Sunday, August 22, 2010

Genes in Populations: a More Modern View

Quicktime Oversimplified Genetics: So What Happens to Genes in Populations?

Since I have spent so much time on eugenics and Alice Stewart and the eugenics Grannies, I want to spend a little time on real genetics. As I mentioned in earlier posts, we all contain less-than-ideal genes. However, such genes don't "build up in the gene pool" as eugenics and Alice Stewart believe. Here's a part of Stewart's quote: It's the genetic damage, the possibility of sowing bad seeds into the gene pool from which future generations are drawn. There will be a buildup of defective genes into the population. It won't be noticed until it is too late.

To understand why defective genes don't build up and fill the gene pool, we have to talk about genes a bit.

Full disclosure: the following sections are oversimplified.

Recessive Genes

Some genes are recessive. They only express themselves when two people carrying the same gene have a child. For example, two people carrying Tay-Sachs gene could have a child who dies young of Tay-Sachs, but if only one parent is carrying the gene, the child will not have the syndrome.

Aside: If the gene leads to a visible result in the child, such as a child afflicted with Tay-Sachs syndrome, it is said that the gene is "expressed" in the child. This is an oversimplification of the term "expression" for genes, but I can't think of a better term. I could begin referring to genotypes and phenotypes, but that would make this post completely unreadable. End aside.

Since there are so many genes, we are all carrying some bad recessive genes. The way to avoid having these genes expressed in the children is to marry someone who doesn't share lots of genes with you. This is why first-cousin marriage is illegal in some states. The two cousins share so many genes that negative recessive genes are more likely to be expressed in the child.

Anthropologists have found many example of matrimonial moieties in societies. The society is divided into two kinship groups by descent (two moieties), and it has a marriage rule that everyone must marry someone from the other kinship group. This type of rule lessens the number of children with a double dose of a negative recessive gene. Other societies have other types of exogamy (marrying out) rules. Not all societies have these types of rules.

Negative recessive genes have been with the human race for a long time, and society has found some ways to mitigate the problems they cause. The usual method of mitigating recessive gene problems is to avoid inbreeding.

Dominant genes

What about negative genes that are "dominant?" A dominant gene expresses itself in the child, even if only one parent has the gene, and gives it to the child. In general, dominant negative genes do not spread widely because they decrease the fitness of the person to have children. They certainly don't cause a "buildup of defective genes in the population" as Alice Stewart claimed. As a matter of fact, negative dominant genes usually die out very quickly in the population because they decrease biological fitness.

Some negative dominant genes remain, however. Huntington's Disease, which killed Woody Guthrie, is caused by a dominant lethal gene. Only one gene from one parent is enough to cause the syndrome and eventually cause death.

However, since the Huntington's gene often doesn't express itself until the affected person is middle-aged (Guthrie died in his fifties), a person with the Huntington's gene can still live to have children. Therefore, the gene continues in the gene pool. It certainly doesn't spread and "contaminate the whole gene pool" and so forth, but it does remain in the population.

I am grateful for music written and sung by Woody Guthrie and his son Arlo. I am glad that no eugenicists were around to make sure that people with Huntingtons didn't contaminate the gene pool!

Dominant lethal genes are just as bad as it gets, genetically. Yet people with such genes can contribute to the strength and joy of society.

To prove it, this video of a song from the man with Huntingtons.

Another aside: Of course, what is positive and what is negative in a gene is not as clear as it used to be, either. Sickle cell disease is caused by a double dose of the sickle cell gene. However, a single dose of the gene, sickle cell trait, is protective against malaria. Modern genetics is not as simple as eugenics, which may be why the Grannies don't quote it. End Aside.

The Conversation is Private Now

In the days when eugenics was popular, the right of a person to reproduce was considered subservient to more important goals of society. This was reflected in the Stewart quote about "genetic damage" and the "buildup of defective genes in the population." At that time, society's goals were stated in terms of protecting the gene pool, ridding society of imbeciles, and so forth. Often, as in Vermont and the Abenakis, the definition of imbecile was "people from another ethnic group." The Stewart quote above and the Oliver Wendell Holmes ruling quoted in earlier posts exemplify this world-view.

Now, things are different. Genetic testing is common. While it is common to abort fetuses that have genetic problems, it is not universal to do so. (I am not arguing for or against abortion here, just pointing out that debates about the subject are NOT debates about saving society and the gene pool from defective genes.) Special-needs children are not called "imbeciles." They are not warehoused in institutions until they die. They are treated as the human beings they are.

In other words, the conversation about genetics has changed. It's a private conversation nowadays, a conversation about individuals. It's a kinder conversation, too. Society has grown in thoughtfulness and compassion as our knowledge has grown. For most people, the cruelty embodied in the early 20th century views of genetics is a thing of the past. Human dignity is now considered more important than protecting society from "defective" genes.

I hope the eugenics Grannies will soon join the twenty-first century, at least in terms of genetics and human dignity.

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