374 LINKAGE PHENOMENA 



recombination types. Other characters show the same 

 sort of phenomena but the strength of linkage varies with 

 the particular characters. 



Crossing-over is the technical name given to the 

 breakage of linkage between characters whose genes are 

 carried in the same chromosome-pair. In the preceding 

 paragraphs the characters, black body and vestigial 

 wings, were shown to be linked and during the formation 

 of gametes usually to remain together, but to separate in 

 a certain proportion of the gametes. Since linkage is due 

 to the genes being in the same chromosome it is evident 

 that there must be some means by which a gene may be 

 exchanged from one chromosome to its mate. 



The Mechanism of Crossing-over. In a very early 

 stage of the reduction divisions in the mother cells of 

 gametes in animals (and spores in plants) the chromo- 

 somes are drawn out into slender threads. At this time 

 they become associated side by side throughout their 

 entire length in the process called synapsis. They become 

 so closely twisted about one another that it is difficult to 

 distinguish one from the other. During their later 

 development they become much shorter and thicker. It 

 is supposed, and there is much indirect evidence to prove, 

 that while they are twisted a part of one may come to 

 replace a corresponding part of its mate. The diagram 

 (Fig. 110) shows how crossing-over affects the position 

 of the black and vestigial genes in a pair of chromosomes 

 6f a hybrid vinegar fly. 



The Fifth Law of Heredity may be stated as follows: 

 Linked genes may cross-over from one chromosome of a 

 pair to its mate by exchange of corresponding segments 

 of the paired chromosomes. 



The Sixth Law^ states that the genes in a chromosome 

 are arranged in linear order. This law rests on the fact 

 that there is a definite percentage of crossing-over between 

 any two pairs of genes and that it is greater the further 

 apart they are. This is, of course, obvious from the 



