One crossover changes two of the four chromatids
- The idea
- Predict
- See it
- Why
- Check
The idea
“When homologs cross over, all four chromatids trade pieces, so every gamete from that cell is recombinant.”
In tetrad drawings the homologs wrap around each other, so it looks like all four chromatids trade pieces. Each crossover involves just two.
The short version
The mistake: When homologs cross over, all four chromatids trade pieces, so every gamete from that cell is recombinant.
What's actually true: One crossover swaps segments between two nonsister chromatids. Their sisters keep the parental alleles.
- The paired homologs put four chromatids side by side. This group is a tetrad.
- A crossover joins two nonsister chromatids, one from each homolog. They break at the same point and swap everything past it. That point is a chiasma.
- The other two chromatids aren't in that crossover, so they keep their parental alleles.
- So recombination frequency between two linked genes tops out near 50%. Even with a crossover between them in every cell, half the gametes stay parental.
On the exam: One crossover touches two of the four chromatids. Two come out recombinant and two stay parental.