76 GENETIC STOCKS AND BREEDING .METHODS 



(reported by Cooper). 227 In 1951, Carter and Falconer 172 reported the development 

 of a new set of stocks incorporating mutants discovered in the intervening period, 

 described their use, and discussed the theory of the design of such stocks. By means of 

 the concept of the swept radius, i.e., the length of linkage map tested by a marker gene 

 in any one cross, they were able to define the principles to be followed in making a 

 choice when two or more linked markers are available. The concept also allows one 

 to calculate the total length of linkage map tested by a particular set of stocks, a value 

 which is useful for comparing the efficiency of different sets of stocks. 



Since 1951, many new mutants and six new autosomal linkage groups have been 

 discovered. It has thus become possible to construct revised linkage stocks which test 

 a greater length than was possible in 1951 . Such stocks are maintained at the Institute 

 of Animal Genetics in Edinburgh, Scotland, the Radiobiological Research Unit at 

 Harwell, England, and the Roscoe B. Jackson Memorial Laboratory, Bar Harbor, 

 Maine. Lists of these stocks, as well as stocks at other institutions, are published 

 periodically in the Mouse News Letter, a mimeographed bulletin produced and dis- 

 tributed by the International Committee on Laboratory Animals, and Laboratory 

 Animals Centre, M.R.C. Laboratories, Woodmansterne Road, Carshalton, Surrey, 

 England. 



The set of stocks for testing linkage now maintained or under construction at the 

 Jackson Memorial Laboratory is as follows: 



Linkage groups IV and XVII are not represented in the stocks, the first because 

 it did not, at the time the stocks were designed, contain a suitable marker, and the 

 second because it contains at present only recessive mutants, which are difficult to 

 incorporate into existing multiple recessive stocks. No linkage is known for SI but it has 

 been tested with nearly all the other markers and has shown no close linkage with them. 



Using the method of Carter and Falconer, 172 the total length of linkage map swept 

 by these stocks can be calculated. The method requires knowledge of the average 

 map length of mouse chromosomes. Counts of chiasmata in the male mouse made by 

 Slizynski (quoted by Carter 159 ) show the average number per autosome to be 1.9. 

 Earlier counts made by Crew and Roller 229 showed the average number of chiasmata 

 per autosome to be 2.4 in the male and 2.8 in the female. Since Crew and Roller 

 were unable to count the chiasmata in complete cells, their counts may have been 

 biased upward if they tended to count the longer chromosomes. The difference 



