Heredity of Twin Births. 



75 



44 (1504) 

 Heredity of twin births. 

 By C. B. Davenport. 



[From the Eugenics Record Office, Cold Spring Harbor, L. I.] 



About 1 per cent, of human births are twin births. However, 

 there are certain families in which the proportion rises to 5, 10, or 

 even 15 per cent. There can be little doubt then that, as in sheep, 

 so in man, there are strains having a special tendency toward the 

 production of twins. 



It is commonly believed that this tendency toward the pro- 

 duction of twins must be wholly a maternal quality, depending 

 upon the inherited tendency to double ovulation. 



The study of the heredity of twins is accompanied by certain 

 difficulties, such as the fact that the occurrence of twins is fre- 

 quently isolated, apparently haphazard, occurring perhaps in 

 only one case in a fairly large fraternity, in which other represen- 

 tatives are single births. It will simplify the matter a little if we 

 consider only those cases in which two or more sets of twins have 

 arisen from a given mating. 



The study of twins is still further complicated by the fact that 

 they are of two types, namely twins derived from a double ovula- 

 tion and twins derived from a single ovulation, there being a sub- 

 sequent fission or budding of the fertilized egg. Such single-egg 

 twins are easily distinguished clinically by being both enveloped 

 in the same chorion. They are also always of the same sex. 



The statement that the mother alone determines the tendency 

 to twins is not, however, supported by the facts. Of the births 

 giving rise to the fraternities of twin-repeating mothers, 4.5 

 per cent, are twin births. Of the births giving rise to fraternities 

 of twin-repeating fathers, 4.2 per cent are twin births. These 

 figures depend upon 355 and 289 labors respectively. 



The sisters of twin-producing fathers have twins in 8.2 per 

 cent, of labors, while the sisters of twin-producing mothers have 

 twins in 5.5 per cent, of labors. Among the children of brothers 

 of twin-producing fathers, 6.5 per cent, are twins, among the 



