184 THE MENDEL JOURNAL 



of Mr. Nettleship's cases are interesting and 

 suggestive, for they show that the abnormal condition 

 may skip a generation.* That is probably the meaning, 

 in the pedigree described by Dr. Rutherfurd, of the 

 appearance of the affected individual No. 21, in generation 

 B, who was the daughter of two unaffected parents. 

 Had we known her grandparents' history, it is probable 

 there would have been evidence of cataract forthcoming. 

 A glance at the history of No. 14 in the B generation 

 renders this consideration almost a certainty. No. 14 

 himself is affected, but both his parents were normal. 

 Nothing is known of his grandparents. But his uncle 

 No. 1, A, was an affected person. There is thus clear 

 history of the existence of the disease in the family, 

 and it may be regarded as certain that if more were known 

 of the history of Nos. 3 and 4, A, the existence of cataract 

 would be manifested somewhere among the members of 

 their family. We can therefore hardly regard No. 21 B, 

 as manifesting a mutation, since it is probable that her 

 defect has not arisen de novo, as Dr. Rutherfurd postu- 

 lates. 



There is another point of interest in Mr. Nettleship's 

 pedigrees. In a relatively large number of cases the 

 disease has appeared in the offspring of consanguineous 

 marriages. The disease, however, can always be traced 

 back to an ancestral stock. The relatively frequent 

 appearance of the disease in cousin marriages 

 suggests that the disease may be due to the presence 

 of not one but of several factors. The association of 

 these factors in the same individuals is, of course, 

 more likel}^ to occur in the cousin marriages of an affected 

 stock than in marriages among individuals selected at 

 random from the general population. If the congenital 

 diseases, retinitis pigmentosa and lenticular cataract, 

 are due to the presence in the same individual of two or 

 more factors, the presence only of any one of these 

 factors being insufficient to cause the disease, then we 

 have an explanation of unaffected pairs of parents, as 

 well as of pairs in which one or both parents are affected, 

 producing a mixed offspring of abnormal and normal 



* See Roj'al London Ophthalmic Hospital Reports, vol. xvii., part i., 

 1907, pages 19 and 20, Figs. 16, 16a, 19, and 20. 



