62 SUMMARY. 



All the brachyphalangous individuals are heterozygous for the gene 

 in question, with one possible exception. This individual resulted 

 from an intermarriage within affected lines, and therefore may have 

 been homozygous for the factor for brachyphalangy. She was a cripple 

 without fingers and toes and was unable to develop. She died at the 

 age of one year. 



Analogous cases from experimental genetic work, demonstrating a 

 distinct difference in the appearance of the somatic character when 

 the genes are homozygous from that of the heterozygous condition, 

 and the lethal effect of many dominant genes when homozygous are 

 discussed in this connection. Emphasis is laid on the importance of 

 this experience from a medical point of view. 



The material includes one case of identical twins, both brachypha- 

 langous of an identical type. 



Based on the numerous measurements from the radiographs, an 

 analysis of the two types of the malformation was possible. Much 

 attention has been paid to this special point. It was found that the 

 B-type of brachyphalangy may overlap the limit of extreme variation 

 in normal hands — i. e., some individuals heterozygous for the gene for 

 brachyphalangy (which generally causes a very characteristic altera- 

 tion of the hand) may be somatically normal. The conclusive demon- 

 stration of this fact was obtained in one case where the radiographs 

 gave normal measures while the genetic test, which was at the same 

 time at hand, proved the individual to be heterozygous for the gene 

 for brachyphalangy. 



A genetic explanation of the occurrence of the B-type and B !-type 

 is suggested. Based on analogies from experimental genetic work, the 

 two types are explained through the presence in some of the normal 

 individuals, married into the family, of a dominant specific modifying 

 gene which enhances the effect of the principal gene for brachyphalangy 

 and changes the B-type into a B !-type. 



Indications in favor of this view are presented from the earlier 

 literature concerning hereditary brachyphalangy in man which deals 

 with types of brachyphalangy different from the one here studied. 

 The review of this literature includes both ''pre-Mendelian" and 

 "post-Mendelian" publications demonstrating inheritance of these 

 malformations in accord with Mendel's laws. 



