Phenotypic variation in sickle cell disease
Sickle cell anemia is an inherited blood disorder in which hemoglobin is defective (Genetic disease profile: Sickle cell anemia). After hemoglobin molecules give up their oxygen, some cluster together and form long, rod-like structures. These structures cause red blood cells to become stiff and assume a sickle shape that makes it difficult for them to squeeze through small blood vessels. As a result, they stack up and cause blockages that deprive organs and tissues of oxygen-carrying blood.
Sickle cell anemia affects millions world wide (Genetic disease profile: Sickle cell anemia). It is the most common among people whose ancestors come from sub-Saharan Africa; Spanish-speaking regions (South America, Cuba, Central America); Saudi Arabia; India; and Mediterranean countries such as Turkey, Greece, and Italy. In the Unites States, it affects around 72,000 people, most of whose ancestors come from Africa. The disease occurs in about one in every 500 African-American births and one in every 1000 to 1400 Hispanic-American births. About two million Americans, or one in twelve African-Americans, carry the sickle cell trait.
People have twenty-two identical chromosome pairs with one of each pair inherited from the father, and one from the mother (How does sickle cell cause disease?, 2002). Mutation involving gene alteration in the exchange between a parent and child occurs only rarely. Most likely, sickle cell disease depends on inherited genes from parents’ the disease cannot be caught, acquired or otherwise transmitted. The disease is caused by a change in a single amino acid difference in the beta chain of hemoglobin. (Malaria, sickle cell anemia, and balancing selection).
Individuals with two copies of the sickle form of the gene have sickle cell anemia (Malaria, sickle cell anemia and balancing selection). Heterozygotes — individuals with one normal and one mutant copy of the sickle gene — appear normal and do not manifest the disease except under very stressful conditions. However, these individuals are carriers of the sickle cell trait. Approximately ten percent of African-Americans are carriers. In Africa and India, the frequencies of the disease and carriers are even higher de to protection against malaria that occurred for people with sickle cell trait discussed later in this paper.
Beta globin is a major component of adult hemoglobin and its gene is located on chromosome 11 with more than 475 allelic variants (Ashley-Koch, Yang, and Olney, 2000). One of these variants, sickle hemoglobin (Hb S), is responsible for sickle cell disease. The most influential risk factor for disease severity is genotype (Sickle cell anemia – description). Individuals who are homozygous for the sickle beta globin gene (b S) have sickle cell anemia (SS disease). Individuals with sickle beta thalassemia have a b S. gene and a gene for beta thalassemia. If no beta globin is produced by the beta thalassemia gene, the individual has Sb o thalassemia (Sb o thal). If some normal beta globin is produced by the thalassemia gene, the individual has Sb + thalassemia (Sb + thal). In the case of hemoglobin (SC disease), the individual has two abnormal beta globin genes, b S. And b C, and makes two abnormal hemoglobins, Hb S. And Hb C.
There are several phenotypes associated with the homozygous sickle genotype (Hemoglobinopathies, 1998). Sickle cell anemia refers to a recessive gene condition in which a person has inherited two genes for hemoglobin S, one from each parent. If two parents who are carriers have a child, there is a one-in-four chance of their child developing the illness and a one-in-two chance of their child just being a carrier. (Sickle cell anemia, Wikipedia). Children who receive one abnormal gene and one normal gene usually have no symptoms and are said to have sickle cell trait. The next two phenotypes, blood cell sickling and altered beta-globin electrophoretic mobility involve partially dominant genes when both the dominant and recessive alleles are present in the genotype. 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 phenotypes because differential reproduction and survivorship depend on phenotype. 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.
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
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