Sickle-cell anemia is caused by a mutant allele of a hemoglobin gene. Individuals with two mutant alleles have sickle-cell anemia
Individuals homozygous and heterozygous for the mutant gene are more resistant to malaria than those with two wild-type alleles. Is this mutation dominant, recessive, or co-dominant?
The classification of all mutations depends on the phenotypic feature under consideration. With regard to the sickle-cell anemia phenotype, the mutant allele is recessive because a heterozygous individual has the same healthy phenotype as a homozygous wild-type individual. With regard to the malaria phenotype, the mutant allele is dominant because a heterozygous individual has the same phenotype as a homozygous mutant individual, namely resistance to malaria. With regard to the phenotype as a whole, the two alleles could be said to be co-dominant.
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