Biology A-Level
crossing over
What crossing over means in Biology A-Level, as defined for AQA.
Definitions of crossing over
Exam boards and our study guides word this term in slightly different ways. Learn the version your board uses.
Check what you remember
Read the wording for your board, then hide it and explain crossing over in your own words. Rough notes are fine. This is a self-check, with no mark or grade.
Reference: AQA
In prophase I, homologous chromosomes form bivalents and sections of chromatid are exchanged at the chiasmata, producing chromosomes that carry new combinations of alleles of linked genes.
A process in prophase one that exchanges equivalent lengths of chromatid between homologous chromosomes, so no gene is lost or duplicated.
In prophase I, homologous chromosomes form bivalents and chromatids twist at chiasmata, exchanging equivalent sections of DNA.
Topics that use crossing over
- DNA, genes and chromosomes (AQA A-Level)
- Evolution may lead to speciation (A-level only) (AQA A-Level)
- Genetic diversity can arise as a result of mutation or during (AQA A-Level)
Related terms
- alleles
- degenerate code
- genetic variation
- new combinations of alleles
- polyploidy
- random fertilisation
- allele frequency change
- allopatric speciation
Using crossing over in the exam
Check the command word and mark scheme for your exact question. After comparing your explanation with the reference, use the term in an answer on the linked topic. This practice does not predict an exam mark.
Practise Biology questions
Create a free account to answer exam-style questions on this topic and get feedback on your answers.
Start free
All Biology A-Level termsA week of Premium from your first lesson · Then the free plan continues · No card, no charge