EEB318H1 Chapter N/A: Perspectives Two Lessons from the Interface of Genetics and Medicine

QUESTION 15 (8 points)

Sickle-cell anemia is an interesting genetic disease. Normal homozygous individuals (SS) have normal blood cells that are easily infected with the malarial parasite. 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?

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. Assuming an African-American child were born each day of the year, statistically, one of these children per year would have a severe form of sickle-cell anemia (ss). Assuming Hardy-Weinberg equilibrium of these alleles, what is the percentage of the African-American population that are sickle-cell carriers? (Note that these carriers will turn out to be more resistant to malaria, should they decide to visit a malaria-infested part of the world; sometimes there are bizarre perks to being a carrier of a horrible disease!). Enter a percentage number (round up to whole number) without percent sign (%)

A human gene called the β-globin gene encodes a polypeptide that functions as a subunit of the protein known as hemoglobin. Hemoglobin is found within red blood cells; it carries oxygen. In human populations the β-globin gene can be found as the common allele known as Hb^A, and it can also be as the Hb^S allele. Individuals who have two copies of the Hb^S allele have the disease called sickle cell disease. Are the following examples descriptions of genetics at the molecular, cellular, organism, or population level?

A. The Hb^Sallele encodes a polypeptide that functions slightly differently from the polypeptide encoded by the Hb^Aallele.

B. If an individual has two copies of the Hb^Sallele, that persons red blood cells take on a sickle cell shape.

C. Individuals who have two copies of the Hb^Aallele do not have sickle cell disease, but are not resistant to malaria. People who have one Hb^Aallele and one Hb^S allele do not have sickle cell disease but are resistant to malaria. People who have two copies of the Hb^Sallele have sickle cell disease, and this disease may significantly shorten their lives.

D. Individuals with sickle cell disease have anemia because their red blood cells are easily destroyed by the body.