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Haemoglobin & natural selection in areas plagued by malaria


FredParisFrance 61 / 7  
Jul 26, 2007   #1
Hello,

Could you please read my essay and give me some feedback? Thank you very much!

The prompt is:

Considering the alleles for haemoglobin (Hba and Hbs), explain why natural selection favours heterozygotes over homozygotes in areas plagued by malaria.

Thank you in advance
Frederic

Since Darwin revealed his theory of evolution conventional wisdom has considered that species could escape the extinction thanks to the transmission of favourable characteristics as conspicuous. However, as a book that cannot be judged by its cover, evolution cannot be only assessed on environmental factors. Indeed, although environmental factors have an important role within the agents that exercise selective pressures, other dynamic systems take place. One of those processes, adaptation, particularly surprises by the courses of action that can happen, and notably in the case of the allele frequency for haemoglobin, which is a protein molecule that occurs in red blood cells and that binds to oxygen molecules and carries them to cells throughout the body, in areas plagued by malaria. The comprehension of the influence of the allele frequency for haemoglobin in areas plagued by malaria is eased thanks to a clear understanding of the haemoglobin molecule nature and functions in addition to the interaction between specific alleles and the disease.

The haemoglobin molecule nature and functions underpin the eventual reproductive success, or even survival, of an individual. The haemoglobin molecule may suffer from a point mutation in the genes, i.e. a change in the sequences of DNA bases that specify the order of amino acids in an entire protein, and thus appears an abnormal form of haemoglobin known as haemoglobin S, written HbS. Moreover, the allele for the haemoglobin S is recessive to the allele for normal haemoglobin HbA, that is to say that HbS has less chances to be transmitted from parents to offspring in comparison with the allele for normal haemoglobin HbA, which is a dominant trait. People who are homozygous for the HbA allele (whose genotype is HbA /HbA) produce normal haemoglobin whereas people who are homozygous for the HbS allele (whose genotype is HbS /HbS) are severely impaired because they suffer from sickle-cell anaemia, which provokes a collapse in red blood cells, with therefore a blockage of capillaries that reduces the blood flow to organs that can even end in the patients' death if they do not beneficiate from a medical treatment. People who are heterozygous (whose genotype is HbA /HbS) have a condition called sickle-cell trait which, nevertheless, is not detrimental under most circumstances.

Surprisingly enough, the distribution of this allele displays high rates in certain areas of the world, despite the HbS mutation is relatively exceptional in the global population, and epitomizes the interrelations between the allele for the haemoglobin S and the malaria. In the subtropical and equatorial regions of the Old World malaria proliferates because of mosquitoes that often spread a pathogen agent, which is made up of single-cell organisms that proliferate in the red blood cells where they obtain oxygen for surviving and reproducing. However, when those parasites attempt to infect an organism they may encounter an unexpected impediment: the haemoglobin S. Indeed, heterozygotes, who are people whose genotype is HbA /HbS, defend better against the contamination than homozygous people, whose genotype is HbA /HbA or HbS /HbS, because their red blood cells do not sufficiently provide oxygen for the parasite to easily live and reproduce.

Because of the competition for resources that happens in ecosystems, dwellers of the subtropical and equatorial areas in the Old World, who have the favourable characteristic embodied by a heterozygous phenotype, tend to avoid contracting the malaria and survive, and have a propensity to pass that trait on to their offspring through interbreeding. Those lacking that beneficial trait, because they have homozygous phenotypes, produce fewer offspring because their organism cannot preserve their healthiness and thus they die early. People who do not undergo a reduced fitness transmit their advantageous trait to their offspring because they benefit from a longer lifespan due to the fact they do not die from neither the malaria nor the sickle-cell anaemia. Therefore, over time, the haemoglobin S tends to appear more frequently in the phenotype and thus the beneficial heterozygous phenotypes accumulate in a population in opposition to the homozygous phenotypes that are less and less present.

The selective agent in this case is the malaria that acts as a pathogen agent and that is conveyed through the mosquitoes, which are the vectors of the disease, exercises a selective pressure on human populations. In order to maintain their fitness and its corollary the reproductive success, the humans who live in those areas have to adapt. Nonetheless, in this particular situation, the adaptation is entirely unconscious because the organisms develop an advantageous trait at a microscopic level that does not necessitate the implication of a cultural adaptation. Finally, the Mendel's principles of inheritance of segregation and independent assortment guarantee the presence of the recessive allele of the altered form of haemoglobin HbS at a moderate rate in a population. Consequently, in a malarial environment, in spite of the loss of their offspring through the genetic disease of sickle-cell anaemia, the sickle-cell allele carriers counterbalance the selective forces of natural selection thanks to their fitness and their higher reproductive success. Finally, when the black bluesman Willie Dixon was singing "you can't judge right from looking at the wrong" in his song "you can't judge a book by its cover", he certainly does not think about the import of what he was singing, because he was far from suspecting that a genetic trait that was surely present flowing in his blood could be far more detrimental to his life than segregation and love affairs he depicted in his lyrics.
EF_Team2 1 / 1,708  
Jul 27, 2007   #2
Greetings!

You manage to make this subject comprehensible even to those of us who know nothing about the topic. Very good!

Some editing suggestions:

Since Darwin revealed his theory of evolution, [comma] conventional wisdom has considered that species could escape [delete "the"] extinction thanks to the transmission of favourable characteristics as conspicuous.

beneficiate - not a word; how about "benefit"? :-)

Surprisingly enough, the distribution of this allele displays high rates in certain areas of the world, despite the fact that HbS mutation is relatively exceptional in the global population, and epitomizes the interrelations between the allele for [delete "the"] haemoglobin S and [delete "the"] malaria.

they benefit from a longer lifespan due to the fact they do not die from either malaria or sickle-cell anaemia. - Because this is a negative statement [they do not die], use either/or instead of neither/nor.

The selective agent in this case is the malaria that acts as a pathogen agent and that is conveyed through the mosquitoes, which are the vectors of the disease, exercises a selective pressure on human populations. - This is hard to follow. I would rewrite it like this: The selective agent in this case is malaria, which exercises a selective pressure on human populations. It acts as a pathogen agent and is conveyed through mosquitoes, which are the vectors of the disease.

"You Can't Judge a Book by its Cover" - capitalize song titles. I like how you always end with some popular reference. This one is just a bit of a stretch ... but I like it anyway. :-))

Thanks!

Sarah, EssayForum.com


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