1 14 HEREDITY AND EUGENICS 



condition is present in the males of three generations, 

 being transmitted in one case through two genera- 

 tions of females before reappearing in a male. In 

 the second family there were two cases of colour- 

 blind females, one of whom had three sons, two of 

 whom were known to be colour-blind. 



Little and Gibbons (1921) have recently considered 

 the inheritance of haemophilia and colour-blindness 

 in relation to the presence of sex-linked lethal factors, 

 which would, of course, follow the same line of in- 

 heritance. If there is linkage between a sex-linked 

 lethal factor and the normal allelomorph to haemo- 

 philia or colour-blindness (the latter being due to 

 different defects in one X chromosome, and the normal 

 to a non-defective X chromosome), then the authors 

 show that there should be an excess of abnormals 

 among the males in pedigrees in which these sex- 

 linked diseases occur, and a decreased proportion of 

 females in families in which an excess of affected males 

 does not occur. From the data of Bulloch and 

 Fildes (1910), as well as other data at the Eugenics 

 Record Office, they find these expectations fulfilled, 

 thus furnishing evidence of sex-linked lethal factors 

 in man.* 



Congenital stationary night-blindness w^as recognised 

 as a Mendelian dominant b}^ Nettleship (1907). 

 It was in existence near Montpelier as early as 1637. 

 Data were afterwards obtained from the parish 

 records, and in 1836 Cuvier published a pedigree of 

 seven generations. The Abbe Capion completed 



* Lethal factors have been most extensively studied in Dvoso- 

 phila. Their presence causes the normal development to go 

 astray, leading to the death of the organism at an early or a later 

 stage in its development. Enriques (191 9) has shown that in 

 the fly Calliphora erythrocephala, certain individuals produce 

 25 per cent, of non-vital offspring. The larvae cease to eat after 

 two or three days and then die. Some of the non-lethal offspring 

 in turn produce lethals in varying proportions. 



