Crossing Over and Chiasma 



111 



combinations, while the other two haploid 

 nuclei contain crossover, nonparental re- 

 combinations. 



Evidence that the crossovers found in 

 gametes have this origin is ordinarily difficult 

 to obtain. This is so because, in females, 

 typically only one of the four haploid prod- 

 ucts, from each nucleus starting the meiotic 

 divisions, is retained as the nucleus of a func- 

 tional gamete, the others being lost (often as 

 polar body nuclei) . Even in those cases where 

 each of the four haploid products becomes a 

 gamete, as in sperm or pollen formation, the 

 four gametes, produced from a cell containing 

 a givenchiasma,mixwith those produced from 

 other meiotic cells which may or may not have 

 had a similar chiasma. As a consequence, 

 therefore, only one of the four meiotic prod- 

 ucts from a given chiasma is observed or 

 identified at a time. Note that the chiasma ex- 

 planation for crossing over would provide 

 equal numbers of the two reciprocal kinds of 

 crossovers, and also equal numbers of the two 

 types of noncrossovers, as is required from 

 the crossover data already presented. How- 

 ever, crossing over during the two-stranded 

 stage I would also satisfy these require- 

 ments. 



Can we obtain genetic evidence as to 

 whether crossing over occurs in the two- 

 strand or four-strand (tetrad) stage? One 

 possible way to determine this would be to 

 use a genetic system whereby not one but two 

 strands of the four in a tetrad are retained 

 in a single gamete. If such a gamete carried 

 one strand which is a noncrossover and an- 

 other homologous one which is a crossover, 

 this would support the four-strand hypothesis 

 exclusively. This possibility becomes a re- 

 ality through the use of Drosophila females 

 whose two X's are not free to segregate from 

 each other because they share a single centro- 

 mere. Such attached-X's are V-shaped at 

 anaphase. During meiosis the attached-X 

 replicates once and the four arms synapse to 

 form a tetrad, yielding two meiotic products 



each of which carries attached-X's and two 

 devoid of X chromosomes. From a genetic 

 analysis of the female progeny, of females 

 whose attached-X's are dihybrid, evidence 

 favorable to the four-strand view has been 

 obtained (Figure 16-6). 



In pursuit of additional evidence regarding 

 the time crossing over takes place during 

 meiosis, the red bread mold Neurospora can 

 be investigated. Neurospora ("nerve spore") 

 is usually haploid and comes in two different 

 sexes. In the sexual process, so-called fruit- 

 ing bodies are formed, composed of cells each 

 containing two haploid nuclei, each of which 

 was derived originally from a different parent 

 (Figure 16-7). Two such haploid nuclei fuse 

 to form a diploid nucleus containing seven 

 pairs of chromosomes, and the cell elongates 

 to form a sac. The diploid nucleus immedi- 

 ately undergoes meiosis, in the manner shown, 

 so that at completion of meiosis the four 

 haploid products are arranged in tandem, 

 i.e., the topmost two nuclei come from one 

 first division nucleus, the bottom two from 

 the other first division nucleus. Subse- 

 quently, each haploid nucleus divides once 

 mitotically, in tandem, so that each meiotic 

 product is present in duplicate within the 

 elongated sac, and each of the eight nuclei 

 becomes encased to form a football-shaped 

 spore. Each haploid spore (ascospore) can 

 be removed from the sac (ascus), grown indi- 

 vidually, and its genotype determined directly. 

 You see, therefore, that these events and pro- 

 cedures make it possible to obtain and iden- 

 tify all of the products of meiosis derived 

 from a single diploid nucleus. 



Using the same symbols as were used in 

 Figure 16-5, let us follow, in Figure 16-8, the 

 genetic consequences of a single chiasma that 

 occurs between the loci under study. This 

 will provide us with the results expected from 

 four-strand stage crossing over. Since only 

 one pair of the seven pairs of chromosomes 

 present are being traced, the others are 

 omitted from the Figure. As a consequence 



