Trisomies 417 



with any degree of reliability by their general phenotypic appear- 

 ance. In fact, in general, an extra chromosome does not produce 

 so striking an effect as a missing one. In plants trisomies 

 are much more frequent than monosomies, and the extra chromo- 

 some does not have the same lethal effect, at least in the female 

 gametophyte, that is observed in monosomic types. 



Chromosome pairing in a trisomic is very interesting, and, to 

 understand it, we must understand three principles. (1) With 

 a very few minor exceptions, chromosome pairing at zygotene is 

 between homologous chromosomal segments only. (2) At any 

 one place, chromosome pairing is between two chromosomes only. 

 (3) Except for mechanical difficulties that might influence two 

 regions near to one another on a chromosome, the two threads 

 that pair at one place on the chromosomes have no determining 

 influence on the threads that pair at any other place. Unless the 

 chromosomes are very short, all three chromosomes will be in- 

 volved in the pairing at various places so that the three chromo- 

 somes will together form a trivalent configuration. If two of the 

 three homologues are paired throughout their length while the 

 third is not paired at any point, the three chromosomes will not 

 form one configuration but will be paired as a bivalent and a 

 univalent. This bivalent will arrange itself on the equator in 

 the same manner as any other bivalent, but the univalent will 

 appear on the equator by itself and behave much as does the 

 univalent in a monosomic. Usually, pairing occurs in such a 

 manner that a trivalent is formed (Fig. 114). 



After pairing has occurred, the chromatids form chiasmata, 

 and the characteristic diplotene appearance of nodes and inter- 

 nodes is observed as in any normal bivalent. Perhaps the process 

 is most easily understood if the figure is not regarded as a tri- 

 valent but as a combination of several bivalent segments. Each 

 bivalent segment then acts like the segments of a bivalent con- 

 figuration, forming chiasmata in the same way. These chiasmata 

 may or may not terminalize. If they do, the chromosomes will 

 be joined together at the ends, and the particular figure will de- 

 pend upon the position of the paired segments. At diakinesis and 

 at the following metaphase, the trivalent may have the form of a 

 chain of three chromosomes, or of a ring of two with the third 

 chromosome attached to the ring at one end (a ring-and-rod) , 



