Question

Consider a trait or phenotypic feature or even a recessive disorder with decidedly non-random geographic patterning to the frequency of the trait, feature, or allele where it is high frequency in some populations and low frequency in other populations (non-random frequency distribution). What are the mechanisms or ways that you could testable hypotheses to explain how that pattern developed and why it is maintained – i.e. forces of evolution? Population histories? Cultural mechanisms? Developmental adaptation? Acclimatization? Describe and illustrate with one of the disease alleles or phenotypes presented in the textbook and lecture or if there is one of particular interest to you that we didn’t discuss, use that one! (Hint: Discuss all the mechanisms and factors that we consider when developing evolutionary explanations for traits, phenotypes, and alleles. Note whether there is always one mechanism? Or, are there multiple forces or mechanisms?)

Answer 1

Sicle cell anemia is common in African countries including African Americans. In African countries nearly 2% of the children born are with the condition. Carriers constitute 20-40% of the equitorial Africa. This distibution in African countries is due to the advantage of carriers getting protection from malaria. It is especially more prevalent in the regions with more malaria. In Ugandan BA=aamba tribe it reaches 45% of the population. The prevalence of the disease in different regions here is indicative of the extent the malaria is pread in the region. Because the sickled RBCs can't be infected by falciparum malarial parasite which is the most deadly variant of malaria. The cases of malaria is an indication for the number of sickle cell anemai cases in the region.

The selection pressure is because of the ability to resist the malarial disease which is fatal disease. Though homozygous allele of the disease is fatal, carriers live normally. and are capable of transmitting the disease to next generation. Carriers are heterozygous for the allele and due to the selective advantage against the disease it still is present more prevalently in population and the evolutionary force acting here is stabilzing selection or balancing selection.

S is normal allele for normal hemoglobin, s is the allele for abnormal sickle shped hemoglobin. So, AA are normal and Aa also normal but these are carriers. aa population is the one which suffer from the disease and children with condition may not survive. Population has many of them with Aa condition which is heterozygous condition but leading healthy life. Due to its heterozygote advantage the allele has not been completely removed from population. So this is an adaptation in these parts of the world which the natural selection has preferred.

The disease is due to a substitution occuring in the 6th codon of the beta chain of the hemoglobin. Due to this mutation, beta chains have valine instead of glutamine which makes the hemoglobin molecules crystalize and distort the shape of the RBCs. Such distorted RNCs can't holf malarial parasites in them.

Migration of Africans to other parts of the world like USA where even though it has no survival advantage to malaria still persists in those populations through heredity.