Summary
Exchange of segments between homologous chromosomes in prophase I that creates new allele combinations.
In depth
In prophase I, homologous chromosomes pair (synapsis) and exchange segments at chiasmata. At least one crossover per chromosome pair helps them align properly. The exchange makes recombinant chromatids, giving new allele combinations.
Crossovers are catalysed by enzymes that cut and rejoin DNA precisely.
Crossovers are not random: they are suppressed near centromeres and telomeres and vary between sexes (human females have about 1.6 times more than males). Crossing over is started by programmed double-strand breaks made by Spo11 protein and repaired using the homologue as a template, and crossover interference keeps events apart.
Gene conversion, a nearby result, copies one allele over another. Low crossover numbers or absent crossovers cause nondisjunction. Recombination hotspots, directed by the protein PRDM9, make genomes shuffle at predictable places.
Example: in a genetic map, recombinant offspring between two genes 10 map units apart make up roughly 10 percent of the progeny.
Frequently asked questions
What is Crossing Over?
Exchange of segments between homologous chromosomes in prophase I that creates new allele combinations.
Which area of biology covers Crossing Over, and how advanced is it?
Crossing Over is one of the 54 topics in Genetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Law of Independent Assortment, Genetic Linkage Mapping, Linked Genes.
Can you give an example of Crossing Over?
It separates linked alleles so offspring can differ from both parents.
What should I learn before Crossing Over?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Crossing Over. Intermediate topics assume the Beginner vocabulary.
What topics are related to Crossing Over?
The closest topics to Crossing Over are Law of Independent Assortment, Genetic Linkage Mapping, Linked Genes, Genetic Variation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Crossing Over?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Crossing Over. To practise the whole of Genetics, use the Genetics quiz.
How is Crossing Over different from Law of Independent Assortment?
Crossing Over: Exchange of segments between homologous chromosomes in prophase I that creates new allele combinations. Law of Independent Assortment: Alleles of different genes are distributed to gametes independently when the genes lie on separate chromosomes. Read both side by side and note what each one explains.
How should I study Crossing Over step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Law of Independent Assortment and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.