Siblings differ by reshuffling, not by new mutations
- The idea
- Predict
- See it
- Why
- Check
The idea
“Sperm, eggs and siblings differ mostly because of copying errors in their DNA.”
New alleles come from mutation, so it's easy to credit mutation for every difference. Most differences between siblings are new mixes of old alleles.
The short version
The mistake: Sperm, eggs and siblings differ mostly because of copying errors in their DNA.
What's actually true: Gametes differ mainly because meiosis reshuffles the alleles a parent already has. New mutations add far fewer changes.
- Mutation makes new alleles, but a new mutation in any given gene is rare. Nearly every allele you carry came from a parent.
- In independent assortment, each pair lines up without regard to the others. With 23 pairs, one person can make over 8 million combinations.
- Crossing over in prophase I trades pieces between partner chromosomes. Then fertilization joins two gametes at random.
On the exam: When offspring of the same parents differ, look to meiosis and fertilization before mutation.