182 Locating Genes on Chromosomes 



vised. Any linkage map, therefore, must be considered only 

 tentative. It has been found that aristaless (al) is 48.5 map 

 units to the left of b. This discovery means that the tentative 

 map just pictured must be revised and rewritten as: 



al h pr V c px sp 



48.5 54.5 67.0 75.0 100.5 107.0 



In considering such linkage maps, we must remember that 

 the actual distances are always subject to revision as more data 

 are accumulated. Drosophila map distances are the result of 

 much careful work involving numerous crosses and many thou- 

 sands of flies. Map distances based on one experiment are un- 

 trustworthy and must never be considered final, but most map 

 distances in Drosophila are the average of so many experiments 

 that the degree of error has been reduced greatly. 



QUESTIONS AND PROBLEMS 



1. By means of a testcross, it was found that the percentage of recom- 

 binations between genes I and m was 13.5; that between m and n it was 

 20; that between I and n it was 6.5. Plot these genes on their chromosome. 

 Could they be plotted if only two of these three values were obtained? 



2. In rabbits, yellow fat (y) is recessive to white (F), chinchilla fur 

 {c''^) to Himalayan (c^), and brown hair (b) to black extremities (B). 

 A heterozygote, Yy c^d^^ Bb, crossed to a triple recessive gave (data 

 from Castle in Proceedings of the National Academy of Sciences, Yo\\xn\Q 19) : 



Determine whether there is linkage, and plot any linked genes on their 

 chromosomes. If there was no linkage at all, what would be the expected 

 ratio? 



3. Could the double crossover classes in problem 2 be determined 

 merely by inspecting the numbers of each class? What is the percentage 

 of double crossovers, and what is the coincidence, if any? 



4. In Pisum, salmon flowers (6) are recessive to purple {B), reduced 



