MEIOSIS 105 



chromosome pairs the number of such possible arrangements is four; hence 

 a nucleus at the close of meiosis might have any one of eight possible types 

 of genome: A B C , a b c, A B c, a b C , A b c, a B C , A b C , a B c. A gi\(Mi 

 (jiuartet would have two of these types. There are both cytological and 

 genetical evidences that this randomness of orientation at metaphase / does 

 prevail. With more chromosome pairs the nimiber of possible types of 

 genome would of course be greater, the formula for this number being 

 2", where n equals the number of pairs. (4) These various genomes differ 

 qualitatively^ and therefore in their effects upon reactions and characters, 

 only to the extent that the chromosomes in the original diploid comple- 

 ment differed from their respective homologues. If there had been no 

 such differences originally, the genomes in the quartets would all be 

 qualitatively alike in spite of differences in the derivation of their mem- 

 bers; hence meiosis does not always result in nuclei differing in actual 

 constitution. Ordinarily, however, there are some original differences, 

 so that the genomes eventually resulting from meiotic chromosome dis- 

 tribution do show qualitative differences. * 



Turning now to the second column of Fig. 75, we see what would 

 result if the tetrads, instead of all separating disjunctionally in division /, 

 were to be so oriented at metaphase that separation would be equational 

 for at least one of them (the Bb pair in the diagram). Sister chromatids 

 of such a tetrad would separate equationall}' to opposite poles, and 

 disjunction would follow at division II. The other tetrads would 

 disjoin at /, wnth equational separation at //. The result would be a 

 quartet of nuclei with four tj'pes of genome rather than two. Equational 

 division of whole tetrads at / may indeed occur, but it is now thought 

 that it must at least be verj^ exceptional, the reported cytological evidence 

 for it having received a new interpretation. 



The third column of Fig. 75 illustrates the interpretation now generally 

 placed upon cytological and genetical evidence indicating the occuri-ence 

 of equational separation in division /. In one of the tetrads is shown a 

 chiasma, i.e., a place at which two of the four chromatids actually 

 exchange corresponding portions. Such crossing over is a normal feature 

 of meiosis in most organisms and commonly occurs in all the tetrads; 

 moreover, a single tetrad may have more than one such exchange. It 

 is only for the sake of simplicity that the diagram shows onl}' one in 

 the whole complement. Crossing over complicates the process of 

 meiosis, but the complication must be faced because it affords an 

 explanation of certain genetical phenomena to be discussed in later 

 chapters. 



If, now, all the tetrads are oriented in the spindle at metaphase / so 

 that kinetochores of sister chromatids face the same pole, the proximal 

 portion (near the kinetochores) of the tetrad with the chiasma will 



