24 HEREDITY AND EUGENICS 



diagram (Fig. 8), but onl}' the sons will be colour- 

 blind. 



The last three lines show how a colour-blind father 

 and a heteroz3'gous mother will have a family in which 

 half the daughters show the defect and half the sons 

 will show it.* If the mother were homozygous for 

 colour-blindness and the father also carried it, then 

 all the children would be colour-blind. There is no 

 instance of a colour-blind father transmitting the 

 condition to the next generation, except in connec- 

 tion with a mother who transmits it. This criss- 

 cross type of inheritance is more complicated than 

 simple Mendelian behaviour in which both parents 

 take the same part in inheritance, but it is simply 

 explained by assuming the behaviour to be due to the 

 transmission of a defective X chromosome. 



It appears, then, that in all these cases the fact 

 that the differences are inherited as Mendelian factors 

 results from the manner of distribution of the chro- 

 mosomes in the reduction divisions at the time the 

 germ cells of the organism are matured. It may be 

 that some of the fundamental resemblances betw^een 

 related organisms are inherited in a different wa}^ 

 Since in experimental breeding it is only possible 

 to study directl}^ the inheritance of differences, 

 evidence concerning the process of inheritance of 

 resemblances must necessarily be indirect and closel}' 

 wrapped up with development itself. 



We ma}^ now consider some of the differences 

 appearing in man which so often follow one of these 

 types of Mendelian behaviour. While dominance 

 is very common, especiall}^ in connection with abnor- 

 malities, it is not by any means universal. There 

 is, for instance, no dominance in such a character 

 as skin colour, but the first generation is intermediate 



* There appear to be some irregular cases in which colour- 

 bhndness shows in a heterozygous woman. 



fnffparr librart 



19, C. Statt Colkj^ 



