194 HEREDITY AND EUGENICS 



Inheritance of Twinning. 



Interesting studies have recently been made on 

 the inheritance of twinning. It is well known that 

 a tendency to produce twins is inherited in sheep, 

 certain breeds nearly always bearing twins. Daven- 

 port (1920) places the frequenc}^ of twin births at 

 about II per cent., and Bonnevie (1919^) finds, from 

 statistics of human births in Norway, that i -34 per 

 cent, of births are twins. In one Norwegian family, 

 however, the frequenc}^ of twin births was as high as 

 19-5 per cent., showing a strong tendenc}^ to in- 

 heritance. It is significant that twinning occurs more 

 often in large than in small families. In the families 

 investigated, 20 per cent, of the twin births were 

 believed to be uniovular — i.e., derived from a single 

 impregnated Qgg. It is suggested that the tendency 

 to produce biovular or non-identical twins is a 

 recessive character, but this conclusion does not 

 appear very probable. 



In a statistical stud}^ of twinning in families, 

 Davenport (1920) considers families in which twin 

 births had occurred more than once. The mothers 

 and fathers of such families are spoken of as repeaters. 

 If inheritance of twinning were onl}^ through the 

 mother, then the relatives of repeating mothers should 

 show a higher proportion of twins than the relatives 

 of repeating fathers. It was found, however, that of 

 355 labours occurring to the mothers of repeating 

 mothers, 16 or 4-5 per cent, were twin labours; while 

 of 289 labours occurring to the mothers of twin- 

 repeating fathers, 12 or 4-2 per cent, produced twins. 

 Thus, twins occur with nearly equal frequency in the 

 fraternities of repeating fathers and mothers, and with 

 about four times their frequency in the general popu- 

 lation. These and similar statistics indicate that, in 

 strains bearing two or more pairs of twins to a family, 



