10. Sickle-cell anemia is an interesting genetic disease. Normal homozygous indi

Question

10. Sickle-cell anemia is an interesting genetic disease. Normal homozygous individuals (SS) have normal blood cells that are easily infected with the malarial parasite. Thus, many of these individuals become very ill from the parasite and many die. Individuals homozygous for the sickle-cell trait (ss) have red blood cells that readily collapse when deoxygenated. Although malaria cannot grow in these red blood cells, individuals often die because of the genetic defect. However, individuals with the heterozygous condition (Ss) have some sickling of red blood cells, but generally not enough to cause mortality. In addition, malaria cannot survive well within these "partially defective" red blood cells. Thus, heterozygotes tend to survive better than either of the homozygous conditions. If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

Explanation / Answer

Sickle cell Anemia is rather a very intersting disorder, it causes shape of RBCs to turn sickle shaped which if do not get proper oxygen tend to collpase very quickly. But these cells do not get infected by malarian parasite, so this seems to be a blessing in disguise especially for people residing in frican countries as these places have more number of mosquitoes and are less hygenic. But patients for sickle cell anemia hav lower probability of survival but hetrerozygotes are at a beifit of not getting malaria and also not getting a complete sickle cell anemia disorder.

In the given question, we have to apply a population genetics principle called as Hardy Weinberg Principle. According to Hardy Weinberg principle, the genetic variation will always remain constant from generetaion to generation. To simplify this it states that allele frerquency/ genotype frequency i.e. no of alleles of a specific gene in a total population will always remain constant in various generations over the years, unless there is some major evolutionary event/change.

eg: if there are two allele p and q of one gene

p+q = 1 (for many many generations), Since there are two alleles in one individual so according to Mendel Genetics, what genotypes we can have with these alleles is

Therefore, according to Hardy Weinberg Principle

pp + pq + pq + qq = 1

or p2 + 2pq + q2 = 1 or

(p+q)2 = 1

This equatiuon is called as Hardy- Weinberg Equation.

Applying the same Equation of sickle cell anemia patients:

ss: Patients homozygous for sickle cell anemia

Ss: Patients heterozygous for sickle cell anemia

SS: Normal individuals

According to Hardy Weinberg rule

S2 + 2Ss + s2 = 1 or 100%

What is given is s2 = 9%, so s = 3

Therefore, (S+3)2 = 100

S+3 = 10

S = 7

Therefore, the hetrozygote population in that African population will be 2Ss i.e. = 2*7*3 = 42%

While homozygote normal individuals will be 7*7 = 49%

We can recheck the hardy weinberg principle

SS + 2Ss + ss = 100%

49% + 42% + 9% = 100%

Alleles p q p pp pq q pq qq

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