Evolution | Genetics | Balanced Polymorphism and Superiority of Heterozygotes
Balanced Polymorphism and Superiority of Heterozygotes
If the heterozygote has higher fitness and enjoys a selective advantage over both types of homozygotes, both recessive and dominant alleles are propagated in successive generations and persist in the population indefinitely, even if one allele is lethal when homozygous.
Such a heterozygote advantage is called Heterozygote Superiority OR Over-dominance OR Single Locus Heterosis. It produces balanced polymorphism for the gene locus and keeps an equilibrium between allele frequencies.
Genotypes that are heterozygous at many gene loci are more fit than more homozygous genotypes. As discussed earlier that being a sickle cell carrier in a malaria infested environment, Hbs allele confers a selective advantage.
Frank B. Livingstone revealed the relationship between the spread of agriculture, malaria and sickle cell anaemia.
• The clearing of forest for agriculture provided new breeding areas for mosquitoes that are responsible for spread of malaria.
• Spread of malaria generated selective pressure in favour of sickle cell allele which in heterozygous state imparts resistance to malaria.
Communities in AFRICA depending mainly on agriculture have the highest frequencies of sickle cell trait, while hunting communities in AFRICA show a very low incidence of malaria and an equally low frequency of sickle cell allele. In case malaria disappears, it will disrupt the Balanced Polymorphism and frequency of allele for sickle cell anaemia will begin to decline.
source: Organic Evolution (Evolutionary Biology), 13th edition, Medtech, by Veer Bala Rastogi , pg 248
If the heterozygote has higher fitness and enjoys a selective advantage over both types of homozygotes, both recessive and dominant alleles are propagated in successive generations and persist in the population indefinitely, even if one allele is lethal when homozygous.
Such a heterozygote advantage is called Heterozygote Superiority OR Over-dominance OR Single Locus Heterosis. It produces balanced polymorphism for the gene locus and keeps an equilibrium between allele frequencies.
Genotypes that are heterozygous at many gene loci are more fit than more homozygous genotypes. As discussed earlier that being a sickle cell carrier in a malaria infested environment, Hbs allele confers a selective advantage.
Frank B. Livingstone revealed the relationship between the spread of agriculture, malaria and sickle cell anaemia.
• The clearing of forest for agriculture provided new breeding areas for mosquitoes that are responsible for spread of malaria.
• Spread of malaria generated selective pressure in favour of sickle cell allele which in heterozygous state imparts resistance to malaria.
Communities in AFRICA depending mainly on agriculture have the highest frequencies of sickle cell trait, while hunting communities in AFRICA show a very low incidence of malaria and an equally low frequency of sickle cell allele. In case malaria disappears, it will disrupt the Balanced Polymorphism and frequency of allele for sickle cell anaemia will begin to decline.
source: Organic Evolution (Evolutionary Biology), 13th edition, Medtech, by Veer Bala Rastogi , pg 248