CXII BOWMAN LECTURE. 



others the age of onset was practically the same in both 

 generations. In only 3 cases was there evidence that 

 the disease began later in the second generation than the 

 first.* In the rare cases where a mother is affected the 

 onset of the disease at an earlier age in her sons than in 

 herself can hardly be called " anticipation/' because the 

 disease usually appears earlier in all males than in all 

 females. 



Anticipation in successively born brothers or sisters is 

 not so frequent, being found in only 29 out of 82 completed 

 sibships containing 2 or more cases of the disease. In 14 

 others the disease began at a later age in each successive 

 birth, in 16 at practically the same age in each case, and in 

 23 the ages of the successive siblings when attacked varied 

 irregularly. The differences of age-onset, are of course 

 much less between successive siblings than between 

 successive generations, the age of onset in the junior 

 sibling being usually about 3 years less than in the 

 senior, and seldom as much as 5 years. t 



It is of interest to inquire whether when a mother 

 suffers from the disease her children will have it in greater 

 numbers or with a different sex- distribution than if she 

 had merely carried it as a potential in the usual manner? 



In the corresponding case of congenital colour-blindness, 

 Professor Bateson finds, as already mentioned, that if a 

 woman be colour-blind the history always shows that her 

 father was so, and that if she have sons they will all be 

 colour-blind ; whereas we know that in the ordinary case, 

 where the mother is unaffected but carries the defect, 

 only a proportion of her sons will have it.J Careful 

 examination of the corresponding data for Leber's disease 

 shows that it does not conform to this rule : I. A man 

 with Leber's disease who has children seldom transmits it; 



* Norris, Amer. Ophth. Soc., iii, p. 673, Cases 58, 75 and 93 in Appendix 

 VI, 6. 



f For the data relating to anticipation see Appendix VI, &. 



Bateson, Mendel's Principles af Heredity, 1909. 



The data for what follows are given in Appendix VI, b. 



