LINKAGE 



the cross of black {bh Vg Vg) by vestigial {B B vg vg), the progeny 

 from back-crossing to the double recessive were as follows : 



Bh Vg vg b^ ^S ^S ^ ^ ^S ^S h b vg vg 



338 1,552 1,315 294 



The preponderant types of gametes are again the parental ones 

 and now 17*9 per cent show recombination (Morgan 1914). Thus 

 the percentage of recombination is a property of the two genes 

 recombining; it is a property fixed within narrow limits for each 

 pair of genes (such as b and vg), and differing between different 

 pairs of genes. All percentages occur up to 50, this limit being 

 indistinguishable from Mendel's case of independence, which appears 

 when the genes are in different linkage groups. 



What then happens when three genes in the same linkage group 

 are recombined in the same experiment? We can obviously work 

 out the recombination percentage or value for each pair of them. 

 Taking a third gene, that for purple eye, pr* in addition to b 

 and vg, Bridges found that it showed 6 '4. per cent of recombination 

 with b and lo- 8 per cent with vg, while b and vg showed 16-3 per 

 cent in this experiment. Thus two of the values add up to the third, 

 or to a little more. It seems as if the genes were arrayed in a line, thus : 



b pr vg 



^ 6-4 >,< 10-8 



16-3 



Now with such a linear arrangement, a recombination between 

 b and pr must also give a recombination between b and vg. Similarly 

 one between pr and vg must give one between b and vg, always 

 provided that the recombinations between b and pr and between 

 pr and vg do not take place simultaneously. Such double recom- 

 binations were in fact found in 0-46 per cent of gametes, so giving 

 a deficit 0-92 per cent (each double cancelling two singles) in the 

 observed recombination of b and vg as compared with the sum of 

 the two smaller recombination values. 



* In designating the gene as pr, the allelomorph which gives the normal or wild-type 

 red eye is assumed. It is this wild-type allelomorph which we should designate by Pr on 

 the convention we have been adopting. In the Drosophila system {see Appendix 2) no special 

 symbol is assigned to the wild-type allelomorph, the gene being designated by the variant 

 allelomorph. 



43 



