( II \I'I ER 9 



univalent is now represented b\ two sister 

 •strands). After crossing over, the tetrad 

 seems to appear at diplonema as depicted in 



stage 11. which shows a chiasma between 

 the a and b loci (the places in a chromo- 

 some containing the genes). Note that 

 when the univalents are initially identical 

 in appearance, a chiasma shows there was 

 a physical exchange of apparently exactly 

 equivalent segments between two nonsister 

 strands oi a tetrad, the strands being just as 

 long after as before the exchange. Stage 

 111 shows the dyads present after the first 

 meiotic division is completed. The upper 

 cell or nucleus contains one -f + noncross- 

 over strand and one + b crossover strand, 

 whereas the lower one contains the recip- 

 rocal crossover strand a + and the non- 

 crossover strand a b. Stage IV shows the 

 four haploid products (cells or nuclei) pro- 

 duced after the dyads form monads, and the 

 second meiotic division is completed. Ac- 

 cording to this hypothesis, if one chiasma 

 (representing a crossing over) occurs in any 

 position between the loci of a and b, two 

 of the four haploid nuclei produced con- 

 tain noncrossover parental combinations, 

 and the other two contain crossover non- 

 parental recombinations. 



Evidence that the crossovers found in 

 gametes originate in this way is ordinarily 

 difficult to obtain because, in females, only 

 one of the four haploid products from each 

 nucleus entering the meiotic divisions is 

 usually retained as the nucleus of a func- 

 tional gamete, the others being lost (as 

 polar body nuclei or cells). Even when 

 each of the four haploid products becomes 

 or gives rise to a gamete, as in sperm or 

 pollen formation, the four gametes — pro- 

 duced from a cell containing a given chi- 

 asma — mix with gametes produced from 

 other meiotic cells which may or may not 

 have had a similar chiasma. For these rea- 

 sons, only one of the four meiotic products 



is normally observed or identified at a time. 

 If each chiasma results from a prior cross- 

 ing over in the four-strand stage, approxi- 

 mate^ equal numbers o\' the two reciprocal 

 kinds o\ crossovers would be expected, as 

 seen in the crossover data already pre- 

 sented. However, crossing over during the 

 two-stranded stage 1 is also expected to pro- 

 duce this result. The occurrence of non- 

 crossover types, which are equally frequent 

 and more numerous than the crossovers, 

 can be explained if crossing over between 

 the loci of a and b occurs less than 509? 

 of the time at the two-strand stage or less 

 than 100% of the time at the four-strand 

 stage. The morphology of a chiasma, how- 

 ever, supports the view that crossing over 

 takes place sometimes, if not always, at the 

 four-strand stage. 



Genetic evidence as to whether crossing 

 over occurs at the two-strand or the four- 

 strand stage might be obtained from gam- 

 etes that retain not one but two or more 

 strands of a tetrad. Finding a gamete that 

 carries one strand which is a noncrossover 

 and another homologous one which is a 

 crossover, would support only the four- 

 strand hypothesis. A suitable system for 

 this test is found in Drosophila females 

 whose two X's are not free to segregate 

 since they are joined and have a single cen- 

 tromere. One type of such attached-X's is 

 V-shaped at anaphase. During meiosis this 

 attached-X replicates once, and the four 

 arms synapse to form a tetrad, yielding two 

 meiotic products each of which carries at- 

 tached-X's and two products devoid of X 

 chromosomes. Using females whose at- 

 tached-X's are dihybrid and scoring their 

 female progeny, one finds attached-X's hav- 

 ing one arm a crossover and one that is not 

 (Figure 9-7). Though this evidence also 

 supports the four-strand hypothesis, it does 

 not eliminate crossing over at the two-strand 

 stage. 



