Extra classes don't mean many genes
- The idea
- Predict
- See it
- Why
- Check
The idea
“More than two kinds of offspring, or an odd ratio, means the trait is polygenic.”
You learned 3:1 as normal, so a third color or a 9:7 split seems to need something bigger.
The short version
The mistake: More than two kinds of offspring, or an odd ratio, means the trait is polygenic.
What's actually true: Many additive genes give a smooth range of in-between phenotypes. A few sharp classes point to one or two genes.
- In a polygenic trait, several genes each add a little. Offspring get many different totals, so they form a graded range.
- One gene can still give extra classes, through an in-between heterozygote or a third allele.
- A dihybrid cross of two unlinked genes gives sixteenths. In epistasis, one gene masks another, so 9:3:3:1 classes merge into 9:7 or 9:3:4.
On the exam: Many additive genes make smooth in-betweens. A few sharp classes mean one or two genes.