Crossing Over 155 



and the other was Rg a / Rg a, and let us compare it with the 

 one by Brink and Senn just cited. The Fi was obviously 

 Rg a / rg A. It was phenotypically like the Fi from Brink and 

 Senn's cross and was genotypically the same to the extent that 

 it had the same genes, but the arrangement of the genes on the 

 two chromosomes was different. In Brink and Senn's cross, 

 the genes of the Fi were linked in a combination of Rg A / rg a 

 whereas in Brink's they were Rg a / rg A. Does this make any 

 difference? When Brink's Fi was backcrossed to the recessive, 

 rg a / rg a, the offspring were: 



Noncrossovers Crossovers 



Rg a rg A Rg A rg a 



Observed frequencies 616 724 480 488 



Expected if 1 : 1 : 1 : 1 577 577 577 577 



Again, the percentage of crossover or recombination types was 

 41.9, but now the Rg A and rg a plants are the recombination 

 types whereas in the other cross they were the parental types: 

 and the rg A and Rg a plants now are the parental types whereas 

 they previously were nonparental types. In other words, when 

 two pairs of alleles are on separate chromosomes, the ratio ob- 

 tained by backcrossing the Fi to the double recessive is exactly 

 the same whether the parents of the Fi were homozygous for 

 the two dominants and the two recessives, or whether each was 

 homozygous for one dominant and one recessive; but when the 

 two alleles are on the same chromosome, and therefore show 

 linkage, the results are very different. 



When genes A and B are linked, and when the original cross 

 is between A B / A B and ah / ah, the AB and ah types are more 

 numerous in the testcross than the Ah and aB types; but when 

 the original cross \^Ah/Ahy^aB/aB, the .46 and aB types 

 are more numerous than the AB and ah types. However, it 

 must be noted that the ^percentage of recomhinations is the same 

 no matter ivhich way the cross is made. The only difference 

 is that the parental types in the one cross are the crossover types 

 in the other. 



Through long usage, it is customary to speak of the cross 

 AB/ABy^ah/ah as being in the coupling phase, and the 

 cross Ah / Ah y, aB / aB as being in the repulsion phase. 



