Are dominant alleles always the most common ones in a population?
Short answer: Dominance is about expression in heterozygotes, not frequency. Many dominant traits are rare.
The trap
The wrong idea
“Dominant alleles are always the most common in a population.”
It sounds reasonable, and a lot of NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
Polydactyly (extra fingers) is dominant in humans. It is:
- Very common
- Rare
- Present in everyone
Show the answer
B. Rare
Dominance is about expression in heterozygotes, not frequency. Many dominant traits are rare.
The click
Dominance describes how an allele is expressed, not how often it occurs. An allele is dominant if one copy is enough to show its trait, so a heterozygote (Aa) has the same phenotype as AA. That says nothing about how common the allele is in a population.
How common an allele is depends on other things: mutation, natural selection, genetic drift, migration and patterns of mating. A dominant allele can be rare, and a recessive allele can be very common.
Polydactyly (extra fingers or toes) is a well-known example of a dominant trait, yet it is rare, because the allele causing it is rare in most populations. Huntington's disease is another dominant condition that is rare. Meanwhile, the O allele of the ABO blood group is recessive to A and B, yet blood group O is the most common blood group in many populations.
If being dominant made an allele spread, every population would drift toward its dominant traits. The Hardy–Weinberg principle shows the opposite: without evolutionary forces such as selection, mutation, migration, drift or non-random mating, allele frequencies stay constant from one generation to the next, whether an allele is dominant or recessive. Dominance gives an allele no built-in advantage; only its effect on survival and reproduction can.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.