74 LINKAGE 



of cases of linkage that is occurring at the present 

 time, when the importance of detecting them has 

 become apparent, and the methods for studying 

 them have been worked out, appears to presage the 

 realization of linkage as a general phenomenon. Its 

 occurrence in such widely separated types is also a 

 sign that it is a constant accompaniment of Mende- 

 lian inheritance. 



The Reduplication Hypothesis 



Linkage has been interpreted by Bateson and his 

 co-workers on a basis entirely different from that 

 adopted in this book. These investigators do not 

 connect Mendelian factors w^ith the chromosomes in 

 any way, and do not suppose that segregation occurs 

 at the reduction division. In a case of hnkage be- 

 tween two pairs of factors, Aa and Bb, the doubly 

 heterozygous individual will have the formula ABab. 

 Bateson supposes that in such an individual segre- 

 gation takes place before the reduction division — 

 perhaps in early cleavage stages, perhaps after the 

 formation of the gonads. Two cell divisions are 

 required for this segregation. In the first, A and a 

 do not divide, but one goes to each daughter cell, 

 i.e., they segregate. B and b, however, both divide, 

 and each daughter cell receives both B and b. The 

 resulting cells then have the formulae, ABb and aBb, 

 respectively. In other words, A and a have segre- 

 gated, but B and b have not. At the next division 

 B and b segregate, giving four cells, with the combina- 



