Gene Arrangement; Crossover Maps 145 



Alfred H. Sturtevant {in 1945). 



SUMMARY AND CONCLUSIONS 



Using crossover frequency as an indication of distance between loci, linked genes are 

 found to be arranged linearly. Observed crossover frequencies fluctuate because of 

 variations in sample size and factors (temperature, age, nutrition, genotype) which 

 affect either the crossing-over process itself or exert their influence after crossing over 

 (differential viability). Standard crossover maps are made under standard (optimal 

 for crossing over) conditions. 



One crossing over (or chiasma) can interfere with the occurrence of another in the 

 same tetrad. This crossing-over (or chiasma) interference diminishes as the distance 

 between the two regions increases. When double crossing over occurs, the chromatids 

 that exchange in one crossing over generally have no influence upon which chromatids 

 exchange in the other. Consequently, there is usually no chromatid interference. 



Recombination with respect to end genes is 50%, maximally, no matter how many 

 chiasmata occur per tetrad. Although the order of linked genes is easily determined 

 by test-crossing trihybrids, the distance between two marked loci is underestimated 

 when multiple crossovers between them are not detected. 



REFERENCES 



Barratt, R. W„ Newmeyer, D., Perkins, D. D., and Garnjobst, L., "Map Construction 

 in Neurospora crassa," Advances in Genetics, 6:1-93, 1954. 



Emerson, R. A., Beadle, G. W., and Fraser, A. C, "A Summary of Linkage Studies 

 in Maize," Mem. Cornell Univ. Agr. Sta., No. 180, 1935. 



