On pages 8-15 of "Survival of the Sickest", Dr. Sharon Moalem addresses Big Idea 1(the process of evolution drives the diversity and unity of life) when he discusses the bubonic plague in Europe. Although the Bubonic plague was deadly, it didn't kill the entire population. Moalem previously discussed on page 6 that "iron helps nearly all bacteria multiply almost unimpeded", but during the plague, people with hemochromatosis were very resistant to the disease.
What is hemochromatosis? Why was the gene for hemochromatosis selected for if it will eventually kill the person who has it? Why is it that people with hemochromatosis were able to effectively fight off the bubonic plague during the middle ages? Why is hemochromatosis still around today?
(Julianna Holzer jholzer4@students.d125.org)
In the introduction of Survival of the Sickest, on page X, Dr. Moalem defines hemochromatosis as “a disorder that causes iron to build up in the body.” This basically means that there is too much iron in the body which can be harmful to organs like the liver and pancreas and heart because all the excess iron is stored in these types of organs (cdc.gov). The way hemochromatosis works is by making the body believe that it lacks iron, allowing it to absorb iron in high amounts to satisfy it. The problem is, the body normally has sufficient iron levels so hemochromatosis patients take in extra, unnecessary iron which leads to the excess buildup (Moalem 2). Hemochromatosis is caused by a mutation of the HFE gene (irondisorders.org). It is inherited through generations and is considered to be of low penetrance, meaning that it requires a host of other circumstances to be expressed (Moalem 3). According to the Center for Disease Control, untreated, hemochromatosis can lead to organ failure (of the heart, liver, pancreas), bronzed skin color, and, eventually, death. Diagnosis is difficult, however, because there are no set symptoms associated specifically with hemochromatosis like with other disorders (cdc.gov). Bloodletting, as mentioned in Survival of the Sickest, is a form of treatment for patients with hemochromatosis. Bloodletting is bleeding. Similar to phlebotomy, bloodletting forces iron to leave the body along with the blood cells thereby decreasing the iron concentration in the body.
ReplyDeleteThe gene for hemochromatosis, the HFE gene, was selected for by natural selection because even though it may kill you in forty years, it is the “only thing that will stop you from dying tomorrow” (Moalem 4). To analyze this statement, Dr. Moalem researches into the time of the Bubonic Plague, an infection of the lymph nodes leading to swelling and death. The plague, while it killed almost anyone who became infected, left survivors. These people were most likely patients with hemochromatosis. This is because hemochromatosis, while it leads to an excess of iron, does not evenly distribute the extra iron throughout the body. In fact, the iron lacks in a white blood cell called macrophages. Macrophages are part of the immune system, locating and surrounding and killing any foreign, harmful substance in the body. Because diseases like the Bubonic Plague feed off of iron much like our bodies do, macrophages, having a lack of iron, are immune to the infection of the plague. This means that the white blood cell can successful surround and destroy the threatening virus without being destroyed. Because the plague can be fought off by iron deficient white blood cells, those with hemochromatosis survive the plague while others die from it (Moalem 12).
The reason hemochromatosis is still around today and inherited is because during the Bubonic plague it was selected for. Because patients with the disease were able to live through the plague, they survived and reproduced (evolution unit of biology). This means that their offspring also possessed the mutated gene for hemochromatosis. Therefore, because natural selection and adaptation happens to a population over a long period of time, the gene has been passed on through generations from parent to offspring. It continues to be prevalent because “we asked for it” (Moalem 3) during the time of the plague to stay alive and, therefore, allowed the mutated gene to be a trait of people who survive and are able to reproduce.
(Morgan Eisenstot – mesiene4@students.d125.org)