Genetics of Sickle Cell Anemia Objectives •To observe how a disease can act as a selective force •To describe changes in allele frequencies in a population as a result of a selective force Hypothesis The starting ratio of HbA to HbS is 3:1; in a typical population, the Homozygous Dominant Gene would become the standard, but because carrying the Heterozygous version of the allele has an advantage to fighting Malaria it will become the dominant Allele for the population.
The HbA/HbS gene will thrive because it helps in the fight against Malaria, turning a typically negative trait into an advantage. The Homozygous Recessive will disappear from the population because either way it has a fatal disease. The Homozygous Dominant has a 50% chance of living, it will continue to populate but the dominant Allele will remain the HbA/HbS Gene. Data Tables F1 Generation Tally SackTotal HbA28 HbA / HbS19 HbS3 F1 Generation Surviving Alleles Surviving HbA51 Surviving HbS19 F2 Generation Tally SackTotal HbA28 HbA / HbS17 HbS1
F2 Generation Surviving Alleles Surviving HbA47 Surviving HbS17 Discussion Questions What happened to the frequency of the HbA allele & the HbS allele over the course of this experiment? As the experiment progressed, the HbA took a sharp dive but quickly evened out; the HbS followed the very same pattern. I think that if it were to continue, the two allele’s would come much closer to the same number, however the HbA would always outnumber the HbS because it still has a 50% chance of possibility of surviving without the aid of the Sickle Cell Gene.
Will the frequencies continue to change as they did in the first three generations (the parental generation as well as the F1 and F2)? Why or why not? The frequencies will even out, become less rapid, and find an equilibrium. The first generation took a very sharp dive in frequency, but as with all populations they will even out soon. The HbS gene is essential to survival for Africans, without it 50% of them would succumb to Malaria; however without the HbA gene, they would all parish due to Sickle Cell Anemia – both of the genes are necessary.
HbA is more necessary, which is why it is more frequent than HbS. If individuals with sickle cell anemia are unlikely to live to reproduce, why hasn’t the HbS allele been eliminated? As stated in my hypothesis, the Sickle cell carries an advantage when not fatal. Assuming the HbS gene is not totally fatal, which it isn’t, there will always be a trace of it left in the population. In this particular population it holds an advantage, it increases the life expectancy of the individual when paired with an HbA, assuring the host survives both Malaria and is not plagued with Sickle Cell Anemia.
Explain the impact of malaria on the frequency of the HbS allele in Africa and the United States. The frequency of Malaria increases the frequency of the HbS allele by supplying it the time to reproduce and pass on that gene to the next generation. The Heterozygous allele is assured life, and therefore assured breading; both forms of the allele are passed to the next generation. If 2 Heterozygous Africans were to mate, 1 of their children would have both HbA alleles, which have 50% survivability due to the likelihood of catching malaria without the proper genes to fight it off. of their children would carry the same alleles as the parents, assuring them safety from both diseases. And the last child would die of Sickle Cell Anemia. This overall increases the number of HbS genes because Malaria provides it an advantage. Conclusion Selective force can shift a population by altering the frequency at which genes are passed on to the next generation. In this example, had Malaria not been present the HbA / HbA allele would thrive and have a much larger population; rather, 50% of those allele’s died off because of their inability to fight of Malaria.
This caused the ratio HbA / HbA to HbA / HbS be much closer to even, changing the path of evolution. Application I can apply Selective Force to my life as a wildlife biologist, I could determine whether an animal population is at the mercy of selective force by tracing animal deaths by disease, and how they correlate to that animals genetic traits. I could research diseases in the local area, and compare them to animals of the same species in different parts of the country where that particular disease does not plague the animals.