Gene Arrangement and Chiasniata 



135 



1.00 



1.00 



FIGURE 17-5. Determination of gene order from 

 a test crossed trihybrid. 



expected frequency of double crossovers 

 would be 0.3 X 0.1, or 0.03, so that the 

 coefficient of coincidence is .02/.03, or 0.66.) 

 For the a-b region the percentage of cross- 

 overs is 40% (the double crossovers are 

 counted twice since each represents two 

 crossovers between the end genes). 



Having established the order of these 

 genes, would it be satisfactory to use these 

 data to construct a standard map for the 

 distances between these genes, assuming 



large numbers of progeny had been scored 

 and standard experimental conditions had 

 obtained? The observed distance between 

 c and b is acceptable for this purpose, since 

 in such a short distance only a single chiasma 

 can be produced. However, the situation is 

 otherwise for the a-c region, which is at least 

 30 map units long. For in such a distance 

 double chiasmata could occur and the double 

 crossovers these produce would be unde- 

 tected, since there are no genetic markers 

 between a and c whose switch would enable 

 us to detect them. You see, then, from this 

 and from what has been discussed previously, 

 why the crossover rates observed for large 

 distances are less than the standard map 

 distances. For standard map distances are 

 always obtained by the summation of short 

 distances within which only a single chiasma 

 can occur. 



You should also realize, even though end 

 genes can show at most 50% recombination, 

 that the length of the crossover map may be 

 more than 50 units. For example, if each 

 tetrad contained an average of two chiasmata 

 (see Figure 17-4), there would be a total of 

 100 crossovers among 100 meiotic products 

 and the map length would be 100 units (even 

 though end genes recombined 50% of the 

 time). In fact, the length of the standard map 

 can be predicted to be equal to the mean 

 number of chiasmata per tetrad X 50. 



SUMMARY AND CONCLUSIONS 



Using crossover frequency as an indication of distance, it is found that linked genes are 

 arranged linearly. Observed crossover rates may fluctuate because of sample size and 

 because of factors acting either after crossing over (differential viability), or on the cross- 

 over process itself (temperature, age, nutrition, genotype). Standard crossover maps are 

 made under standard conditions. 



The presence of one chiasma interferes with the occurrence of a second one nearby in 

 the same tetrad, this chiasma interference diminishing as the distance between the two 

 regions increases. Whenever double chiasmata do occur, the chromatids of one chiasma 

 typically have no influence upon which chromatids form the other chiasma, so that there 

 is no chromatid interference. 



Recombination between end genes is 50% maximally, no matter how many chiasmata 

 occur per tetrad. While the order of linked genes is easily determined by test crossing 

 trihybrids, the distance between two genes will be underestimated when they are far apart. 



