GLOSSARY 



Alio -polyploid : An organism with more than two haploid 

 sets of chromosomes which have been derived from 

 two or more ancestral species, by hybridization. Cf. 

 auto -polyploid . 



Amitosis : A form of nuclear division in which no spindle 

 mechanism is present, so that the chromosomes are 

 not necessarily equally distributed to the two resulting 

 nuclei. 



Anaphase : The stage of mitosis w^hich follows on metaphase 

 and precedes telophase. During anaphase the chromo- 

 somes move from the central region of the spindle 

 towards the poles. 



Aneuploid : An organism in whose somatic chromosome set 

 one or more chromosomes are represented more times 

 than the rest ; consequently an irregular polyploid. 



Auto-polyploid : An organism with more than two haploid 

 sets of chromosomes which have been deriv^ed from tho 

 same parent species. 



Autosome : Any chromosome which is not a sex-chromosome. 



Bivalent : Two homologous (or at any rate partly homologous) 

 chromosomes which have paired at meiosis and are 

 held together, either by a mutual attraction or by 

 chiasmata. 



{Trivalent : a similar association of three chromo- 

 somes ; quadrivalent, an association of four ; multivalent, 

 an association of more than two chromosomes.) 



{Unequal bivalent : a bivalent in which one of the 

 constituent chromosomes is longer than the other, so 

 that it has an unpaired region at one end.) 



Centromere : See Spindle attachment. 



Chiasma : A visible change of pairing affecting two out of 

 the four chromatids in a bivalent at meiosis ; an out- 

 ward sign that a genetical cross-over has taken place. 

 (Chiasma -frequency : the average number of chias- 

 mata formed in a particular chromosome or in a par- 

 ticular organism under given circumstances.) 



(Terminal Chiasma : an association of four chroma- 

 tids end-to-end which results from the shifting of a 

 chiasma until it reaches the end of the bivalent or multi- 

 valent — see Fig. 14c.) 



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