Question

Sickle cell anemia is inherited as an autosomal, recessive disorder. Individuals who inherit the mutated gene from both parents will show symptoms of sickle cell, including a stiffening of the red blood cells when the individual is under conditions with low oxygen levels in the air (e.g. on top of a mountain). This can lead to low red blood cell count (anemia), shortness of breath, fatigue, jaundice, and joint pain. Most critically, the stiffened red blood cells can clog small blood vessels, blocking blood flow to critical organs (e.g. brain, lungs, liver, etc.) that can lead to death. Curiously, heterozygous carriers have a certain amount of resistance to malaria. The malaria parasite must invade red blood cells in order to develop and apparently the cell surface is altered in heterozygous individuals, thereby limiting the ability of the malaria parasite to enter the red blood cells. Recall that the rules for autosomal recessive diseases are that only the homozygous recessive individuals experience the symptoms of sickle cell anemia.

C.    Determine the results of a mating between two parents that are both heterozygous for sickle cell anemia.

1. Provide the Punnett square and the potential offspring phenotypes and genotypes.
2. What percentage of children from this mating will develop sickle cell anemia?
3. What percentage of children from this mating will carry at least one sickle cell allele?

Answer 1

Sickle cell anemia is an autosomal dominant condition. Means, both the alleles must be affected for the disease to be expressed.

Let A be the normal hemoglobin and S be the allele for sickling.

Then AA is Normal; SS is sickle cell disease; AS is sickle cell carrier .

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i)

Now, the cross is between two heterozygous individuals (AS)

A S x A S

The punnet square is :

x	A	S
A	AA	AS
S	AS	SS

Here, the results are 1 AA : 2 AS : 1 SS

ie. 3 normal : 1 sickle cell disease.

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ii)

From the punnet square, 1 out of four children developed sickle cell anemia.

So, percentage of children who will develop sickle cell anemia is 1 / 4 = 25 %

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iii)

From the punnet square, the genotypic ratio is :: 1 AA : 2 AS : 1 SS

From this AS and SS carries atleast one sickle cell allele. [total 3]

So, percentage of children who will carry at least one sickle cell allele is 3 / 4 = 75 %