Gamete combinations come from the pairs
- The idea
- Predict
- See it
- Why
- Check
The idea
“I get gamete combinations by counting chromosomes, squaring the pairs, or using 2 to the chromosome number.”
Any formula that grows with the chromosome count looks about right. The count really comes from the pairs, and each pair can face only two ways.
The short version
The mistake: I get gamete combinations by counting chromosomes, squaring the pairs, or using 2 to the chromosome number.
What's actually true: Each homologous pair lines up one of two ways in meiosis I, on its own. So n pairs give 2^n combinations.
- Homologous chromosomes come in pairs, one copy from each parent. In metaphase I, each pair lines up at random, on its own. That's independent assortment.
- Each gamete gets one copy of every pair, and the choices multiply. So n pairs give 2^n combinations. Humans have 23 pairs, so about 8.4 million.
- Crossing over in prophase I swaps pieces between homologs and adds even more variety.
On the exam: Halve the diploid chromosome number to get n pairs, then use 2^n.