SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 7 



Of course if a dominant character is advantageous in itself, it 

 will have a better chance of spreading through the race, than will 

 an advantageous recessive character, because every hybrid that 

 carries one dominant gene shows also the character, which in- 

 creases the chance that it will propagate and spread the genes. 

 But, on the other hand, if a dominant character is injurious it will 

 have a smaller chance of spreading than will an injurious recessive 

 character; for, the recessive may be carried by the hybrid without 

 showing itself, and therefore will not place the hybrid individual 

 at a disadvantage. 



An excellent illustration of dominance is that recently pub- 

 lished by Mohr. He has traced, through five generations of a 

 Norwegian family, the inheritance of a shortened first digit. In 

 the history of this case there is one record that is extraordinarily 

 interesting. A child was born that was so completely crippled 

 that it died in infancy. One parent was short fingered; the other, 

 a cousin, was probably also short fingered. It is possible that the 

 child had a double inheritance of this character ; it was a pure 

 dominant. If this is true, then it appears that this character can 

 survive to maturity only in the hybrid condition. As a matter of 

 fact, in other animals there are some well-recognized cases of this 

 sort. That of the yellow mouse is the best known. Yellow is a 

 dominant and in double dose it kills; therefore when yellow is 

 bred to vellow all the pure yellows die. The hybrid yellows and 

 the pure blacks (in Fig. 4) survive. Here yellow is dis- 

 criminated against in the embryo ; but, being dominant, it still ap- 

 pears twice as frequentlv in each generation as does the alternate 

 character (here black). In the fly, Drosophila, we have at least 

 25 dominant lethal characters, but as yet we have no knowledge 

 as to why such a high percentage of dominant characters should 

 be lethal when homozygous. 



In man there are no certain cases known of lethal dominants 

 unless some of the short-fingered types come under this heading. 



Dominant and recessive characters have been so much dis- 

 cussed in modern Mendelian literature that it is popularly sup- 

 posed that all Mendelian characters must lie either dominant or 



