Dermatoglyphs in duplication of the thumb.

The dermatoglyphs of 13 Japanese (eight males and five females) with duplication of the thumb were studied. The dermal patterns on the thumb were compared with those on the extra digit. A whorl on the thumb was associated with a whorl on the extra digit in two cases, a radial loop in four cases, and an arch in two cases. Where the thumb pattern was a loop the pattern on the extra digit was a loop in all three cases. Where the thumb and extra digit are both well developed the pattern tends to be the same on both digits. When the extra digit is poorly developed it tends to have a different pattern from the thumb.