CHAPTER VIII 

 THE CHROMOSOMES IN HEREDITY : PHYSIOLOGICAL 



Differentiation of Behaviour — Specificity — Nucleoli — Condensation — 

 Differentiation of Activity — Balance — Inert Chromosomes — The Gene. 



Besonders aussichtsreich scheint mir gerade bei diesem Problem, das wir 

 ihm gewissermassen von zwei Seiten her beikommen konnen, einmal durch die 

 Beobachtung der Vererbungs-Erscheinungen und dann durch die Beobachtung 

 der (uns jetzt ja bekannten) Vererbungs-Substanz. 



Weismann, 1892. 



I. THE TWO METHODS OF ENQUIRY 



The modern study of heredity has been built up by the study of 

 differences. The mendelian technique of experimental breeding 

 depends on the classification and comparison of differences between 

 parents and offspring and between brothers and sisters. We have 

 now seen how closely this differential method is paralleled, in the 

 deductions it makes possible, by cytological observation. The same 

 modes of inheritance may be inferred from mendelian and from 

 cytological analysis, although the special advantages of each method 

 enable us to go further with it in certain directions than with the 

 other. The direction in which the c3^tological method is particularly 

 advantageous is (as pointed out in Table 44) that it can dispense 

 with experimental differences. It enables us to study the hereditary 

 properties not only of hybrids too sterile to have offspring, but also 

 of homozygous organisms having no differences to segregate. We 

 can therefore proceed to use the study of the chromosomes for 

 finding out the hereditary properties of organisms, as Weismann, 

 in some respects prematurely, attempted to do. We can proceed 

 to deduce from the principles now induced. 



One of the first problems to which this method can be applied 

 is the problem of differences within the hereditary materials of the 

 individual. With regard to this, alternative inheritance tells us 

 the cardinal fact that different " loci " can be defined by their 



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