My Science Box

Sickle Cell Anemia
Sickle-shaped red blood cellsSickle cell disease is a disorder that affects the red blood cells. Red blood cells use a protein called hemoglobin to transport oxygen from the lungs to the rest of the body. Normally, red blood cells are round and flexible so they can travel freely through the narrow blood vessels.

Patients with sickle cell disease have a mutation in a gene that codes for part of the hemoglobin protein. As a result, hemoglobin does not form properly, causing red blood cells to be oddly shaped. These irregularly shaped cells get stuck in the blood vessels and are unable to transport oxygen properly, causing pain, frequent infections, and damage to the organs. Patients with sickle cell disease only survive to be 20 to 30 years old. About 1 in 500 babies born in America has the disease.

The normal hemoglobin nucleic acid sequence looks like:
T A C C A C G T G G A C T G A G G A C T C
A T G G T G C A C C T G A C T C C T G A G

Use the genetic code below to decode the top strand of the DNA.

Genetic Code

• The messenger RNA would read:
• The protein message would read:

The mutant form of the hemoglobin gene is shown below with the mutation highlighted:
T A C C A C G T G G A C T G A G G A C A C
A T G G T G C A C C T G A C T C C T G T G

• Describe the difference between mutant allele and the normal allele.
• How would this mutation affect the hemoglobin protein that would normally be produced? In your answer, explain exactly what would happen to the amino acid sequence and how that might affect hemoglobin’s ability to do its job.

Sickle cell disease is a recessive genetic disease. The normal hemoglobin allele can be represented by “H” and the mutant allele can be represented by “h”.

• If you have sickle cell disease, what is your genotype? _____
• If you do not have sickle cell disease, what could your genotype be? _____ or _____

Jack and Jill were married one year ago and are thinking of starting a family. Neither has sickle cell disease. However, Jack’s younger sister is very sick with sickle cell disease. This has made Jack and Jill worried that a baby they have together may be born with sickle cell disease. Jack and Jill decide to get genetic testing to see whether either of them has a mutant hemoglobin gene. The results come back saying that both Jack and Jill are both Hh.

• What are the chances that their kids will have sickle cell disease. Explain completely how you know. Use a Punnett square to help you show the possibilities.
• In general, is having sickle cell disease an advantageous trait to have?
• In terms of natural selection, what should happen to the prevalence of sickle cell disease in a population over several generations?
• Should the prevalence of the mutant form of the hemoglobin gene (h) increase or decrease over time?

Sickle cell disease is most common in people of African, Indian or Middle Eastern descent. Africa, India and the Middle East all have a big problem with another disease - malaria. If you get malaria, you become extremely sick with a high fever, vomiting, convulsions and possible organ failure. Infants and children are particularly vulnerable. Almost 1 million children die from malaria each year.

Interestingly, people that only have one copy of the mutant hemoglobin gene are more resistant to malaria. If infected, they become only slightly sick (with symptoms more like the common cold) and children that are heterozygous rarely die from malaria!

• What does it mean for someone to be “heterozygous”?
• In areas where there is a lot of malaria, is being heterozygous for the sickle cell gene an advantageous trait?
• In terms of natural selection, what should happen to the prevalence of the mutant form of the hemoglobin gene (h) over time in areas with a big malaria problem?