Gene Arrangement; Crossover Maps 



139 



of double crossovers, .02, must be doubled 

 and added to the frequency of single cross- 

 overs to obtain the total crossover frequency 

 between a and b. The genetic map based 

 on crossover frequency becomes linear 

 (0.30 + 0.10 = 0.36 + 0.04), therefore, 

 when double crossovers are taken into ac- 

 count. The expected frequency of double 

 crossovers is 0.3 times 0.1 or 0.03, so that 

 the coefficient of coincidence in this case is 

 0.02 0.03, or 0.66. ( In the y w spl example 

 discussed earlier in this chapter, the longest 

 region, y-spl, was too short for double cross- 

 ing over.) 



Would it be satisfactory to use the data 

 in Figure 10-6 to construct a standard link- 

 age map for the distances between these 

 genes, assuming that large numbers of prog- 

 eny had been scored and standard experi- 

 mental conditions had been used? For this 

 purpose, the observed distance from c to b 

 is acceptable since only a single chiasma can 

 occur in such a short interval. The situa- 

 tion is otherwise for the a-c region, however, 

 which is 30 map units long in the present 

 experiment. Double chiasmata are expected 

 to occur under these circumstances, yet the 

 absence of genetic markers between a and c 

 prevents their identification. Therefore, the 

 standard map distance for a-c must be longer 

 than 30 map units (and a-b longer than 40) . 

 Note that the identical error foreshortens the 

 a-c and the a-b distances; therefore, for the 

 distances observed, {a-c) plus (c-b) is 

 equal to (a-b). Whether or not the chro- 

 mosome is genetically marked so that all 

 multiple crossover strands are detected, the 

 correct order of three linked genes can al- 

 ways be determined, provided that two are 

 not 50 map units away from the third. 



It should now be clear why the crossover 

 frequencies observed for large distances are 

 less than the standard map distances, and 

 why the standard map distances are always 

 obtained by the summation of the short dis- 



tances in which only a single chiasma can 

 occur. 



Although end genes can show at most 

 50% recombination, the length of the cross- 

 over map may exceed 50 units. For exam- 

 ple, if a given pair of homologs contains an 

 average of two chiasmata in each tetrad (see 

 Figure 1 0-5 ) , a total of 1 00 crossovers will 

 occur among 100 meiotic products, and the 

 map length will be 100 units even though 

 the end genes will have recombined 50% of 

 the time. In fact, it can be predicted that 

 the length of the standard map is equal to 

 fifty times the mean number of crossing-over 

 events (or chiasmata) per tetrad. 



Crossover Maps 



Utilizing crossover frequency, genetic maps 

 cf chromosomes have been made for a num- 

 ber of multicellular organisms. Figures 10-7 

 through 10-10 give the linkage maps for a 

 considerable number of genes in man, mouse, 

 maize, and Neurospora. 



29 



G6PD Deutan Hemophilia A 

 ► e5->|«— 12 • 



I 



3 



38- 

 -41 



V 



figure 10-7. Tentative linkage map of a 

 segment of the human X chromosome. The 

 numbers given are the values for the map 

 distance found in five separate studies. The 

 loci mapped are the Xg {blood group) locus, 

 the G6PD (glucose-6-phosphate dehydrogen- 

 ase) deficiency locus, the deutan (green) color- 

 blindness locus, the classic hemophilia locus. 

 (Courtesy of V. A. McKusick. From Human 

 Genetics, 1964, Prentice-Hall, Inc., Englewood 

 Cliffs, N.J.) 



