Crossing over or independent assortment?
- The idea
- Predict
- See it
- Why
- Check
The idea
“Crossing over and independent assortment both shuffle alleles, so either can explain any new mix.”
Both happen in meiosis I and make new mixes of alleles, so they blur together. The difference is which genes each one can reach.
The short version
The mistake: Crossing over and independent assortment both shuffle alleles, so either can explain any new mix.
What's actually true: Independent assortment shuffles genes on different pairs. Only crossing over can split the alleles of genes on one chromosome.
- At metaphase I, each pair lines up either way, whatever the others do. So genes on two different pairs give four kinds of gametes equally.
- At anaphase I, homologs move whole, so lining up alone keeps genes on one chromosome together. Those genes are linked.
- In prophase I, a chromatid from each homolog trades matching segments. That makes new mixes of linked alleles. Genes farther apart recombine more often.
On the exam: Only crossing over splits genes on one chromosome. Independent assortment shuffles genes on different pairs.