Lesson 4 of 5 / Variation, selection and species
Explain why harmful alleles can persist
Why does selection not remove every recessive harmful allele?
I can explain preservation of genetic variation, including masking of recessive alleles and context-dependent heterozygote advantage.
I can explain preservation of genetic variation, including harmful recessive alleles.Syllabus
Syllabus 8876, Core 4(d). Explain preservation of genetic variation, including harmful recessive alleles.
About 5 min
Make a guess. You are not marked.
Selection strongly removes people with a harmful recessive condition (aa). Will the allele a soon disappear?
Show the answer
Healthy Aa carriers pass it on.
Most copies of a rare recessive allele sit in heterozygotes, who are not selected against.
The key idea
A harmful recessive allele can be carried without its recessive phenotype in heterozygotes. Different conditions and heterozygote advantages can also maintain more than one allele.
Explore the idea
Remove aa: does a disappear?
Inspect genotypes before judging which allele copies selection can remove through their expressed effects.
Explanation
Selection acts through expressed characteristics that affect reproductive success. A recessive allele whose harmful effect is expressed in aa can still occur in Aa individuals that do not show that recessive effect. Such heterozygotes can survive, reproduce and transmit a, so selecting against aa need not eliminate a.
Masking is not the claim that the allele has physically disappeared. If two carriers reproduce, some offspring can inherit two copies. When a harmful recessive allele is rare, many of its remaining copies may be in heterozygotes rather than exposed in recessive homozygotes.
Variation can also be maintained when heterozygotes have an advantage over either homozygote in a particular environment. The sickle-cell example links haemoglobin variation to malaria: HbA/HbS individuals have protection against severe malaria relative to HbA/HbA in relevant settings, while HbS/HbS carries the serious cost of sickle-cell disease. This is not complete immunity or a claim that the allele is beneficial in all circumstances.
Different environments may favour different variants, and changing conditions may alter the advantage over time. Continued mutation can also introduce alleles. There is therefore no universal rule that selection must leave a population with only one allele at every locus.
Step by step
- 1
Find where the allele is carried
A recessive allele can occur in both Aa and aa.
- 2
Find where the harmful effect is expressed
Selection against the recessive phenotype does not directly remove every Aa carrier.
- 3
Consider the environment
An advantage in one genotype or setting can help preserve variation.
Worked example
The hidden copies remain
A simplified population contains 40 AA, 40 Aa and 20 aa. Suppose the aa individuals leave no offspring. Have all a copies been removed from the breeding population?
One way to explain it
No. The 40 Aa individuals still carry 40 copies of a. They can pass those copies to offspring even though they do not show the recessive harmful phenotype in this model.
Why this answer works
- Remove the selected-against genotype, not every occurrence of its allele.
- Count the allele copies remaining in carriers.
Watch out for this
A student says: "Recessive means the allele is weak and will disappear in the next generation." What is wrong with this?
Show the answer
Recessive describes a phenotype relationship in heterozygotes. It does not predict inevitable disappearance or low frequency.
Check your understanding
Why can selection against aa fail to remove a completely?
Show the answer
Aa individuals can transmit a without expressing the recessive harmful phenotype.
The allele can persist in carriers.
Quick recall
Why does selection not remove every recessive harmful allele?