The doctor comes into the exam room and looks at you with that stern but sympathetic look; his hands hold your test results and you feel the worst must be coming.
“I’m sorry Mrs. Smith, your test results came back you have Phenylketnonuria, more commonly known as PKU. With proper diet the effects on your neurological system can be controlled. (Then he grins) On a brighter note it protects you from miscarriages so you and the mister don’t have to worry about that family line!”
“I’m sorry Mr. Greenspan, your son Samuel has Tay-Sachs. As lipids begin to build and store in your sons brain and nervous tissue, he will become blind, deaf, and eventually be unable to swallow. (Then he grins) On a brighter note because you and your wife are both carriers of the gene you are both well protected against Tuberculosis!”
“Mr. Humanitarian, I’m sorry to say your breathing problems are due to Cystic Fibrosis, or CF. You will probably experience more repeated lung infections during your life which may lead to severe lung damage. (Then he grins) but your upcoming mission to Africa don’t you worry about those Cholera outbreaks, while you may catch it like other people you CF will protect you from it it killing you! Have a fun trip!”
All of these sound like horrible jokes; yet there is research that shows different allele mutations that occur in populations lead to immune protection against other diseases and conditions. In fact there is a strange effect that occurs that may even explain why some diseases are prone to certain groups or communities and not others. Factors that are related not only to the diseases themselves but the environment that they interact in also. In a 2008 article by Dr. Pardis Sabeti entitled Natural Selection: Uncovering Mechanisms of Evolutionary Adaptation to Infectious Disease, he shows the relation between sickle-cell anemia and its resistance to Malaria. As those that have sickle-cell escape death from Malaria those without the condition die off. This causes an increase in sickle-cell in the surviving group, because those without could not fight the Malaria. The population left with only a members that most likely carry sickle-cell now has a disease or condition that can be attributed to their group whereas it might not another where Malaria is not prevalent in the other groups environment weeding out non-sickle cell members.
This can also be shown in why Tay-Sachs is a condition that is normally associated with Jewish populations versus other groups and communities. In Jewish History living in ghetto like conditions was not something that singularly arose during WWII. Those conditions had occurred before, yet if for this post one wants to focus on just WWII the point still can be driven across. Crowded living conditions with bad air is a perfect breeding ground for Tuberculosis (TB). The mutation that causes Tay-Sachs has a built in defense against TB. As those in the ghettos who did not have Tay-Sachs to defend their health against TB outbreaks died, the remaining Jewish survivors were more likely to have Tay-Sachs as a result of its ability to side step the TB. Other communities or groups that did not have both conditions present in their enviroment would not have the build up of Tay-Sachs in their genetic community. This leads to Tay-Sachs associations more with people of Jewish heritage then perhaps other groups.
Some of the reasons for cross-disease protection are a result of the specific way a mutated allele may affect the surface of a blood cell to how it changes the way ones body performs a specific function i.e. an organ or its ability to breakdown specific chemical. Why some of these diseases though are more prevalent in some groups versus others though does bring into the wonders of its protection from other diseases causes an increase of its occurrence in certain populations. Remembering outside factors that contribute to a disease being carried on to future generations while non-carriers die out should be an important thought when focusing on the how and a why one person or group has/gets something while others seem to breeze through unaffected.
Two webpages at this moment discuss in small detail these occurrences, I will find some journal articles that also detail this and tag them on later. To get started though is this interests you:
Dr. Sabeti’s article from Nature Education can be found at:
PBS also has a quick comment page on a few conditions that provide protection from other, it can be found at:
http://www.pbs.org/wgbh/evolution/educators/course/session7/explain_b_pop1.html
Medical Anthropology 