Lesson 5 of 5 / Population genetics
Why harmful alleles can persist
Why does selection not remove every harmful recessive allele?
In this lesson: Explain how recessive masking and balancing selection preserve genetic variation.
About 6 min
The key ideaSelection acts through phenotypes. A recessive allele can persist in heterozygotes, and some environments favour heterozygotes over either homozygote.
Explore the idea
Follow the allele that selection cannot always remove
With a completely recessive harmful effect, Aa can transmit a while that effect is masked. Selecting against aa does not automatically remove copies in Aa.
Schematic for following allele copies. It does not assign numerical survival probabilities or predict any individual medical outcome.
Explanation
Natural selection does not inspect allele labels. Its effect depends on how a genotype influences survival and reproductive success in a particular environment. An allele that is harmful in one genotype or environment need not have the same effect in another.
For a completely recessive harmful allele, the associated harmful phenotype occurs in aa but is masked in Aa. Selection against the aa phenotype can therefore leave a copies in heterozygotes. When a becomes rare, most remaining a copies may be in Aa individuals, so eliminating every copy is difficult.
Heterozygote advantage provides another mechanism. In some malaria-endemic environments, individuals heterozygous for the sickle-cell allele have greater protection from severe malaria than individuals with two usual haemoglobin alleles, while avoiding the usual severity of sickle-cell disease associated with two sickle-cell alleles. This can favour retaining both alleles in the population.
The advantage depends on the environment. It does not mean a harmful allele is always beneficial, that every carrier is unaffected, or that one genotype guarantees survival. Changes in disease exposure and other circumstances can change the balance of selection.
Variation can also be replenished by mutation and by gene flow from other populations. Selection may favour different variants in different places or at different times. Distinguish creating a new allele through mutation from rearranging existing alleles through sexual reproduction.
Step by step
- 1
Identify the phenotype under selection
Explain which genotype expresses the harmful effect.
- 2
Find where copies remain
Consider heterozygotes and their reproductive contribution.
- 3
Consider the environment
Explain a specific selective advantage without treating it as universal.
Worked example
Work through the evidence
An aa condition strongly reduces reproductive success, but Aa individuals reproduce normally. Explain why allele a may remain in the population.
One way to explain it
The a allele is present in Aa individuals without the harmful recessive phenotype. These heterozygotes can reproduce and transmit a to their offspring. Selection against aa therefore need not remove all a copies, especially when a is rare and many copies occur in heterozygotes.
Why this answer works
- Link selection to the expressed phenotype.
- Identify heterozygotes as a reservoir of allele copies.
- Explain transmission rather than saying the population needs variation.
Is this true? "A population keeps harmful alleles because it knows they may be useful later."
Populations do not plan. Alleles persist through inheritance, mutation, migration and the present effects of chance and selection.