H. E. JOKDAN 79 



In cuiiclusion; the data with reference to hereditary lefthandedness 

 are overwhelmingly in accord with Mendelian theory. Certain real 

 exceptions appear; those, however, are no more forbidding than those 

 relating to certain other human characteristics, and readily permit of 

 complete reconciliation by aid of the suggested hypotheses of degrees 

 of intensity of bias, and mild dominant and intense recessive strains 

 of lefthandedness. Whether this interpretation of " exceptions " be 

 thought admissible or not, the mass of strictly conformable evidence 

 and data is still so considerable as to compel the conclusion of the 

 general validity of Mendelian law in the inheritance of lefthandedness. 



A Note on Twinning. 



Davenpoi't has called attention to the inheritance of this tendency, 

 and illustrates the phenomenon by two suggestive pedigrees recorded 

 by Stocks and by Wakley. Dr James Oliver has more recently 

 published abbreviated pedigi'ee charts of 28 sets of twins, showing an 

 hereditary tendency to twinning. The data are perhaps not yet of 

 sufficient quantity to warrant .deductions as to laws of twin heredity- 

 Charts 79 and 80 show perhaps simply that t^vins in one's ancestry 

 gives a fair prospect of twins in one's progeny. The fact that " normal " 

 parents may have twins suggests that the factor (or factors) for 

 twinning is a Mendelian recessive. This is further suggested by the 

 circumstance that a normal (A) x twin (B) cross produced a fraternity of 

 10, all normal (Fig. 79). However, the pedigrees cited by Davenport 

 seem to leave little doubt that the twinning tendency is in some 

 instances " dominant " ; probably a complex of factors is involved. 

 Moreover, contrary to Dr Oliver's data, which shows a preponderance of 

 female to male twins to the extent of 2 to 1, chart 79 shows a prepon- 

 derance of males in almost the reverse ratio, 7 females to 11 males. 

 The same deduction is indicated in a pedigree (Fig. 80)' supplied by 

 Mr R. G. Reaves, of Greenville, Tennessee, in which there arc 14 pairs 

 (1 with sex unknown) including 18 males and 8 females (6 male pairs, 

 1 female pair, and 6 bi-sexual pairs). 



1 This is of conrse only a partial pedigree in the earlier generations ; it includes, 

 however, all known pairs of twins ; the total number of descendants of the first mating 

 charted is approximately 1100. Childship (M) fulfils Mendelian expectancy for the 

 probable cross, DR x ER. 



