Questions and Problems 469 



no effect on the segregation of chromosomes for the associated 

 bivalents are not joined together in any way. However, it is 

 believed to indicate some homology and has been used as a 

 criterion of remote polyploidy in some plants. Apparently sec- 

 ondary association can occur only in plants with small chromo- 

 somes. In some diploid hybrids, such as the diploid Raphanus- 

 Brassica hybrid from which the amphidiploid arose, there is no 

 chromosome pairing because the chromosomes of the two sets 

 are too dissimilar. Yet there may be enough resemblance to 

 cause the univalents to tend to lie together in pairs on the meta- 

 phase plate, although since they are not united by chiasmata 

 these univalents pass to the poles independently of one another. 

 Secondary association occurs in allopolyploids with small chro- 

 mosomes and even in plants that have been believed to be 



diploids. 



QUESTIONS AND PROBLEMS 



1. In a certain genus there are six chromosomes in a genome. Species 

 L has the genomes Ai, B, and C; species ]\I has the genomes A2, B; 

 species N has genomes A^, C. All combinations of these species are 

 made. What would be the chromosome behavior at meiosis (a) in 

 these three combinations and (6) in allopolyploids derived from each 

 of the three combinations? 



2. One dominant gene mutates to a recessive allele in a plant which 

 reproduces readily by self-fertilization. Would the recessive character 

 appear most easily if the plant were (a) a diploid, (6) an autotetraploid, 

 (c) an allotetraploid? Explain. 



3. Some of the more desirable apples are triploids. If you had an 

 especially valuable mutation in one of these triploid trees, how would 

 you establish an orchard of that variety? 



4. In apples, the basic chromosome number is 17 and diploid varie- 

 ties usually have seven bivalents. They are often grouped as the result 

 of secondary association into seven groups of bivalents. What light, if 

 any, does this throw on the original basic number of chromosomes in 

 apples? Triploids have occasionally multivalent configurations, some- 

 times involving as many as nine chromosomes. Does this support or 

 reject your other position? Explain. 



