Sister chromatid swaps don't make new allele combinations
- The idea
- Predict
- See it
- Why
- Check
The idea
“Any exchange between chromatids shuffles alleles, even an exchange between two sister chromatids.”
Prophase I diagrams show four chromatids pressed together, so any swap looks like it should shuffle alleles. Sisters are copies, though. Their swap trades B for B.
The short version
The mistake: Any exchange between chromatids shuffles alleles, even an exchange between two sister chromatids.
What's actually true: Sister chromatids carry the same alleles, so only crossing over between homologs makes new allele combinations on a chromosome.
- Sister chromatids are the two copies a chromosome makes when its DNA replicates. They carry the same alleles.
- Homologs are a matched pair of chromosomes, one from each parent. They can carry different alleles.
- Crossing over in prophase I swaps pieces between nonsister chromatids of homologs. That can join A with b.
On the exam: Ask who swapped. Sisters trade identical pieces. Only a swap between homologs makes new allele combinations.