48 AN INTRODUCTION TO MODERN GENETICS 



some considerable time before gamete formation (Bateson 1930). This 

 theory had, however, to be abandoned, both on account of the evidence 

 that segregation takes place at meiosis, after which only a single division 

 occurs before the gametes are formed, and because it could not be 

 made to fit the numerical data; if the different frequencies of the new 

 and the old combinations are due to differential multiplication, the 

 ratio of the two sorts of gametes must be 2^ : 2^, where a and b are small 

 integers (some modifications of the theory would allow the ratio to be 

 a : b where a and b are small integers such that a -\- b = a power of 2) . 



Fig. 15. Theories of Linkage. — The old theory (on the left) supposed that after 

 segregation had occurred one set of gametes, in this case ab and AB, multiplied 

 faster than the other aB, Ab set. The new theory supposes that A and B are linked 

 If they lie in the same chromosome, but that they sometimes separate when the 

 chromosomes break and rejoin in a different way. 



AaBb AaBb 



1.1 



I 



aB aB Ab Ab ab ab ab ab AB AB AB A8 aB Ab ab AB 



The ratios actually found are not limited in this way and may have any 

 value. 



The alternative theory of linkage, proposed by Morgan in 1911/ 

 supposes that factors are linked when they He in the same chromosome 

 and behave independently when they He in different chromosomes. The 

 possibility of recombination of linked factors depends on the breakage 

 of the chromosomes and their rejoining in such a way as to exchange 

 parts. On this theory the genes in any organism should fall into a 

 certain number (= to the haploid number) of linkage groups. This is 

 true in all cases which have been sufficiently tested. Usually we know 

 fewer linkage groups than the haploid number of chromosomes, but 

 this is clearly due to a mere lack of knowledge of sufficient genes. In a 

 few cases (e.g. the pea) it has at various times been claimed that although 

 a large number of factors were known, they fell into more Hnkage 

 groups than should be expected; but later work has always shown that 

 two of the supposed linkage groups were not in fact independent but 

 were very weakly Hnked. When, in Drosophila melanogaster, a faaor 



' The theory had been put forward on teleological grounds by Janssens 1909. 



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