NATURE OF THE GENETIC EFFECTS 367 



somes then unites by its broken end with a fragment of the other, the 

 event — as well as, by an elision of speech, the resulting chromosome 

 configuration — is called a translocation. It is probable that in many of 

 these cases the other two fragments fail to find each other. If these 

 uncombined fragments should later undergo healing or if they should 

 finally have their mother and daughter chromatids unite with one 

 another to form acentric and/or dicentric isochromosomes, the descend- 

 ant cells would be aneuploid, provided they had survived bridge forma- 

 tion. In this case (supposing that these events took place in germ cells) 

 it is very unlikely that the chromosome which did undergo the transloca- 

 tion would be able to get as far as the adult stage of the next generation. 



It often happens, however, that the broken ends of the other two frag- 

 ments likewise find one another and form a union. Such a case is known 

 more specifically as one of mutual or reciprocal translocation or segmental 

 interchange (these three terms being synonymous), and when the word 

 translocation is used without qualification it usually refers to one of this 

 type. In the formation of such a translocation, it is largely a matter of 

 chance whether (1) the centric fragment of one chromosome happens to 

 join the centric fragment of the other, so as to form a dicentric chromo- 

 some, while the other two fragments on uniting form an acentric chromo- 

 some, or (2) each centric fragment joins on to the acentric fragment of 

 the other chromosome (see Fig. 7-2, l-3a, b). The first contingency 

 gives rise to what is called an aneucentric configuration, and eventually 

 leads to the loss of all parts concerned, and therefore to the loss of all the 

 material of both original chromosomes, by the mechanism already 

 explained for acentric and dicentric chromosomes — unless before this 

 happens it kills the descendant cells by bridge formation (Fig. 7-2, 

 3a-5a). If any fertilized eggs were thereby produced which lacked two 

 chromosomes, they would, in the great majority of species at least, except- 

 ing some polyploids, die at an early stage of embryogeny. The second 

 contingency, on the other hand, involving a eucentric configuration, 

 results in two monocentric chromosomes, both of which are transported 

 in a regular manner at mitosis (Fig. 7-2, 3b-5b). In this case all 

 descendant cells derived by mitosis from the cell in which the transloca- 

 tion occurred contain all the chromosome and gene material which is 

 normally present. This second contingency then provides translocations 

 which can be transmitted to descendants, which are viable, i.e., able to 

 survive, and which can be bred and studied genetically. 



The descendants, inheriting the two translocated chromosomes from 

 the parents having the al)erration, and two normal chromosomes, con- 

 taining homologous genetic material in its original arrangement, from 

 their unaffected parent, do not themselves suffer from abnormalities 

 caused by the structural change (except in the cases, very rare for most 

 species, of ''position effect," discussed in Sect. 9). For they possess two 



