Genes that cause disease

The medical school lecture hall was surprisingly full for a Monday at eight A.M. The lecture dealt with nearsightedness. As the room darkened, the overhead spotlights glinted off the eyeglasses worn by nearly half the students. “So that’s why so many showed up,” murmured the professor.

“The facts are clear,” he summarized an hour later. “Myopia is caused by excessive growth of the eye. When it gets too long from lens to retina, the focal point remains above the surface of the retina, so that the image is blurred. Refractive lenses, in the form of glasses or contact lenses, can refocus the image a bit further back so we can see clearly, overcoming nature’s inexactitude.”

Some hands began to wave. “But what causes the eye to grow too long?” asked one student.

“Genes,” he said. “It’s as simple as that. Some of us were just unlucky enough to get bad genes. If your identical twin is nearsighted, you will almost certainly be also. If your sibling is nearsighted, the likelihood is high, but not as high. Pulling all the figures together, myopia seems to be a genetic disease with a heritability of over eighty percent.”

“But how could such genes survive before glasses were invented?” asked another student. “Without my glasses, I wouldn’t last a day on the African plains.” The class laughed uneasily.

“Well, the genes might be recent mutations,” said the professor. “Or perhaps Stone Age myopic people worked in camp sewing and weaving. In any case, the facts make it clear that myopia is a genetic disorder.”

“But how could that be?” the student persisted. “The force of selection against it would be enormous. If such a severe defect can persist, then why aren’t our bodies riddled with defects?”

“In fact, our bodies don’t work very well,” the professor said pointedly. “As you have been learning, we are bundles of genetic flaws. The body is a fragile, jury-rigged device. Our job as physicians is to fix Mother Nature’s oversights.”

The medical students grumbled a bit more among themselves but did not persist further.