Dividing recombinants by the wrong total
- The idea
- Predict
- See it
- Why
- Check
The idea
“Recombination frequency is recombinants divided by parentals, or by every offspring the cross made.”
You just sorted offspring into two groups, so recombinants over parentals looks like a natural ratio. Counting every offspring feels careful. Both give the wrong number.
The short version
The mistake: Recombination frequency is recombinants divided by parentals, or by every offspring the cross made.
What's actually true: Recombination frequency is recombinant offspring divided by all the offspring you could score, parental plus recombinant.
- A recombinant offspring has an allele combination that neither chromosome of the heterozygous parent had. Crossing over between linked genes makes it.
- The frequency is the share of offspring that are recombinant. So divide by every offspring you can classify. Dividing by parentals leaves the recombinants out.
- Leave out offspring you can't classify, like daughters whose father's X carries the dominant allele of both genes. They all look alike.
- In meiosis questions, divide by gametes, not cells. A cell with one crossover between the genes makes 2 recombinant gametes out of 4.
On the exam: Recombinants go over every offspring you could score, not over parentals or offspring you can't classify.