Natural Selection In Humans (Sickle Cell Anemia)
Background Information Hemoglobin is a protein found in red blood cells Transports oxygen to body tissues Individuals homozygous for the sickle cell allele possess abnormal red blood cells Heterozygous individuals produce both normal and mutant hemoglobin proteins Treatments include medications, blood transfusions, and daily regimen of antibiotics Malaria is a mosquito-transmitted disease caused by a parasite
Summary Sickle cell anemia is a genetic disease Red blood cells become misshapen (crescents instead of discs) Prevents oxygen from being delivered to other parts of the body Immigration has played a big role in the amount of people in the U.S. who have it It is most popular in the African American population 1 in 500 people
Summary In the 1950s, as Dr. Tony Allison worked in East Africa, he became the first researcher to find a connection between malaria and sickle cell anemia. He discovered that a high incidence of sickle cell character meant a high incidence of malaria. Children carrying the sickle cell character had a lower parasite count They were partially protected against malaria
Summary Heterozygous parents: ¼ chances that the child will be sickle cell homozygous 2/4 chances that the child will be heterozygous ¼ chances that the child will carry two copies of the normal gene In a malaria environment individuals with two copies of the sickle cell gene and two copies of the normal gene are at a disadvantage Heterozygotes have an innate resistance to malaria Protection of malaria comes with the risk of the sickle cell disease in a population Dr. Tony Allison Dr. Tony Allison
Summary How natural selection works in humans Sickle cell mutation has made it difficult for the parasite to reproduce A mutation that creates one genetic disease can also protect against another disease Other parts of the world where malaria is popular include Southern Europe, Southern India and other parts of Africa. This mutation is not a selective advantage in parts of the world where Malaria is nonexistent.
Key Concepts Sickle Cell Anemia is a genetic disease, not an infectious one If individuals are homozygous for the SCA allele or for the normal allele, then they face consequences in a malarial zone If an individual is heterozygous, they are protected from malaria AND do not have the genetic disease in a malarial zone
Key Concepts It still affects the rest of the population since there are individuals who are not heterozygous for the sickle cell allele The sickle cell allele was brought into the gene pool as a random mutation in the hemoglobin gene.
Discussion Points The word trait refers to the possible phenotypes that can occur. In this case, we are talking about the sickle cell anemia trait or the heterozygous individuals that have an advantage Why is the sickle cell trait still in non malarial zones? There has not been a sufficient amount of time to eliminate the allele
Discussion Points Why did Dr. Allison experiment on children instead of adults? Children have not developed immunity against the parasite unlike adults who have This means there are higher levels of parasites in the blood cells of children
Map of the Sickle Cell Anemia allele frequency
Review Questions 1. Consider the statement: Sickle cell disease is a(n) disease. 2. If a person has sickle cell anemia, what must be true about their parents? a. One parent has at least one copy of the sickle cell allele b. Both parents have at least one copy of the sickle cell allele c. Both parents have sickle cell disease d. One parents has sickle cell disease 3. Where did Sickle Cell Anemia originate from?