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 with the chromosomes in 
any way, and do not suppose that segregation occurs 
at the reduction division. In a case of linkage 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, 
1.e., they segregate. B and b, however, both divide, 
and each daughter cell receives both B and b. The 
resulting cells then have the formule, 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- 
