128 PHYSICAL BASIS OF HEREDITY 



ing over takes place between G and H. Stated in another 

 way, crossing over in one region protects neighboring 

 regions from crossing over. Moreover, this relation fol- 

 lows a perfectly definite law according to the '^distances,** 

 as determined by linkage relations of genes outside of the 

 region of crossing over. If we take two pairs of factors 

 ^ closely linked together we find that the genes lying 

 immediately to the right and left of ^ never cross over 

 independently of - and ~ at the time that a crossover 

 separates ^ ^^^ f • I^i other words, the genes imme- 

 diately to the right of H always go over with H, and those 

 to the left of G always go over with G, when G separates 

 from H, 



If we consider genes that are less closely linked with 

 G and with H, we find that while their crossing over is 

 interfered with by the crossing over between G-H, it is 

 affected to a limited extent. Genes still less linked with G 

 or with H are still less interfered with ; until finally there 

 is no relation at all between crossing over between G-H, 

 and other more loosely linked genes, i.e., crossing over 

 between G-H is found to have no relation to crossing over 

 between L and M. Put in another way, one may say that 

 crossing over at L and M is no more likely to take place 

 when none occurs between G-H, than when it does. 



For different pairs of chromosomes the regions that 

 bear this relation to each other have been found to be 

 different. Even within the same chromosome this rela- 

 tion may be different at the ends and in the middle. There 

 are also special factors that affect special chromosomes 

 and special regions of chromosomes. An example will 

 illustrate this relation that is called interference. If in 

 a group of genes A B C D E F a break occurs somewhere 

 between A and D in 6 per cent, of cases, and if between 

 M and T in the same series (M N P Q R S T), in 10 per 

 cent, of cases, a double break involving both regions simul- 

 taneously should, if the breaks occurred independently of 



