300 HEREDITY AND DISEASE 



If we suppose that the chromosomes are all of equal value, there 

 is always a theoretical possibiUty in a human family of 169 

 different combinations of the grandparental contributions. It 

 is a well-known fact that certain predispositions to disease may 

 be seen in two or three children in a household and be quite 

 absent from other two or three. 



The chromosomes of the four grandparents (P^) are made up 

 of contributions from eight great-grandparents (P^), and if the 

 reduction processes were always quite regular, the 24 chromo- 

 somes in a fertilised egg-cell should contain in the 12 of paternal 

 origin, 6 grand-paternal and 6 grand-maternal ; and either of these 

 groups of six should contain 3 great-grand-paternal and 3 great- 

 grand-maternal contributions. But if the reduction-processes 

 do not exhibit this improbable regularity, we may look for a 

 great variety of possible mosaic arrangements, — as indeed we 

 find. If we accept the chromosome theory, we can readily 

 understand how an innate defect or morbid predisposition in, 

 let us say, a grandfather, may be sifted out of the lineage ; 

 and similarly for a virtue ! 



The business becomes more complicated when we notice that 

 in a number of cases there are differences in the size of the 

 individual chromosomes ; it may be that particular characters 

 are bound up with particular chromosomes, and are not repre- 

 sented even by analogous items in others. Thus a particular 

 predisposition to disease in a particular organ may be embodied 

 in a particular chromosome, which might be thus conceivably 

 sifted out of the lineage altogether. In man, however, the 

 chromosomes are approximately of equal size. 



Ziegler supposes that in man each chromosome has the same 

 value and influence, that each is capable of influencing the whole 

 organism, and that they differ only inasmuch as they are derived 

 from different ancestors, and thus embody diverse hereditary 

 tendencies. 



The chromosomes of an individual usually represent eight 



