Learning focus
Use precise biological vocabulary, interpret diagrams and data, explain mechanisms as linked sequences, and evaluate biological applications and environmental decisions.
Chromosome mutation
A chromosome mutation is a change in chromosome number or structure. It affects much more DNA than a single-gene mutation and can influence many genes simultaneously.
Down syndrome
Down syndrome is used as the syllabus example of a chromosome-number mutation. The individual has 47 chromosomes instead of 46, commonly because an extra copy of chromosome 21 is present.
Origin
The extra chromosome can result when chromosomes fail to separate normally during meiosis. A gamete then contains an additional chromosome; after fertilisation the zygote has three copies of that chromosome.
Sensitive biological language
Chromosome conditions should be described factually and respectfully. A chromosome difference is not a measure of a person’s value. Examination answers should focus on the genetic mechanism specified in the question.
Practical or data skill
Interpret simplified karyotypes and count chromosome copies. Avoid attempting medical diagnosis from incomplete images.
Examination tip
A gene mutation changes a base sequence; a chromosome mutation changes chromosome number or structure.
Review questions and suggested answers
Question 1
How many chromosomes are associated with Down syndrome in this syllabus example?
Suggested answer
47.
Question 2
How can the extra chromosome arise?
Suggested answer
Failure of chromosomes to separate normally during meiosis.
Question 3
State one difference from a gene mutation.
Suggested answer
It changes chromosome number or structure rather than a short base sequence.
