Polymorphism: the situation in a species or population where there are 2 or more alleles of a gene, and at least 2 of the alleles have a frequencies greater than or equal to 1 percent. The mutation rate for any gene is very low, so it cannot get an allele up to 1%, so something else has to be acting on the allele to reach the threshold of 1%. Polymorphism is explained by natural selection acting on mutations. In our species, approximately 28% of our genes are polymorphic.
Let’s say there are 50,000 genes in our genome and 28% are polymorphic. This means there are approximately 14,000 polymorphic genes. Genetic variability allows a species to go through natural selection and adapt.
Example: A, B, O blood system: The genotypes are AA, A0, AB, BB, BO, and OO, and these produced proteins. The proteins that can be produced are A,A, A and B, B, B, and none for the OO. If you are a type A blood at birth, you have the anti-B antibody. This means it produces antibodies against the B protein. If you are type AB blood, you produce no antibodies. If you are type B blood, you produce antibodies against the A protein. If you are type OO blood, you produce antibodies against both A and B proteins.
Balanced Polymorphism: situation where there are 2 or more alleles of a gene in a population or species, and at least 2 of the alleles have frequencies equal to or greater than 1%, and the polymorphism is due to heterozygotes having a higher fitness (natural selective advantage) than both homozygotes.
Example: Red blood cell variants and Malaria:
Sickle Cell Allele
What is sickle cell? Hemoglobin binds oxygen, and it is a protein that is comprised of 2 alpha and 2 beta polypeptide chains. Each alpha polypeptide chain has 141 amino acids, and each beta chain has 146 amino acids. Sickle cell is a result of a single point mutation affecting 1 amino acid.
Despite natural selection disadvantage to the sickle cell allele, the allele is maintained in high frequencies at polymorphic levels from 5-15% in some population.
Malaria is caused by a parasite, which is transmitted from an infected individual to an unaffected individual by a mosquito. When it gets into your system, it gets into the red blood cells.
Relationship between Sickle Cell Allele and Malaria
Geographical correlation between the distributions of polymorphic levels of the sickle cell allele and Malaria.
Clinical and experimental studies show that individuals heterozygous for the sickle cell allele have a lower rate of malarial infection compared with individuals homozygous for “normal” hemoglobin.
Why heterozygosity for sickle cell allele is advantageous?
Heterozygotes for the sickle cell allele show selective destruction of red blood cells. Red blood cells uninfected by the parasite (Malaria) deliver oxygen to tissues. Red blood cells that are infected by the parasite sickle, and then kill the parasite. The sickle-shaped red blood cell provides a lethal environment for the parasite. Therefore, heterozygotes for sickle cell can conjointly suppress parasitic proliferation and deliver oxygen to the tissues.
In a Malarial environment, heterozgotes for sickle cell allele have a natural selective advantage (fitness) over homozygotes for “normal” hemoglobin and for homozygotes for sickle cell.
Therefore, the sickle cell allele is maintained at relatively high frequencies, and this is an example of balanced polymorphism.
Evolution of relationship between sickle cell allele and malaria:
Spread of slash-and-burn agriculture is responsible for the selective advantage of the sickle cell allele
Slash-and-burn agriculture was introduced in rainforest areas of Africa approximately 3,000-4,000 B.C. The agricultural landscape created the conditions for proliferation of mosquitoes. Mutations for the sickle cell allele arise periodically but