Figure 2. Genetic interactions between Twist1^CC and Hand2.

(A) Twist1^CC/+ hindlimb polydactyly is sensitive to Hand2 dosage. Hindlimbs of F1 progeny of Twist1^CC/+; Hand2^+/+ X Twist1^+/+; Hand2^+/− intercross scored for preaxial polydactyly (arrowhead) show complete suppression of polydactyly in double heterozygotes (p<0.001, chi squared).

(B) Twist1^CC forelimb and pelvis phenotypes are sensitive to Hand2 dosage. Hand2 null allele was crossed onto compound Twist1 genotypes as indicated. Twist1^CC/CC limbs with reduced Hand2 gene dosage (row 2) have less severe pubis hypoplasia (P columns, arrows) but still display radial aplasia (FL columns, arrows). Twist1^CC/− limbs with reduced Hand2 gene dosage (row 3) resemble less severe Twist1^CC/CC limbs, with duplicated ulnae replaced by radial aplasia (FL columns, arrows and arrowhead), increased digit number, and reduced severity of pubis hypoplasia (P columns, arrows). FL, forelimb; P, pelvis; il, ilium; pu, pubis; is, ischium.

(C) Twist1^CC pubis defects were scored on a scale of increasing severity from 0 to 4 (left panels) and plotted as a percent fraction of total pelvises scored for each genotype. Reducing Hand2 gene dosage in Twist1^CC/CC or Twist1^CC/− pelvises significantly shifts pubis scores to lesser values (p<0.001 for each, Mann-Whitney).