Splitting sister chromatids doesn't mix alleles
- The idea
- Predict
- See it
- Why
- Check
The idea
“Pulling sister chromatids apart in meiosis II gives each gamete a new mix of alleles.”
Meiosis II is the last split in meiosis, so it looks like where the mixing happens. By then, every chromatid's alleles are set.
The short version
The mistake: Pulling sister chromatids apart in meiosis II gives each gamete a new mix of alleles.
What's actually true: Splitting sisters only hands out alleles already on them. Crossing over and independent assortment in meiosis I make new mixes.
- In prophase I, paired homologs cross over. Non-sister chromatids trade segments, making mixes like A b that neither homolog had.
- At metaphase I, each pair lines up facing either way, independently of other pairs. This independent assortment mixes whole chromosomes.
- Sisters start as copies. After crossing over they differ, and meiosis II sends them apart. That split delivers the new mix but doesn't make it.
On the exam: Meiosis II hands out allele mixes. Crossing over and independent assortment in meiosis I made them.