THE PHYSICAL BASIS OF INHERITANCE 299 



assumptions about possible happenings in maturation. One marked 

 difficulty was that it depended upon an active process, the twining of 

 chromosomes and the exchanging of genes, which could not be watched 

 under the microscope, since the chromosomes, in order to be seen in 

 detail, had to be fixed and stained, with all processes stopped. 



The establishment of this hypothesis involved many technical details 

 that are beyond the scope of our treatment. A new type of genetic re- 

 search was developed, in which appropriate breeding experiments were 

 checked and compared with detailed microscopic studies of the chromo- 

 somes of the breeding stocks. These investigations, which are still actively 

 in progress, have confirmed the supposition that the genes lie within the 

 chromosomes. They have demonstrated the processes that provide and 

 account for nonindependent assortment. They have also established a 

 number of other principles that appear to be as fundamental as the origi- 

 nal "laws" of Mendel. Some of the more important of these are sum- 

 marized below: 



Linkage. Many groups of two or more genes do tend to be inherited 

 together. Such genes are said to be linked, or to constitute a linkage 

 group. Four linkage groups have been found in Drosophila, and three of 

 them comprise more than 100 genes each. Linkage groups have also been 

 found in most of the other organisms that have been extensively utilized 

 for breeding experiments — corn (maize), garden peas, sweet peas, rabbits, 

 mice, guinea pigs, and others. The mechanical explanation for linkage is 

 now clearly established to be the fact that all the genes within any one 

 linkage group reside within the same chromosome. 



Crossing over. Linkage was defined as the tendency for two or more 

 genes to remain together without segregation. Most linked genes do 

 not show an absolute or invariable linkage. As we saw in Drosophila, 

 if the genes for black and vestigial enter a cross together, they tend to 

 remain associated, but the linkage is occasionally broken (17 per cent 

 of the time, in this instance), and black becomes associated with long 

 and gray with vestigial. Such breakage of a former linkage association, 

 with the consequent formation of a new linkage, is termed crossing over, 

 and the new combinations that result are termed crossovers. Crossing 

 over was once thought to be rather exceptional, but we now know that 

 the processes that bring it about are a normal event in the very early 

 stages of meiosis of many, and probably of all or nearly all, organisms. 

 The essential features of this process and of linkage are shown in Fig. 19.4. 



Linear order of the genes. All the genes within any one chromosome 

 are arranged in a definite linear order within (or along) that chromosome. 

 Each gene thus has its own precise location, or locus, that will be occupied 

 only by that definite gene orbits allele, and the characteristic crossover 

 percentage shown by any two linked genes is a function of the distance 



