A dominant trait can be rare
- The idea
- Predict
- See it
- Why
- Check
The idea
“If most individuals have a trait, it must be dominant.”
Dominant sounds like winning. A cross of two heterozygotes even gives 3 dominant to 1 recessive. So it seems dominant should mean common.
The short version
The mistake: If most individuals have a trait, it must be dominant.
What's actually true: Dominance only says which allele shows in a heterozygote. How common a trait is depends on allele frequency.
- Dominant describes a heterozygote. One copy of the dominant allele is enough to show its trait. It says nothing about how many carry it.
- Being dominant doesn't make an allele spread. Huntington's disease comes from a dominant allele, and it's rare.
- A trait that appears in the young of two parents who both lack it is recessive.
On the exam: A count shows how common a trait is. A cross or a pedigree shows which allele is dominant.