CHAPTER 6 



CONSEQUENCES OF CHANGE 



Meiosis and Fertility in Polyploids Structural Hyhridity 

 Units of Recombination Allopolyploidy 

 The Integration of Differences 



The different polyploid and polysomic types, and such of the 

 new structural types as are viable, behave normally at mitosis. 

 Their vegetative life follows an even course. At meiosis, in their 

 germ cell formation, however, we fmd new types of behaviour 

 which lead to abnormalities of breeding and the loss of fertility. 



Meiosis and Fertility in Polyploids. 



Triploids, or trisomies which are merely triploid for one chromo- 

 some, show the principles governing all abnormalities of meiosis, 

 and indeed governing meiosis itself There are three chromosomes 

 of each type capable of pairing with one another throughout their 

 length at prophase. In the polytene nuclei of a triploid Drosophila 

 this expectation is fulfilled. But in all triploids, at the prophase of 

 meiosis, association is limited to two. One is left out, but this is 

 usually a different one at different places; the chromosomes make 

 contact in pairs at random either near the ends or near the centro- 

 meres; each association runs along the chromosomes until it meets 

 another and they consequently exchange partners according to the 

 chances of their original contacts. Crossing-over takes place and 

 chiasmata are consequently formed which hold together the paired 

 chromosomes at the points of crossing-over. If all three chromosomes 

 have been paired, and if one has formed chiasmata with both the 

 other two, an association of three, a trivalent, is held together until 

 metaphase. If one fails to cross-over with either of the other two 

 it is left out completely, a univalent, while the other two form a 

 bivalent (Fig, 27). 



At first metaphase in triploids the bivalents arrange themselves 

 on the plate, and the trivalents also with their centromeres either 



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