A white-eyed fly

To scientists, the most important mutation that ever took place happened inside a milk bottle - in a ordinary little fruit fly.

For a year, starting in 1909, Thomas Hunt Morgan, professor of zoology at Columbia University, had been breeding this little fly called Drosophila. It is a small, ordinary looking insect the sort you often find in grape arbors.

One day in 1910 professor Morgan noticed a very unusual sight in one of his fly-filled milk bottles. There, among all the red-eyed Drosophila was one with white eyes!

Was the white-eyed fly really something new, or would its offspring go back to the red eyes of the rest of the flies? Professor Morgan bred his white-eyed fly and waited to see what colour the eyes of the breed would be. Some were white! He had discovered a real mutation.

This single white-eyed fly started professor Morgan and his co-workers off on eighteen years scientific work. They are known to have studied 15 million flies and found about 500 mutations. The mutations affected the development of every part of the flies' bodies, their legs, their shape and colour, their internal organs. Through the long years of work in the now famous "fly-room" at Columbia, Professor Morgan and his colleagues were able to show that the genes were arranged on the chromosomes like beads on a string.

Drosophila seems almost made to order for scientists to study mutation. The flies are known to grow very easily on bananas or on other simple food. They are hardly little creatures and will stand up under all kinds of treatment. They are known to have a great many clear, easy to recognize features. They have a very small number of chromosomes - only 8 (man has 46 and the crayfish has 200). Most important of all, these flies breed very rapidly. It takes Drosophila only 12 days from the time an egg is laid to grow into mature fly ready to lay eggs in its turn. And under the right conditions, a single fly may lay over a thousand eggs.

Drosophila, trees, bacteria, molds, man and every other living thing - all have genes which pass along from parents to offspring generation after generation. In 1933, twenty three years after the first white-fly appeared in the milk bottle, Professor Morgan was awarded the Noble Prize in medicine for his part in showing how characteristics pass from one generation to another.

A few mutations are very striking and most mutations have little influence on development. Their results are so tiny that we never notice them. Occasionally, however, a mutation may have an important effect because it occurs under just the right set of circumstances.

Mutations occur rarely, but over the years they begin to pile up. Remember that in evolution we deal with many thousands, even millions of years. After a million years, offsprings begin to have quite a few genes that are different from their ancestors.

But what causes mutations? Unfortunately scientists still know very little about what actually does make genes mutate. We do know that mutations can pile up in any directions. By X-rays or other treatments, scientists have made mutations take place much more often than they do naturally. So far, they have not been able to control the direction in which mutations take place. By even this may become possible before long. But treating such things as bread, mold and bacteria with certain chemicals scientists are known to make them mutate in direct way. The possibility of directing mutations in more complicated plants and animals will certainly increase as we learn more about the gene's chemistry and understand better what causes mutations in nature.

("Biology and Human Progress"

by L.Eisman and Ch. Tanzer)