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. 1

    Count chromosomes

    Ask whether an entire chromosome is extra or missing.

  2. 2

    Compare segment order

    Track labels through deletion, duplication, inversion or transfer.

  3. 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.

Watch out for this

A student says: "Trisomy 21 means one base has been inserted into gene 21." What is wrong with this?

Show the answer

It means three copies of chromosome 21, a chromosome-number change affecting many genes.

Check your understanding

Which changes the number of a particular whole chromosome?

  1. Trisomy 21
  2. A base substitution in one gene
  3. An inversion within chromosome 2
Show the answer

Trisomy 21

There are three copies of chromosome 21.

Quick recall

Card 1 of 3. Answer in your head, then check.

How does losing a chromosome differ from deleting part of one?

You can return to this lesson any time.