In the classification of the brachydactylies, the analysis by Bell (1951) proved highly useful. The type A brachydactylies of Bell have the shortening confined mainly to the middle phalanges. In the A1 type, the middle phalanges of all the digits are rudimentary or fused with the terminal phalanges. The proximal phalanges of the thumbs and big toes are short. Genetic Heterogeneity of Brachydactyly Type A1 BDA1B (607004) has been mapped to chromosome 5. BDA1C (615072) is caused by mutation in the GDF5 gene (601146) on chromosome 20q11. BDA1D (616849) is caused by mutation in the BMPR1B gene (603248) on chromosome 4q22.

Brachydactyly type B2 (BDB2) is a subtype of brachydactyly characterized by hypoplasia/aplasia of distal phalanges in combination with distal symphalangism, fusion of carpal/tarsal bones, and partial cutaneous syndactyly (summary by Lehmann et al., 2007).

Proximal symphalangism-1A (SYM1A) is an autosomal dominant disorder characterized by ankylosis of the proximal interphalangeal joints, carpal and tarsal bone fusion, and, in some cases, conductive deafness (Strasburger et al., 1965). Genetic Heterogeneity of Proximal Symphalangism Another form of proximal symphalangism (SYM1B; 615298) is caused by mutation in the GDF5 gene (601146).

Carpenter syndrome is a rare autosomal recessive disorder with the cardinal features of acrocephaly with variable synostosis of the sagittal, lambdoid, and coronal sutures; peculiar facies; brachydactyly of the hands with syndactyly; preaxial polydactyly and syndactyly of the feet; congenital heart defects; growth retardation; mental retardation; hypogenitalism; and obesity. In addition, cerebral malformations, oral and dental abnormalities, coxa valga, genu valgum, hydronephrosis, precocious puberty, and hearing loss may be observed (summary by Altunhan et al., 2011). Genetic Heterogeneity of Carpenter Syndrome Carpenter syndrome-2 (CRPT2; 614976), in which the features of Carpenter syndrome are sometimes associated with defective lateralization, is caused by mutation in the MEGF8 gene (604267).