THE CHROMOSOME BASIS 



Now this proportion, 0*46 per cent, is instructive. If double 

 recombinations occurred in the proportion expected with random- 

 ness there would be (6-4 X 10 -8)/ 100 or 0*69 per cent. The fact 

 that only 0-46 per cent are recovered {i.e. found in the progeny) 

 shows that recombination between b and pr reduces the chance of, 

 or, as Muller put it, interferes with, that between pr and vg. 



It is an easy step from this kind of experiment to the business 

 of testing and arranging in a line all the genes of one group at 

 intervals proportional to their recombination values. In this way we 

 get a map on which distance between any two points stands for 

 frequency of recombination between the two genes they represent 

 (Fig. 9). From this map the linkage properties of all the mapped 

 genes can be, and nowadays regularly are, predicted in combinations 

 which have not been tried. 



The Chromosome Basis 



The system of map prediction is useful but it is not enough. We 

 want to know why it works. It must have a material basis, and this 

 becomes plain when we return to the cell. The chromosomes of the 

 fruit fly fall into four pairs. One of these is a very small pair (Figs. 4 

 and 5), just as one of the linkage groups contains very few genes. 

 Two of the other chromosomes are roughly equal and somewhat 

 longer than the remaining one. Again just like the linkage maps. 

 Both the chromosomes and the linkage maps are linear. Both the 

 chromosomes and the genes are double in the zygote, single in the 

 gamete. The chromosomes, or rather their constituent chromatids, 

 cross-over and separate in germ cell formation just as the genes 

 segregate and recombine. And the recombination limit of 50 per 

 cent follows simply from the fact that two of the four chromatids 

 cross over at each chiasma. 



All in all we have to take it that we are observing the same 

 chromosomes in two different ways, under the microscope and in 

 the breeding experiment, cytologically and genetically. In other 

 organisms than Drosophila the comparison can be carried even 

 further. In maize {Zea mays), for example, not only can each of the 

 ten linkage groups be shown to correspond with one of the ten 

 pairs of chromosomes ; these chromosomes can be seen to pair and 



45 



