(A-C) Hindlimb phenotypes in domestic pigeon breeds, including scaled (A), grouse (B), and muff (C). (D) Experimental approach to identify differentially expressed genes at three stages (HH21, HH25, and HH29) of pigeon FL vs. scaled HL development. (E) PCA based on top 500 most variable genes in FL and scaled HLs. PC1 (77% of variance) separates samples by stage, PC2 (10% of variance) separates samples by limb type. (F) Number of genes that show FL-enriched or scaled HL-enriched expression pattern at HH21, HH25, or HH29. Genes are grouped based on Log2FC. FL = forelimb; HL = hindlimb; HH = Hamburger-Hamilton stage. Photo credits: A, Sydney Stringham; B and C, Thomas Hellmann.