For example, in the case of sickle beta thalassemia, the individual has inherited a gene for hemoglobin S. from one parent and a gene for beta-thalassemia from the other. Or, in the instance of SC disease, the individual has inherited a gene for hemoglobin S. from one parent and a gene for hemoglobin C. from the other. The sickle cell trait in heterozygous carriers confers the resistance to malaria phenotype characterized by a dominant gene (Sickle cell anemia, Wikipedia). Because people with sickle trait were more likely to survive malaria outbreaks in Africa than those with normal hemoglobin, it is believed that sickle hemoglobin evolved as a protection against malaria.

Haplotypes of sickle cell disease are polymorphic restriction endonuclease sites in and around the mutant beta-globin gene Acording to Fields (2000), the existence of haplotypes specific to certain regions of the world suggests that the mutant beta globin gene arose separately in these locations. Because all of the areas in question are now or have been endemic locations of malarial infestation, the high incidence of sickle mutation is most often attributed to natural selection. Selection that actively maintains two or more alleles at a locus is called balancing selection (Malaria, sickle cell anemia, and balancing selection). Balancing selection can arise by the heterozygotes having a selective advantage, as in the case of sickle cell anemia. It can also arise in cases where rare alleles have a selective advantage. In extreme cases, balancing selection can maintain alleles in populations long enough for speciation to occur.

The mutation that produces sickle hemoglobin occurs spontaneously at a low rate (Fields, 2000). People with one sickle hemoglobin gene and one normal hemoglobin gene are somewhat more resistant to malaria than people with two normal hemoglobin genes. Therefore, people with sickle cell trait would have a better chance of surviving an outbreak of malaria.

Conclusion/Implications phenotype is equivalent to a genotype plus its development in a given environment. In only a narrow genetic sense does the genotype define the phenotype (Phenotype and genotype). Selection acts on...

If the phenotype affecting reproduction or survivorship is genetically based, then selection can eliminate genotypes indirectly by winnowing out phenotypes. Because, malaria is prevalent in Africa and India, heterozygotes can maintain sickle allele in balance with normal allele (Malaria, sickle cell anemia, and balancing selection). However in the United States, where malaria is rare, the carriers possess no such advantage and may even have a small selective disadvantage. Therefore, due to the strong selection acting against those with the anemia, the frequency of sickle cell anemia should decline in the United States over time provided that there are not outbreaks of Malaria. Further, reducing malaria worldwide should have the same positive implications.
Bibliography

Ashley-Koch, A., Yang, Q. And Olney, R. (2000, May 1). Sickle hemoglobin (Hb S) allele and sickle cell disease. American Journal of Genetics 151(9): 839-845.

Fields, E.L. (2000, October 27). Phenotypic variation in sickle cell disease: An analysis. Harvard University. Retrieved February 5, 2005 from Web site: http://sickle.bwh.harvard.edu/sickle_heterogeniety.html

Genetic disease profile: Sickle cell anemia. Retrieved February 5, 2005 from Web site: http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/sca.shtml

How does sickle cell cause disease? (2002, April 11). Harvard University. Retrieved February 5, 2005 from Web site: http://sickle.bwh.harvard.edu/scd_background.html

Malaria, sickle cell anemia, and balancing selection. Retrieved February 5, 2005 from Web site: http://www.learner.org/channel/courses/biology/textbook/humev/humev_8.html

Phenotype and genotype. Brown University. Retrieved February 5, 2005 from Web site: http://biomed.brown.edu/Courses/BIO48/5.Geno.Pheno.htmL

Sickle cell anemia. Wikipedia. Retrieved February 5, 2005 from Web site: http://en.wikipedia.org/wiki/Sickle_cell_anemia

Sickle cell anemia - description. Retrieved February 5, 2005 from Web site: http://www.blackhealthcare.com/BHC/SickleCell/Description.asp