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CHAPTER 16 



exactly two of the four meiotic products are 

 crossovers and exactly two are not. Note 

 on the other hand, that had crossing over 

 occurred in the two-strand stage (in the top- 

 most nucleus), all the meiotic products in a 

 single sac would be recombinant. 



The result actually observed in some cases 

 is that either none of the eight spores in 

 an ascus is a crossover between a and b, or 

 exactly four of the eight are; never are all 

 eight spores, from a single sac, crossovers. 

 This demonstrates conclusively that crossing 

 over occurs in the four-strand stage, as shown 

 in Figure 16-8. 



It has already been implied that chiasma 

 formation is a normal part of meiosis (p. 26). 

 One of the consequences of chiasma forma- 

 tion is to prevent the premature separation 

 of dyads by holding them together as a tetrad 

 until anaphase I. (There is usually at least 

 one, and there may be as many as six chias- 

 mata, per tetrad.) Therefore, crossing over 

 too is a normal part of meiosis. The 

 fact that there are numerous places along a 

 chromosome where a chiasma can form has 

 a very interesting consequence. A single 

 chiasma formed at any position between two 

 loci will result in crossovers for these loci. 

 Accordingly, it is reasonable to believe that 

 the greater the distance between two loci, the 

 greater will be the chance for a chiasma to 

 occur between them, and the greater will be 

 the frequency of crossovers for them. Re- 

 ciprocally, the closer two loci are, the smaller 

 will be the chance a chiasma will occur be- 

 tween them, and the smaller will be the num- 

 ber of crossovers for them. According to 

 this view, the frequency of crossovers can 

 be used as an indication of the relative dis- 

 tances between loci. (The results presented 

 in Figure 16-4 should now have additional 

 meaning for you.) 



Suppose, in our model, that no chiasma 

 occurs in 80% of spore sacs in the genetically 

 marked region we have been considering. 

 These sacs would produce 80% of the total 



number of spores carrying only parental, 

 noncrossover genotypes. From the 20% of 

 spore sacs which do contain such a chiasma, 

 one would find half of the spores to be of 

 the parental types and half to be recombina- 

 tional. So, a chiasma frequency of 20% 

 would result in 10% of all spores being of 

 crossover type. We can express the distance 

 between the loci of a and b as being 10 cross- 

 over map units, where a map unit is that dis- 

 tance which gives one crossover per hundred 

 (spores, in the present case). It is generally 

 true, then, that in the simple case (where the 

 genes are sufficiently close together, as in the 

 present example), crossover frequency (map 

 distance) is just one-half chiasma frequency. 



What procedures may be used to measure 

 crossover frequency in Neurosporal There 

 are several. First, this can be done in terms 

 of spore sacs. As many spores are tested 

 from each sac as are required to decide 

 whether or not the sac carries a crossover in 

 the region tested. (No more than five spores 

 need to be tested per sac.) Speaking in terms 

 of the a-b model already discussed, 20% of 

 sacs would have crossovers, 80% would not. 

 And since each sac containing crossovers has 

 four spores that are crossovers and four that 

 are not, crossing over frequency would be 

 10%. Second, all the spores from many sacs 

 are mixed and then a random sample of spores 

 is taken and tested. This method would give 

 10%, recombination with our model, and is 

 like the sampling procedure involved in deter- 

 mining crossover frequency in animal sperm. 

 A third procedure would be to test one ran- 

 domly chosen spore from each sac and dis- 

 card the others. Again, 10% recombination 

 would be obtained. This method resembles 

 the situation in many females (including Dro- 

 sophila and human beings) where, normally, 

 one random product of meiosis enters the 

 egg, the others being lost. 



In what has been discussed relating chias- 

 mata and crossing over, no study was de- 

 scribed that directly correlated a genetically 



