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Sickle cells in the blood (foreground) result from two inherited copies of the gene, and cause anemia. One copy confers resistance to malaria. Photo: <a href="http://www.flickr.com/photos/wellcomeimages/">Wellcome Images</a>, CC <a href="http://creativecommons.org/licenses/by-nc-nd/2.0/">some rights reserved</a>
Sickle cells in the blood (foreground) result from two inherited copies of the gene, and cause anemia. One copy confers resistance to malaria. Photo: Wellcome Images, CC some rights reserved

Natural Selections: When evolution GOES WRONG!

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Not all evolutionary change is good. Genetic changes can be neutral or harmful, as well as beneficial. And some change can be both, conferring benefit when a single copy of a gene is present, and causing a life-threatening disease when copies are inherited from both parents. Martha Foley and Dr. Curt Stager roll the dice on evolution.

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Is everything that happens beneficial to the thing it happens to? Usually we talk about something neat and cool an animal or plant does. However, that is not always the case, and an evolutionary biologist would say not everything is beneficial.

“In fact, evolutionary biologists assume that there are some mutations things get that don’t have any real affect at all; it’s a little bit complicated,” Stager said.

The mutations can serve as little “time keepers”, and can be used to analyze the sequence of the genes to tell relatedness when looking at two species.

“So, it’s actually not assumed, in evolution, that every feature on an animal or plant or person would have to necessarily always give benefits. They could be neutral, some of them could even be harmful.”

Some mutations can be advantageous in some circumstances and not others. An example is hair color.

Hair color comes from two genes, one from each parent. A brown hair color gene from mom and blonde hair color gene from dad—brown hair will dominate.

“If you get two blonde parent genes in there, that’s the only time suddenly in this lineage there is someone with blonde hair. It’s called a recessive trait; you get it very rarely.” Situations occur with recessive traits where there’s no obvious benefit--or there’s a disadvantage which normally is hidden.

With hair color there is obvious advantage, but you could be at a disadvantage in some other cases, such as with the blood disease sickle cell anemia. “It [the responsible gene] actually has an advantage in the place where it’s from in Africa, because it gives you resistance to malaria.”

If one parent gives you this trait, it mingles with the regular gene and you have a mild case of sickle cell. “But if you are unfortunate in that both parents had it, so you have two copies instead of one copy of the gene, it dominates, and… you get much more serious symptoms of this illness.”

Wouldn’t a recessive gene eventually just peter out?

In the case of sickle cell, the gene persists, Stager said, because of the advantage (resistance to malaria) of having a single copy of the gene. This outweighs the disadvantage to the fewer individuals who have serious illness from having two copies of the gene.

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