CHAPTER VII 

 THE CHROMOSOMES IN HEREDITY : MECHANICAL 



The Theory of Heredity — Segregation — -Crossing-Over — -The Chiasmatype 

 Theory — Structural Hybrids — ^Dyscentric Hybrids — ^Secondary Structural 

 Change — Cytological Investigation of Crossing-Over — -Its Universality and 

 Biological Effect. 



Fit quoque ut interdum similes existere avorum possint et referant pro- 

 avorum saepe figuras propterea quia multa modis primordia multis mixta 

 suo celant in corpora saepe parentis, quae patribus patres tradunt ab stirpe 

 profecta.^ 



Lucretius, Dc Rentyn NaUtra IV. 



I. THE CHROMOSOME THEORY OF HEREDITY 



The chromosome theory of heredity, which we owe chiefly to 

 Weismann, is the hypothesis (and the corollaries of the hypothesis) 

 that :— 



the permanence of the physiological properties of organisms 

 which is manifested in heredity is determined by the permanence 

 in the structure of their chromosomes. 



In order to examine the theory it is necessary first to recall the 

 special properties associated with heredity and shown by genetical 

 analysis, i.e., by the comparison of parents and their offspring in 

 their higher or super-chromosomal organisation. 



They may be conveniently arranged under five heads, as follows : 



I. Potential Permanence. Heredity consists in the same properties 



or characters being reproduced in successive generations ; a causal 



explanation, therefore, requires the assumption of permanence in 



the cause, although, since the character is only liable to be repro- 



1 Commonly also children may resemble their grandparents or even repeat 

 the characters of remoter ancestors for this reason that the parents often 

 conceal within their bodies many primordia combined in many ways which, 

 derived from the stock, are handed down from generation to generation. 



244 



