Genes Seen?

In 1922 the foremost scoffer at the gene theory of heredity, England's formidable, bushy-browed Biologist William Bateson, went to the Columbia University laboratories of Thomas Hunt Morgan, examined the data, looked at the jars of fruit flies, stared down the microscopes, announced his conversion. Since then there has been little doubt among geneticists that the chromosomes in the germ cells are the theatres of heredity, that the ultimate agents, called genes, which transmit unit characters, occupy definite and fixed positions along the spindly, crooked chromosomes. Since then fame has come to Dr. Morgan and his flies, and to some of his early laboratory helpers, notably to affable, shock-haired Calvin Blackman Bridges of the Carnegie Institution of Washington.

Born in Schuyler Falls. N. Y. 47 years ago, Bridges found himself some years later in a zoology class at Columbia which Dr. Morgan had agreed to take while the regular professor was away. Dr. Morgan changed the young man's general interest in science to a specific one in genetics. Bridges haunted Dr. Morgan's office, asked if help was not needed with the bottles of flies. Morgan replied gently that trained assistants were what he needed. Undaunted, Bridges kept coming back. One day he spotted a fly with vermilion eyes instead of the normal red. Dr. Morgan had had to etherize the insects and go over them with a magnifying glass to verify the mutation. He admired Bridges' eyesight, gave him a job nursing the flies forthwith.

No geneticist will say that he has seen a gene, but last week Dr. Bridges would not say that he had not seen one. The chromosomes themselves in the fruit fly's germ cells are no larger than .00015 inch long, minute dots and streaks under the best microscopes. The chromosomes in the fly's salivary glands, however, are 70 times bigger than those in the germ-plasm, and two years ago Dr. Theophilus Shickel Painter of University of Texas took pioneer photographs of these tiny giants showing cross-bands. Then Dr. Bridges made such good photomicrographs of the salivary chromosomes that the crossbands marking gene locality came even more clearly to view, and the bands themselves were seen to be dotted or segmented (TIME, Oct. 1, 1934).

Last week Dr. Bridges published in Science further researches with charts clearly showing a gene maneuver associated with the appearance of abnormally small eyes in the fly. This mutation, discovered in 1913, was christened "Bar." Some true-breeding strains showing the Bar mutant developed other mutations in which the eye was even further reduced ("Bar-double") or returned to normal ("Bar-reverted").

Down toward the base of the X-chromosome is a sudden bulge followed by a gradual tapering to normal diameter. This is the "turnip segment." It contains, first, a heavy band in two segments, then a faint dotted line, a stronger dotted line, a diffuse double line, a faint dotted line. In the Bar flies Dr. Bridges found this arrangement in duplicate. In the Bar-double it was tripled. In the Bar-reverted it returned to the single sequence.

These observed phenomena check with previous explanations, ventured by Dr. Morgan and others, that such two-way changes could occur during "crossing over." Before fertilization, opposed pairs of chromosomes in single cells twist around each other, then separate. During this performance they may exchange genes. If the exchange is unequal, a duplication of the gene results in one chromosome, a loss of it in the other.

The dots and segments in Dr. Bridges' crossbands may be the actual genes. Whether they are or not, he seems to have taken the closest look to date into the operations of the tiny mechanism which not only distorts the eyes of a pestiferous insect but may cause the first-born son of a royal family to be a genius, an albino or an idiot.

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