Question

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Although the following scenario is hypothetical, it is based on actual facts. Sickle cell anemia is an inherited form of anemia. Those with sickle cell anemia do not have enough healthy red blood cells to adequately carry oxygen to all the cells in their body. Normal red blood cells are flexible and round, and can easily move through the blood vessels. However, in sickle cell anemia, the red blood cells are rigid and shaped like sickles (crescent moons). These irregularly shaped cells often become lodged (or stuck) in the small blood vessels, which can slow or block blood flow preventing oxygen from reaching parts of the body. aa ΑΑ Susceptible to malaria but no sickle cell disease Aa Resistant to malaria and only mild sickle cell disease Resistant to malaria but has fatal sickle cell disease The disease is caused by a mutated version of the gene that helps produce hemoglobin, a protein that carries oxygen in the red blood cells. The specific gene defect is a single nucleotide mutation (GAG codon is changed to GTG) in the B-globin gene, which results in the amino acid glutamic acid being substituted by the amino acid valine in the resulting protein (we will learn more about this when we study molecular biology in the next set of modules). Individuals with sickle cell disease have two copies of the sickle cell allele and therefore have abnormally shaped red blood cells. Not only will these individuals have blocked blood flow, but they are also at risk for organ damage and other complications. They often have a life expectancy of 40 years. Individuals that are carriers of sickling hemoglobin (ie., heterozygous at this gene locus), have symptoms that are

Answer 1

13. OVERDOMINANCE

14. Overdominance provides a slecetive advantage to heterozygous over Homozygous i.e why llele frequency of ALAD is 1 .

15. The alleles are at Equillibrium in this population and lead to balanced polymorphism .

16. Yes overdominance is the reasonable in this instance beacuse overdominance maintains both allele In the population to achieve maximum overall fitness for a population. Equillibrium at this point is stable . This is an example of stablizing selection.