Mon 20 Mar 2006
There has been a lot of research into the human genome. A few very interesting things came to my attention in reading about recent “evolutionary improvements” to some human Genes. It appears that evolutionary pressures can encourage genetic shortcuts that help the survival of one group. Sometimes these shortcuts can cause a lot of havoc.
In tropical Africa malaria is the source of much sickness. Malaria is a disease that can make life miserable with treatment, and without often causes death. The disease is caused by a parasite, Plasmodium falciparum, which is carried by mosquitos. Through part of its life cycle the parasite inhabits the human red blood cell.
Since people who have malaria often died, or were so sick that they did not have children, people who were resistant to malaria would have a clear advantage. Several thousand years ago someone in Africa had a genetic mutation that made this person somewhat resistant to malaria. The expression of the gene changes the red blood cell to have a slightly modified hemoglobin, which carries less oxygen and is less hospitable to the malaria parasite.
Genes come in pairs, and if you lived in Africa, you were lucky to get one of these genes, because you were likely to live long enough to have children because you probably would not succum to malaria. This condition is called Sickle Cell Trait. Unfortunately, there is a down side. Too much of a good thing is very bad. Two copies of the gene causes Sickle Cell Anemia – a particularly nasty genetic disorder where a different form of hemoglobin the stuff that carries oxygen in the blood, hemoglobin S is in the red blood cells.
The disease usually shows up in children at about four months of age. Because the hemoglobin is different, the red blood cells are sickle shaped, and do not carry oxygen very well at all. The person with sickle cell anemia has many symptoms of illness, including pain, jaundice, fatigue, and is subject to crises that could result in death.
When a person’s mother and father each have sickle cell trait, ie. one gene for the special hemoglobin, there are four combinations of this genetic mix that can occur with about equal probability. 1) Neither the mother nor father contribute a gene for the special hemoglobin. The child is normal and has no improved resistance to malaria. 2) The mother contributes a normal gene and the father contributes a gene for the special form of hemoglobin. The child will have enhanced resistance to malaria, and will have sickle cell trait. 3) The reverse of case 2, where the mother contributes a gene for hemogolbin S, and the father contributes a normal gene. The outcome is the same as case 2. 4) The mother and father both contribute genes for hemoglobin S. This is bad, the child will have sickle cell anemia.
In the case where either the father or the mother carries one hemoglobin S gene, and the other parent carries none, half of the offspring will carry the trait, but none of the children will have the disease.
In the case where the father or mother has sickle cell anemia, and the other parent has no hemoglobin S genes, then all the children will carry the sickle cell trait, and none will have the disease. This is a lower likelihood case because an ill person is less likely to live to childrearing age, and sickle cell anemia also delays puberty.
In the case where the father or mother has sickle cell anemia, and the other parent has sickle cell trait, then there is a 50 percent chance that the child will have sickle cell trait, and a 50 percent chance that the child will have the anemia.
This explanation may seem rudimentary for some readers, but it is important to understanding my next point: In a population of several thousand persons, in a malaria infested region, this genetic shortcut, in lieu of other ways of controlling malaria, is good for the population. Lots of people with sickle cell trait will fare better when infected than those without the resistance. They are likely to be able to provide for their families in spite of the illness (and not be as likely to die). For those without the trait, you take your chances with malaria.
Since most of the children of parents with sickle cell trait will have no anemia, and some of those will fare better than others, this genetic variation seems good. The problem is that 25% of the children where both parents have sickle cell trait will have a devastating disease, nearly as terrible as malaria. They don’t have to “catch” it, they are born with it.
This genetic shortcut is good for the overall population, but if you happen to be the one who has the anemia, you are screwed!
Another genetic disease that has the same recessive pattern is Tay-Sachs disease. In medeval times, Jews in Europe were not permitted to own or farm land, and were forbidden from practicing many occupations. A Jewish person had to be smart to be effective in the few businesses that were open to him or her, such as banking and sales. It was difficult living for Jews, and many children did not survive.
In eastern Europe, several hundred years ago, some Jewish person had a mutation on chromosome 15 that changed his or her neurological makeup to increase intelligence, or let him handle problems in a more efficient manner. This meant that he could live better, because this was a critical skill, and he was “gifted”. Since he was more likely to become wealthy, while his peers were struggling, he was more likely to have children, and have them live to maturity.
Once again, the children of parents who both had the Tay-Sachs trait had a 75% chance of normalcy, and a 50% chance of being above average. The problem is the poor child that received both Tay-Sachs genes.
The child appears normal at birth through six months of age. At two years of age they have limited peripheral vision, frequent sesiures, and reduced mental function. Eventually, the child becomes blind, mentally retarded, paralyzed, and non-responsive to his or her environment. This is a devastating disease for the child, and is a great stressor on the family, since, like in Sickle Cell Anemia, the child starts out seeming normal and the family bonds with the child.
These diseases come from a “short cut” mutation that provides a benefit to the population as a whole, to the great detriment of the individuals who inherit a double copy of the gene. It seems as though it would be great to always “dodge the bullet” but the genetic dice are thown, and the mix is what you get.
How can we improve the human condition, in light of these and other recessive genetic diseases? We can now screen for sickle cell trait and Tay-Sachs trait. People who might be carriers can be tested to determine with a level of certainty whether they are or not. With this knowledge, they can decide whether a given individual is a suitable mate. Knowing that a potential partner is also a carrier could be information important enough to break off a relationship or could be grounds for divorce.
A fetus can be tested to determine if it has a pure strain of genetic code and would be certain to have the disease. Is this an appropriate basis for abortion? What other measures are appropriate in this age of knowledge of our genetic makeup. Can this knowledge lead to eugenics and compulsion to break up a relationship or abort a fetus? Would this necessisarily be a bad thing? Who should have a say in the decision?
How can human society in control of so much knowledge use it to better the human race? It is desirable that we have smarter people and people who can resist the effects of malaria, but how do we deal with the incredibly bad effects that happen to a small minority?
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