Picking p², 2pq or q² for the wrong genotype
- The idea
- Predict
- See it
- Why
- Check
The idea
“p², 2pq and q² all come from p and q, so whichever one I work out answers the question.”
By the time you've squared and taken a square root, every number on your page looks like an answer. It's easy to report the one you found first.
The short version
The mistake: p², 2pq and q² all come from p and q, so whichever one I work out answers the question.
What's actually true: p² is the share with two dominant alleles, q² the share with two recessive, and 2pq the share with one of each.
- Under random mating, each gamete carries A with chance p and a with chance q. An offspring gets one gamete from each parent.
- Two A's is p × p, or p². Two a's is q². One of each can happen two ways, so it's 2pq.
- The share showing the recessive trait is q². The share showing the dominant trait is p² + 2pq, because heterozygotes show it too.
On the exam: Write down the genotype the question asks for, AA, Aa or aa, before you pick p², 2pq or q².