Sickle Cell Anemia Frequency and Malaria Effects

Question

Sickle cell anemia is a genetically inherited autosomal recessive trait in which results in a condition that there are not enough healthy red blood cells to carry oxygen throughout the body due to the production of red blood cells that are sickle-shaped. The frequency of the sickle cell condition is as high as 10% in Central Africa compared to 0.5% in the United States.

a. Calculate the frequency of the normal and carries of the sickle cell condition.

b. Malaria is a disease caused by a parasite that infects healthy red blood cells and can lead to death. However, the parasite is not able to infect sickle cell red blood cells. Scientists claim that the higher frequency of sickle cell carriers is due to the presence of malaria. Provide reasoning to support the claim.

Answer 1

Hi Ashley T.,

p(sickle allele) = p (sickler individuals)^0.5 since if allele has probability p, then homozygous_sicklers = p*p

You should be able to figure therest.

The second part of the question is trickier. I'd bet that heterozygous sickle hemoglobin allows infection (the parasite only has to enter the cell, after all; the cell can't exactly evade!), but not replication. I subtle distinction, you might think. But a lot of medical practice is built on subtle distinctions! So the standard argument would run, that the presence of malaria selects for resistant individuals, hence those heterozygous for sickle trait, by killing or incapacitating the normals. But if this were the case, the prevalence of normal alleles would be about 0.5, not 0.7 -- all normal persons would be killed before breeding, and the population would reflect 0.25 normal (before dying!), 0.5 hetero, and 0.25 homo sickler (also dies, the disease is quite incapacitating).

Here's another item for you to think about: intestinal parasitic worms. It's epidemiologically known that women WITH roundworms (data is from S. America) have a higher fertility rate (and more kids) than those WITHOUT. So what is going on?

" https://www.sciencenews.org/article/having-parasites-can-boost-fertility

Having parasites can boost fertility

Women infected with giant roundworms have more babies

By Meghan Rosen November 19, 2015

Parasitic worms may cause baby booms.

Amazonian women infected with giant roundworms could bear up to two more children during their lifetime compared with uninfected women, an analysis of nearly a decade of medical data suggests. Hookworms, on the other hand, might act as birth control. Women with these parasites could have three fewer children than uninfected women, researchers report in the Nov. 20 Science.

No one knows how exactly worms tweak fertility. The parasites could tinker with immune cell numbers, making conditions ripe (or wrong) for pregnancy, speculate behavioral ecologist Aaron Blackwell of the University of California, Santa Barbara and colleagues.

Blackwell’s team studied worm infections in Tsimane women in Bolivia. Hookworms — bloodsucking parasites about the length of a grain of rice — seemed to postpone women’s first pregnancies. But women infected with giant roundworms — sleek, whitish parasites that can be thicker than a straw and longer than two pencils — started having babies earlier with shorter breaks between pregnancies. "

Also:

" The Parasite Underground

A shadow network of patients are trying to treat their own debilitating diseases — by infecting themselves with gastrointestinal worms.

http://www.nytimes.com/2016/06/19/magazine/the-parasite-underground.html "

includes detail: " To survive for years in another animal, parasitic worms, known as helminths, counter their hosts’ defenses. Because an out-of-control immune response against native bacteria was thought to drive inflammatory bowel disease, Weinstock’s insight was that parasites’ ability to disarm the immune system might prevent the disorder. "

So you see, sometimes there are multiple connections between causes and effects! If you are interested in more details, email me directly for full-text ([email protected])

-- Cheers, --Mr. d.