88 



The Journal of Heredity 



tion, a condition not under control. 

 But the fact remains that there is a 

 possibility of any of the phenotypes 

 not already observed appearing sooner 

 or later. So that whenever such ap- 

 pear, they are easily accounted for. 

 Further, we must note that in the 

 practical observations that are known 

 to us, there is nothing that can not be 

 accounted for; there is nothing that 

 conflicts with our theoretical expecta- 

 tions. 



(3) The Mendelian ratio of 3 : 1 in 

 the Fj generation is apparent, but we 

 have no data as to how far it is attained 

 in actual practice. However, we must 

 remember that this ratio should be 

 looked for only in those organisms in 

 which a reasonably large number of 

 individuals are born in a generation. 



(d) Facts Nos. 4 and 5 are inter- 

 related, one being the cause, and the 

 other its effect. A toothless female 

 (bX) (bX) can only result when a 

 heterozygous female (BX) (bX) is 

 mated to a toothless male (bX) Y, a 

 condition not yet attained in practice. 



CONCLUSION 



(1) The toothless man seems to be an 

 example of regressive mutation. 



(2) The phenomenon of heredity in 

 the toothless man is strictly a case 

 of sex-linked inheritance, as all the 

 existing facts are in conformity with 

 our theoretical expectations based on 

 Mendelian laws of Segregation and 

 Dominance. 



PRACTICAL SIGNIFICANCE 



Since heredity in the toothless man 

 obeys the laws of Segregation and 

 Dominance, we can predict some of the 

 important facts summarized below. 



(1) A normal male, even if he is a 

 son of a toothless man, has no contami- 

 nation of this defect in his germplasm; 

 no fear need be entertained about his 

 carrying the defect in his germ without 

 he himself showing it. 



(2) The daughter of a toothless man, 

 altho she does not show the defect in 

 her body, carries the contamination 

 hidden in her germplasm and will 

 produce, on union with a normal male, 

 some toothless children, and these will 

 be males, with the complete exclusion 

 of the toothless female children. But 

 if she marries a toothless male there is a 

 possibility of her producing both tooth- 

 less and normal females, and also both 

 toothless and normal males. 



(3) Should, at any time, a toothless 

 female appear, and should she marry a 

 toothless man, then all their children, 

 whether males or females, will be 

 toothless without exception. 



INFORMATION WANTED 



The writer of this article would be 

 glad to hear from anyone who can 

 report if, at any time, any of the 

 toothless men ever had (or has) a 

 brother (from the same mother) who 

 was normal, that is to say, was not a 

 "Bhuda" ; if any of his sisters (from the 

 same mother) had all toothless sons 

 while another had some normal sons. 



The;Diseases 



LeITFADKN DKR KlXDKRIlEILKUNDE f iir 



Studierende und Aertze; erster Teil, 

 Sauglingskrankheiten; 4te, ver- 

 besserte Auflage. Von Dr. Walter 

 Birk, professor d. Kinderheilkunde 

 a.d. Universitat Tul)ingen. l*p. 269, 

 broschiert M. 12.50, gebunden 

 M.15.50. A. Marcus and E. Webers' 

 Verlag, Bonn, 1920. 



Dr. Birk lays out his subject method- 



of Infancy] 



ically and sticks to it in a practical way 

 that gives little opportunity for excur- 

 sions into the field of genetics; although 

 much nn'ght be written on the part that 

 luTedity plays in the diseases of the 

 first year of life. His brief remarks 

 on the frequency with which twins 

 show marked difference as regards 

 constitutional disease, make one desire 

 further elaboration on this point. — P. P. 



