Sickle cell anaemia: The selective forces that a population experiences can vary

Question

Sickle cell anaemia: The selective forces that a population experiences can vary by context, especially by geographic place.

A.Explain in your own words how natural selection affects the evolution of hemoglobin in humans and how and why the selective pressures differ among human populations.

B.For your answer to part a, which are examples of disruptive, stabilizing or directional selection?

C.For human populations living in regions of the world without malaria, do you expect the HS allele to eventually go extinct? Explain.

Explanation / Answer

From the given data,

a)

Natural selection is the selection by nature in which there is increased chances of survival of any organism with some special adaptations. It helps the organism to survive as well as to increase in number. Natural selection occurs through environmental pressure selecting for advantageous traits in individuals, and those individuals then reproducing preferentially over other members of their population who do not possess the advantageous trait. Artificial selection occurs when a non-natural pressure causes such a traits to be selected.

The natural selection select for evolution of hemoglobin based on the environmental conditions that the population lives in. Other aspects that affect its evolution are concentration of oxygen, carbon dioxide, etc. For example, the atmosphere contains about 20% of oxygen in the atmosphere. However, the levels of oxygen vary from altitude to altitude by its presence being scanty at 36000 ft. (feet) above sea level. The natural selection favors the evolution of hemoglobin that can take enough oxygen in the presence of low oxygen environment or high altitudes.    

b)

The ratio of Ka/ Ks is the number of non-synonymous substitutions per non-synonymous site (Ka) to the number of synonymous substitutions per synonymous site (Ks). This measure is used to identify selective pressure acting on hemoglobin protein (Hb) coding gene. If the Ka/ Ks is less than one, the selection is stabilizing selection, equal to ne, neutral selection, greater than one implies positive selection. So, in this case, the selection is positive selection.

Evolution of hemoglobin is associated with homologous genes that are included both paralogous and orthologous genes. Both the genes relate and represent common ancestral origin. For example, of paralogous relate to same species but it is not necessary in case of chimpanzee’s myoglobin and human hemoglobin (Hb) genome represent most of the similarity which is often considered as paralogous. In case of orthologous, two distantly related species genome is nearly identical that represents the common ancestral origin with two separate species which were due to speciation or continental drift.

Codominance is expressed by heterozygous alleles in which one is normal and another is mutant form that coexist, thereby the heterozygous for sickle cell allele result in both normal and abnormal hemoglobin in the circulating blood. This codominance associated with the increased prevalence of sickle cell disease among African Americans actually has more to do with the environment than skin color or other phenotypes used to define races.

Sickle cell anemia:

Point mutation - result in sickle cell anemia (11th chromosomal co dominance effect)

147 beta chain of the hemoglobin- 6th glutamic acid replaced by valine (E6V)

Codominance is expressed by heterozygous alleles in which one is normal and another is mutant form that coexist, thereby the heterozygous for sickle cell allele result in both normal and abnormal hemoglobin in the circulating blood.

Codominance is expressed by heterozygous alleles in which one is normal and another is mutant form that coexist, thereby the heterozygous for sickle cell allele result in both normal (HA) and abnormal (HS) hemoglobin in the circulating blood.

The inheritance of recessive genes (disease causing) can remain prevalent if presence of two copies of an allele is disadvantageous and presence of one copy of the allele may be advantageous.

For example in case of sickle cell anemia, two alleles may cause sickle cell anemia, but carrying one allele is advantageous in regions where malaria is common because this single copy of allele offers resistance against malaria (common in African human phenotypes irrespective of color). Thus, many people still carry the allele of sickle cell anemia, as natural selection does not remove it; this is called “balanced polymorphism.” HBf- The genes of fetal hemoglobin is genetically regulated protein in the predominant characteristics of sickle cell anemia. The differential level of expression of fetal Hb is varies according to the sickle RBC.

Going to higher altitude make sickle cell disease worse to the patients the reason, going to higher altitudes result in lowering blood pressure, difficulty in breathing and thereby insufficient oxygen-Hb capacity further leads to lower supply of oxygen to the living cells such as cardiac muscle cells.

In the context of sickle cell, (sickle shaped anemia) + malaria, Chloroquine is not advisable because it may produce aplastic anemia as side effect.

Native Americans have very high frequencies of O allele and almost no B allele in their blood group.Frequency of sickle cell allele is high in Central Africa and Arabian Peninsula, where the occurrence of malaria caused by plasmodium falciparum is highest. In case of sickle cell anemia, two alleles may cause sickle cell anemia but carrying one allele is advantageous in regions where malaria is common because this single copy of allele offers resistance against malaria. Thus, many people still carry the allele of sickle cell anemia, as natural selection does not remove it; this is called “balanced polymorphism.”

Sherpa from Tibet are adapted to high altitudes and therefore can tolerate oxygen deficient conditions. This is explained by natural selection. Natural selection is a process in which individuals of certain phenotypes have better chances of survival and reproduction than others to the environmental change.

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.