Question

Sickle-cell Anemia Study Sickle-cell anemia or Sickle-cell disease, is a hereditary disorder. characterized by aryabnormality in the oxygen-carrying hemoglobin molecule in erythrocytes. Sickle-cell conditions have an autosomal recessive pattern of inheritance from parents. The sickle cell defect is a mutation of a single nucleotide of the hemoglobin B gene, which results in glutamic acid being substituted by a different amino acid at position 6. Hemoglobin with this mutation is referred to as Hbs, as opposed to the normal hemoglobin HbA. Sickle-cell disease (sickle cell anemia) occurs when a person inherits two abnormal copies of the hemoglobin gene, one from each parent. This causes the cells to assume an abnormal sickle-like shape under certain circumstances. A person with a single abnormal copy of the gene does not experience symptoms and is said to be a carrier or have sickle-cell trait. Due to the adaptive advantage of the heterozygote, the disease is still prevalent, especially among people with recent ancestry in areas where malaria is found. Plasmodium, the malaria parasite has a complex life cycle and spends much of it in red blood cells. In a carrier, the presence of the malaria parasite causes the red blood cells with defective hemoglobin to rupture prematurely, making the Plasmodium parasite unable to reproduce. Therefore, in areas where malaria is a problem, an individual's chances of survival actually increase if they carry (are heterozygous for) sickle-cell trait. A genetic testing result table with the DNA triplets of four patients is provided below. One of the patients has symptoms showing sickle cell anemia. In the process of doing genetic testing of the patients, you discover that another patient is a carrier (has sickle cell trait). Position 6 Amino acids Person 1 Person 2 Person 3 Person 4 Position 6 DNA alleles CTT / CTT CTT / CTC CTT / CAT CAT / CAT Person 1 Person 2 Person 3 Person 4 Position 6 mRNAs GAA / GAA GAA / GAG GAA / GUA GUA / GUA Person 1 Person 2 Person 3 Person 4 13. Based on the passage above, which person has sickle-cell anemia a) Person #1 b) Person #2 c) Person #3 @ Person #4 14. Based on the passage above, which person has the sickle trait a) Person #1 b) Person #2 C) Person #3 d) Person #4 12. Which amino acid is present at position 6) in the individual with sickle cell anemia (a genetic code table is on the next page) a) glutamic acid 6 valine c) alanine d) glycine 15. Why are persons 1 and 2 still making the same amino acid 6) changes in the third nucleotide of a codon may not cause a change in the product b) proofreading functions associated with DNA polymerase will restore the original base c) they have the same codons d) changes in the third nucleotide of a codon never cause a change in the product

Answer 1

Hi there,

Answer 13) The correct option is D) Person 4. If you look closely at the mRNA sequences of alleles of all four persons, it is person third and fourth that are having Valine codon in their alleles. But as sickle-cell anaemia occurs only when two abnormal copies of valine codon are present, it is person 4 who has sickle-cell anaemia (GUA/GUA= Valine-valine) than person 3. All other persons code for glutamic acid.

Answer 14) The correct option is C) Person 3. Keeping same reason as above, however, to carry a sickle-cell anaemia trait, one copy of abnormal gene coding valine is required. Hence it is person 3 who has only one copy of mRNA codon that codes for valine (GAA/GUA= glutamic acid- valine).

Answer 12) The correct option is B) valine. Looking at the universal genetic code, GUA codes for valine and GAA/GAG codes for glutamic acid.

Answer 15) The correct option is A. This is often called the wobble position or degeneracy. Single nucleotide substitutions at the third position may not lead to a change in the amino acid encoded. This is called silent substitution. The first two positions are sufficient to specify a unique amino acid, and any nucleotide at the third position encodes that same amino acid.

Hope this helps.