70 Hereditary Lefthdiidedness 



Four fraternities give the expected proportion for a Dli x RR cross ; 

 several others may be said to approximate expectancy. But only by 

 assuming that a certain number of the normal consorts were duplex for 

 dextrality (-D-D), and that the character is occasionally imperfectly 

 dominant, can the ratio of the totals (1 : 2'55) be made to suggest 

 Mendelian principles. 



Six blanks record lefthandedness appearing in great-grandparent, 

 parent and child (charts. Figs. 17 to 22). Pedigi-ee chart. Fig. 22, is of 

 especial interest in that it gives a final generation of two pairs of twins, 

 all lefthanded. Since both pairs are severally of the same sex, they are 

 presumably identical ; the fact that identical twins are rarely unlike 

 with respect of use of hand, suggests very strongly the hereditary 

 character of this trait. The facts shown in this chart as also in chart 

 Fig. 17, again suggest dominance of the lefthandedness factor in 

 certain strains; or "imperfection of dominance" of the righthandedness 

 factor. 



Fourteen histories show transmission from great-grandparent to 

 child (Figs. 23 to 36). The absence of information concerning collateral 

 lines, and of complete childships in every instance, forbids deduction 

 beyond the general statement that lefthandedness in the ancestiy 

 presages a certain amount of lefthandedness among ofl'spring. The 

 inference is suggested that the normal parents are heterozygous, in 

 which event the Mendelian pioportion of 1 lefthanded to 3 righthanded 

 would be expected. This expectation is practically met in charts 24 

 and 33. With the exception of charts 35 and 36, the remaining charts 

 of this group also are in close accord with Mendelian formulae. 



Four blanks I'ecord a four-genei-ation history of lefthaniledness with 

 the direct ancestors of the penultimate generation normal (Figs. 37 to 40). 

 All show a lefthanded offspring from a double "normal" mating. The 

 ancestry of the normals concerned, however, suggests their heterozygous 

 nature ; one in every four would therefore be expected to be lefthanded. 

 The histories as given are in close accord with this ratio. 



Sixteen blanks record lefthandedness in three consecutive genera- 

 tions (Figs. 41 to 56). Chart, Fig. 41 is interesting in that the direct 

 ancestors in the two earlier generations were both ambidextrous, 

 suggesting a specificity of this condition as compared with dexterity 

 and sinister] ty. In charts, Figs. 44, 50 and 52, the lefthanded condition 

 is distributed in the fraternities approximately as would be expected in 

 a DR X RR cross, most probably the actual fact. 



