In this assignment you are a genetics counselor. You met with two parents whose
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Question
In this assignment you are a genetics counselor. You met with two parents whose first child has sickle cell anemia. (The parents show no signs of the disease themselves). The mother comes to you in the third month of her second pregnancy, and wants to know if this child will also inherit the disease. As a genetics counselor, you are ready to evaluate this couple's case by doing the following: 1. Determine what the genotype of each parent is. Is this disease autosomal or sex-linked? Dominant or recessive? 2. Describe the type of test(s) would you give the mother to evaluate if the fetus is also carrying the gene for this disorder. 3. What is the probability that her 2nd child will also have the disorder? 4. Explain to them why is this condition so common in the African-American community. 5. Describe the type of mutation in the hemoglobin gene that causes sickle-cell anemia. Write a two-page report as part of the medical record that describes what you told the couple to explain the above five points. Submit it as a Word document.Explanation / Answer
Sickle cell anemia (SCA) is an inherited disease characterized by the formation of non-functional RBCs. The RBCs in SCA patients fail to carry oxygen properly and they suffer from anemia, bacterial infections and swelling in hands and feet.
With this background, let us answer the given questions:
The answer to the question 1.
The genotype of people with normal hemoglobin is shown below
HbA HbA ----------> Here HbA is a dominant allele and hence this genotype is called homozygous dominant
The genotype of people with sickle cell trait (these people do not suffer from SCA) is shown below
HbA HbS ----------> Here HbS is a recessive allele and this heterozygous condition acts as a carrier
The genotype of people with SCA is shown below
HbS HbS ----------> Here both are recessive alleles and hence they are called homozygous recessive.
In the question, it is mentioned that the parents are not showing any symptoms. So, from the above explanation, the genotypes of parents will be
Father - HbA HbS
Mother - HbA HbS
HbA HbS x HbA HbS
(HbA HbA, HbA HbS, HbA HbS,) (HbS HbS) ----> this represents a SCA condition
Due to this heterozygous condition, there is a 25% chance that their offspring will suffer from SCA (shown above).
Hence, the genotypes of parents are
Father - HbA HbS
Mother - HbA HbS
Sickle cell anemia is an autosomal recessive disease.
2. The screening for sickle cell anemia is challenging. The hemoglobin variations are more than nine hundred in number. So the screening must be precise and accurate to confirm the presence of hemoglobin S in the fetus. Following are some of the tests used for the prenatal screening of sickle cell anemia:
a. Hemoglobin solubility test - This hemoglobin S screening test is based on the relative insolubility of hemoglobin S when combined with sodium dithionite, a reducing agent. When whole blood is mixed with the reducing agent, saponin lyses the erythrocytes and hemoglobin is released. If hemoglobin S is present, it will form liquid crystals and give a turbid appearance to the solution. A transparent solution is seen with other hemoglobins that are more soluble in the reducing agent.
b. Hemoglobinopathy (Hb) Evaluation – The normal and abnormal hemoglobin types are varied and many in number. So the relative amounts of each of them are evaluated through a series of tests. The Hb evaluation method can specifically identify the particular variant of hemoglobin so that the identification and quantification become easier. The evaluation involves the following methods:
c. Hemoglobin Electrophoresis – This is a traditional common method that identifies the presence of various hemoglobin types.
d. Hemoglobin Fractionation – This is a popular method that screens hemoglobin types and particularly screens for Hb S.
e. Isoelectric Focusing – This is a very sensitive method that is practiced to obtain similar results as the above two. Prenatal testing for sickle cell currently makes the use of the Hb isoelectric focusing in most of the countries.
f. Analysis of DNA by PCR, SNP, sequencing etc., - This is a very effective evaluation process as it can correctly examine and scrutinize the alterations and mutations taking place in the genes that are responsible for the production of the varied hemoglobin components. The test can specify whether the individual has one or more than one copy of the Hb S mutation. This, therefore, gives a very precise result regarding sickle cell.
The answer to the question 2.
The genotypes of parents are
Father - HbA HbS
Mother - HbA HbS
HbA HbS x HbA HbS
Offsprings are as follows:
(HbA HbA, HbA HbS, HbA HbS,) (HbS HbS)
From the above explanation, 1 offspring is normal, 2 are carriers and 1 is affected.
So, the probability of SCA in the second child will be 1/4 = 0.25 or 25%.
The answer to question 4.
People of African descent such as African-American community are prone to sickle cell anemia because in Africa malaria is the most epidemic disease. In order to protect from the severity of malarial infections, the gene has undergone a variation from normal HbA to a variant HbS. The sickle cell condition protects the Africans from malaria. The African-American community through their ancestors have inherited this gene and hence they are more prone to sickle cell anemia.
The answer to question 5.
Single nucleotide polymorphism or point mutation of Adenine (A) to Thymine (T) in the beta-globin gene of the hemoglobin causes sickle cell condition (HbS). This results in the substitution of glutamic acid by valine in position 6 of the protein. Although this does not affect the functionality of hemoglobin in heterozygous condition, in homozygous condition, it causes sickle cell anemia with complicated clinical symptoms.