Evolution What the Fossils Say and Why it Matters

(B)

FIGURE 4.5. Homeotic mutants show that big developmental changes can result from small genetic mutations,

giving rise to dramatic differences in body plan. (A) The antennipedia mutation, where a leg grows instead

of an antenna. (Photo courtesy F. R. Turner, Biology Department, Indiana University) (B) The bithorax mutant

fly, which has a second pair of wings instead of the halteres normally found behind the front pair of wings.

(Photo courtesy W. Gehring and G. Backhaus)

FIGURE 4.6. Map of the locus of action of the Hox genes in the fly and in the mouse. Note that the basic Hox
genes are similar in almost all bilaterally symmetrical animals, so the system goes back to the very origin of
complex animals. Small changes in any of these Hox genes make big differences in body plans. (Illustration
by Carl Buell)

Head Thorax Abdomen

Drosophila Hox C

Hypothetical

common ancestor

Mouse hox

A

B

C

D

Fly (Drosophila) embryo

Mouse embryo