Pairing without Chiasmata 213 



will be single crossovers, and 8 will be triple crossovers. As all 

 the noncrossovers and double crossovers appear as parental 

 types, whereas all the single and triple crossovers appear as 

 recombinations, there will be 32 parental and 32 recombination 

 chromatids, and crossing over will again be no greater than 50 

 per cent. 



If the two genes are so close on a chromosome that one chiasma 

 fails to form between them in every meiocyte, that is, every cell 

 undergoing meiosis, the percentage of crossing over will be less 

 than 50. If one forms in every meiocyte, the percentage w^ill 

 be 50. For all the cells with two, three, or more chiasmata, the 

 percentage of crossover gametes that come from them will be 50. 

 Thus, although the percentage of crossing over may vary be- 

 tween none and 50, it can never exceed 50 no matter how many 

 chiasmata are formed. No matter how many chiasmata are 

 present, the number of gametes with an odd number of cross- 

 overs will equal the number with no crossovers or with an even 

 number. Those with an odd number will be recombination 

 gametes; those with an even number or none wdll be parental 

 gametes. 



Pairing without Chiasmata 



We mentioned in Chapter 10 that there is no crossing over in 

 the males of the Diptera or in the females of silkworms. In 

 spite of this, the chromosomes appear to be paired at meta- 

 phase in a more or less normal fashion and they separate at 

 anaphase in a regular manner. This suggests two questions. If 

 there is no crossing over, are chiasmata formed? If there are 

 no chiasmata, what forces are responsible for chromosome pair- 

 ing at the first meiotic metaphase? Answers have been given 

 by several investigators but they are not all in agreement. 



In most plants and animals, the chromosomes during somatic 

 mitoses are distributed on the metaphase plate in a purely ran- 

 dom manner with reference to one another so that a given chro- 

 mosome does not tend to lie alongside its homologue any more 

 frequently than it lies next to any other chromosome. However, 

 in many of the Diptera, the two chromosomes of each homologue 

 tend to lie near one another at metaphase. Corresponding parts 

 of the two homologues lie opposite one another, but the chromo- 

 somes are not actually in contact at any point. Such an ar- 



