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Cystic fibrosis, Tay-Sachs, and hypertension are three very different, yet deadly diseases. The question arises how these diseases have survived for so long. Three articles attempt to uncover the mystery behind these diseases. It has been found that there were benefits to each of these diseases, which allowed the allele to remain in the community for such a long period of time. Natural selection plays a large part in the success of these conditions.
First, cystic fibrosis has been found to be prevalent in Caucasians. It is the most fatal genetic disorder among this group. It is difficult to understand how this disease has survived for 52, 000 years, especially before the development of modern medicine. The disease generally killed people before they could reproduce. In a normal situation, natural selection should have gotten rid of this defective gene; however, it did not. This suggests that cystic fibrosis may have some usefulness in society that we were unaware of until now.
The author of this article suggests that those who carry the cystic fibrosis gene are protected against diarrhea. In order to understand this, it is important to know how cystic fibrosis affects the body. The gene for cystic fibrosis codes for a protein that forms channels in cell membranes. These channels funnel chloride ions out of a cell, make the surroundings saltier, and draw water out of the cell by osmosis. The fluid is useful in getting rid of unwanted debris in the lungs. For sweat glands, these channels recycle salt out of the glands and back to the skin before the salt is lost.
People with cystic fibrosis have excessively salty sweat and they lack functional chloride channels. Another symptom of cystic fibrosis is thick, sticky, dry mucus clogging the lungs because the cells are not ejecting water. Medical geneticist Xavier Estivill concluded from experiments with mice that mutations of cystic fibrosis offer some resistance to cholera and other diarrhea-inducing bacteria. Persons who are heterozygous for this gene will not have all the characteristics of an individual with cystic fibrosis, but they will have one defective gene coding for chloride channels. When the bacteria causing cholera gets into the small intestines, it releases a toxin that causes the cells to permanently open the chloride channels in an attempt to flush out this poison. The rapid loss of salts and fluids often cause the person to die of dehydration.
When the cholera was injected into mice who had cystic fibrosis, not fluid was secreted at all. Mice heterozygous for the gene secreted half the amount of fluid as normal mice. This might be enough for the heterozygous individual to rid itself of cholera without dying of dehydration. Cholera epidemics were common in Europe's history; however, it is possible these epidemics did not kill persons of had one copy of the cystic fibrosis gene. In this way, natural selection has occurred. The gene helped keep people alive and therefore, many copies of it were made and the allele survived.
Secondly, Tay-Sachs disease is a condition that appears after the first few months of infancy. At approximately six months old, the baby starts to lose control of his head and is unable to sit up without support. He will begin to drool, break out into random bouts of laughter and suffer convulsions. The child will most likely die before the age of four. This disease appears in one of 400, 000 births for people around the world.
However, in people of Eastern European Jewish descent, it appears once in ever 3, 600 births. Again, like cystic fibrosis, there has to be a reason why this deadly gene has been successful even though its victims die before they are able to reproduce. Tay-Sachs individuals lack the enzyme hexosaminidase A, which breaks down a fatty substance called Gm 2 ganglioside. The pile up of this substance causes all the symptoms of the disease. A questionnaire in 1972 asked U.
S. parents of Eastern European Jewish descent what their parents had died of. Only one out of 306 grandparents died of tuberculosis, one of the biggest killers at that time. This suggests that Tay-Sachs heterozygote's may have some sort of built-up immunity against TB.
When Jews and non-Jews from the same city, of the same class and occupational groups were compared, Jews had half the TB death rate of non-Jews. Once again the question arises as to why this specific group of people carries the disease more than others. Every human populations is especially susceptible to certain diseases, not only because of its life-style, but also because of its genetic inheritance. Statistics showed that Tay-Sachs was nearly three times more frequent among Jews originating from Austria, Hungary, and Czechoslovakia. In these areas, 9 - 10 percent of the people were heterozygous for the disease. However, old records from a Jewish TB sanatorium show that patients from Austria and Hungary were present in large numbers, thus proving how much more common TB was in those countries.
It seems then that there is no way that Tay-Sachs prevents TB because the place with the highest frequency of Tay-Sachs also produced the most cases of TB. Researchers have been mistaken when concluding that Tay-Sachs prevents TB. What truly happened was that natural selection produced Tay-Sachs as a response to the region where TB was the most prevalent. Al this information is still under speculation. Tay-Sachs can now be tested so parents will know the chances of producing a child homozygous for this gene.
Finally, different ethnic groups face different health problems for some reason. Hypertension is the leading kill of American blacks and has been for many generations. American Blacks have on the average, higher blood pressure, double the risk of developing hypertension and nearly ten times the risk of dying of it. Hypertension most often causes heart disease and especially kidney failure. Naturally your blood pressure varies with each stroke of your heart, so the higher your blood pressure, the more likely you are to die of a heart attack, stroke, kidney failure, of ruptured aorta. Why do some of us have higher blood pressure than others?
Environmental factors contribute greatly to high blood pressure. High intake of salt, alcohol, or saturated fats, and low calcium intake lead to high blood pressure. A difference between hypertension and the other diseases previously discussed is, while the others are due to a single gene, hypertension involves several different genes whose molecular products remain to be identified. Also, different hypertensive patients may owe their condition to different gene combinations. Hypertension is a shared set of symptoms produced by heterogeneous causes, all involving an interaction between environmental agents and a susceptible genetic background.
According to Jared Diamond, factors have shown why Blacks appear to carry this disease more: blacks consume less potassium and calcium, do experience more stress associated with more difficult socioeconomic conditions, have much less access to medical care and are therefore much less likely to be diagnosed or treated until it is too late. Physiological differences seem to contribute as well. On consuming salt, blacks retain it on average far longer. Salt has been in very short supply for much of recent human evolutionary history. Those with efficient kidneys able to retain salt were better able to survive the episodes of sodium loss.
This may explain why hypertension has existed for so long. Frequencies of hypertension have actually shot up during times of salt loss because the kidneys are trying to retain the salt for longer periods of time. There have been two evolutionary theories about hypertension, however the main point is that they both lead to the same conclusion; New World blacks are more susceptible than whites to hypertension. This disease has lasted because the genes were passed on to following generations and because of the environmental conditions which furnace its continuance. Although Cystic fibrosis, Tay-Sachs, and hypertension appear to have no importance in society except that they are deadly, evidence hints at the fact that both conditions may have been useful at some point in time, thus providing for their continuance throughout generations. Natural selection selects for those alleles that will give an organism the best chance for survival.
In protecting against cholera, TB, and sodium droughts in heterozygous individuals, these diseases have survived and been passed on.
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Research essay sample on Natural Selection And Diseases
