Natural Selection In Humans (Sickle Cell Anemia)

Abigail Morales, Tyler Lewis, Jennifer Lopez, and Irais Perez

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 allelec. Both parents have sickle cell diseased. One parents has sickle cell disease

3. Where did Sickle Cell Anemia originate from?