308 EVOLUTION AND GENETICS 



unit vehicle of transmission, all genes included in a given chromo- 

 some must necessarily retain their association with each other 

 during reproduction. Modification of this relationship cannot 

 occur without irregular behaviour of the chromosomes. 



In the pomace fly, Drosophila melanogaster, many cases of 

 linkage have Ijeen observed. Morgan and his associates record 

 among these the case of l^lack bod}^ color and vestigial wings 

 with details of the behaviour of these characters when combined 

 in hybrids with their normal allelomorphs, gray body color and 

 normal wings. 



Members of the Fi generation of such a cross are all gray and 

 long-winged. When the Fi males are crossed with recessive, i.e., 

 black- vestigial, females, two types of offspring are produced. 

 One half are black-vestigial and one half gray-long. This propor- 

 tion is exactly the same as would lie expected if only a single 

 character were involved, and indicates that the genes for these 

 characters are located in the same chromosome. Linkage implies 

 no functional relationship between these genes. 



The reciprocal cross of an Fi gray-long female with a black- 

 vestigial male, however, gives wholly different results. In this 

 case the offspring are of four kinds, black-vestigial, gray-long, 

 black-long, and gray- vestigial. The ratio in which these kinds 

 appear in experiments is not, however, even remotely similar to 

 the Mendelian dihybrid ratio. Black- vestigial and gray-long 

 appear in equal numl^ers, making up a total of 83 per cent of the 

 generation; the remaining 17 per cent are also equally divided 

 between the characters l^lack-long and gray- vestigial (Fig. 175). 

 This case involves an interchange of normally associated genes, 

 and implies also an interchange of substance between the chromo- 

 somes involved. 



Crossing over is the term applied to this phenomenon and to 

 the accompanying chromosomal behaviour. The fact that inter- 

 change of the characters occurs in a majority of cases points to it 

 as the abnormal type of inheritance, which would also be true of 

 interchange of substance between synaptic mates. The equality 

 of distribution of the reciprocal combinations is also exactly what 

 would result from an interchange of chromosomal substance. 



During synapsis the related chromosomes have been repeatedly 

 observed tightly coiled about each other. McClung states that 

 this coiling is readily visible at one stage of synapsis in the Orthop- 



