Breitenbecher: Short Thumbs 



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INHERITED FOR FIVE GENERATIONS 



Figure 11. Circles represent women, squares, men. Short thumbed individuals are indi- 

 cated by solid black; a question mark shows an individual who died in infancy, about whose 

 thumbs nothing is known. It is evident from the chart that the character is as likely to 

 atppear in one sex as in the other. A study was also made of the members of the family 

 who had normal hands, and among their descendents not a single individual having short 

 thumbs was found. 



Abnormal 8, daughter of 5, married 

 a normal man. Their offspring con- 

 sisted of three abnormals (Nos. 11, 

 13 and 13), son, daughter, and son, 

 respectively, and one normal daugh- 

 ter. 



Abnormals 9, 10, 11, 12 and 13 are 

 not old enough to be married. 



To summarize the appearance of ab- 

 normals with reference to generations : 

 the first generation begins with No. 



1 and in the second generation Nos. 



2 and 3 appeared. Abnormals 4, 5, 

 and 6 were in the third generation, 

 and in the fourth generation were 

 Nos. 7, 8, 9, and 10. Lastly, Ab- 

 normals 11, 12, and 13 appeared in 

 the fifth generation. Thus for five 

 consecutive generations the trait has 

 appeared. 



The writer also obtained complete 

 pedigrees of normals mated with 

 normals in this family, but to give 

 the complete data and family histories 

 for such normals would require too 

 much space and possibly be of little 

 value. The results of this investiga- 

 tion can be summarized by stating 

 that whenever normal members of 



the family married other normal in- 

 dividuals there were no abnormal 

 children. Every individual with an 

 abnormal hand had one abnormal 

 parent. As the other parent, in every 

 case, was normal, all of the abnormal 

 individuals must have been hetero- 

 zygous. 



Discussion 



It seems that this character is the 

 result of a mutation which appeared 

 more than five generations ago. If 

 the abnormality in this family is a 

 part of the normal variation in length 

 of thumbs that is to be expected, it 

 is true that some short-thumbed in- 

 dividuals should be discovered among 

 the hundreds of people examined. It 

 further appears that shortness of 

 thumbs is a mutation because its range 

 of variation differs from the normal. 



The results given in Table 1 indi- 

 cate that this character is a Mendelian 

 dominant, the normal being recessive. 

 If we represent the homozygous domi- 

 nant trait by (AA) and the recessive, 

 normal, by (aa), it is apparent that 

 all offspring from the modification 



