Fig. 16.—If the hands be clasped naturally with fingers alternating, as shown in the above illustration, most people will put the same thumb—either that of the right or that of the left hand—uppermost every time. Frank E. Lutz showed ( American Naturalist , xliii) that the position assumed depends largely on heredity. When both parents put the right thumb uppermost, about three-fourths of the children were found to do the same. When both parents put the left thumb uppermost, about three-fifths of the children did the same. No definite ratios could be found from the various kinds of matings. Apparently the manner of clasping hands has no connection with one's right-handedness or left-handedness. It can hardly be due to imitation for the trait is such a slight one that most people have not noticed it before their attention is called to it by the geneticist. Furthermore, babies are found almost always to clasp the hands in the same way every time. The trait is a good illustration of the almost incredible minuteness with which heredity enters into a man's make-up. Photograph by John Howard Paine.
The sweet pea offers a good illustration of the widespread effects which may result from the change of a single factor. In addition to the ordinary climbing vine, there is a dwarf variety, and the difference between the two seems to be proved, by exhaustive experimental breeding, to be due