The progress of Genetics since the rediscovery of Mendel's papers. 405 



It should be distinctly understood that there is as yet no warrant 

 for the assertion that these diseases are transmitted on this system, 

 but if we suppose that haemophilia and colour-blindness were domi- 

 nant in males and recessive in females, this description will include 



f 



* 1 



sUghi/ 



Fig. 20. Pedigree of colonr-hlindness after Burckhardt. The black symbols show 



the affected. 



a large part of the observed facts. This hypothesis does not ag-ree 

 well with such numerical results as exist, for I find that many more 

 males are affected (with haemophilia) in these families than would 

 be expected on a simple Mendelian scheme, and too many females 

 transmit. Nor is it always easy to suppose that this is due to 

 omission of normal persons by the recorders. 



Other cases of Mendelian Heredity in IVIan. 



r>f characters having an obviously Mendelian inheritance in 

 Man I can give two only. In Farabee's cases reduction in the 

 number of phalanges is plainly a dominant (Figs. 6, 21, 22), and 

 from the pedigrees of congenital cataract collected byNettleship (80) 

 there can I think be no doubt that this condition behaves generally, 

 if not perhaps always, as a Mendelian dominant, unaffected members 

 of affected families, with very rare exceptions, do not transmit it; 

 but when affected persons marry normal persons — the usual mating 

 — their offspring generally consist of affected and unaffected in about 

 equal numbers (Fig. 22). As I have mentioned elsewhere (7, p. 176) 

 there are many nervous diseases in which transmission is exclusivelj^ 

 or almost exclusively through the affected, but the ratio of affected 

 to unaffected in the offspring is almost always too high. 



With regard to recessive characters in Man there is no quite 

 satisfactory evidence yet published. Albinos have been recorded 



