34 Congenital Niglit-Blindness 



dependent upon or associated with the presence or absence of the 

 X chromosome in the spermatozoa. Regarding the ova as homozygous 

 and the spermatozoa as heterozygous for sex, we may explain the 

 inheritance formula of night-blindness by assuming that the character 

 is a Mendelian recessive, which has probably arisen by the loss of some 

 factor necessary for normal vision, and that the defect has its seat in 

 the X element of the affected male. Probably night-blindness arose in 

 a male as a mutation. 



On this assumption eggs fertilized by spermatozoa with the X 

 element will develop exclusively females, and therefore the X element 

 of the affected father, together with whatever defects or peculiarities 

 may be associated with it, is transmitted only to the female offspring. 

 This explains why all of the sous of affected men are normal. The 

 daughters, however, all inherit the factor for night-blindness, but fail 

 to show it because it is recessive to the normal character carried by the 

 normal X element of the normal mother. The maturation of the ova 

 of these night-blind carrying daughters will result in the formation of 

 gametes of two kinds in equal numbers, half with the normal X element 

 and half with the defective X element (= x) carrying night-blindness. If 

 these daughters are ujated with normal men we should expect equal 

 numbers of male and female offspring, half of each of which would 

 contain the factor for night-blindness. The females of course would 

 always appear to be normal, but half of the sons would be night-blind. 

 The following diagram (Fig. 2) will illustrate graphically the inheritance 

 mechanism just outlined. 



This diagram is a modification of that used by E. B. Wilson ('11) 

 in explaining his idea of the probable mechanism of the heredity of 

 white eyes of Drosophila. He states that the same type of explanation 

 may be applied to the inheritance of colour-blindness in man. Since 

 this case of night-blindness is inherited precisely as is colour-blindness, 

 there can be little hesitation in adopting the scheme outlined above 

 for its explanation. 



Wilson points out, furthermore, that " any recessive mutation should 

 exhibit sex-limited heredity when crossed with the normal or dominant 

 form, if it be due to a factor contained or omitted from the X element." 

 It is now known that a number of human traits of an abnormal or 

 defective nature are inherited in the fashion exemplified by the present 

 pedigree. Davenport ('11) gives the following list of such characters: 

 multiple sclerosis, hereditary atrophy of the optic nerve, colour-blind- 

 ness, myopia, ichthyosis, muscular atrophy, haemophilia. Probably 



