Figure 1.

Digit length is additively determined by the dose of Hoxd-11, Hoxd-12, Hox-13, and Hoxa-13 gene products. The length of forelimb digit IV (arrowheads), including the metacarpal bone, was taken as a reference measure in mice of different genotypes. Compound mutant genotypes are indicated on the left, black rectangles indicating the loss of one dose of the corresponding gene (shown at the top). In this view, the wild-type hand (A) had 10 doses of active products. From B to G doses were progressively removed by using the corresponding genotypes: (B) HoxD^Del/+; (C) HoxD^Del/+;Hoxa-13^+/−; (D) Hoxd-13⁻/HoxD^Del; (E) HoxD^Del/Del; (F) Hoxd-13^−/−/;Hoxa-13^+/−; (G) HoxD^Del/Del;Hoxa-13^+/−. (H) The length of digit IV was measured, expressed as fraction of wild-type digit length, and plotted against the Hox dose (see Materials and Methods for the calculation of the respective dose per gene). In this way, the length of digit IV varied as a linear function of the dose, regardless of the nature of the combination. (A–G) Anterior is up, posterior is down. I–V indicate digit number with, by convention, digit I being the thumb. In mutants with more or less than five digits, the phalanx pattern makes individual homologization impossible, but the digit found at the position corresponding to wild-type digit IV was always the longest.