GLOSSARY 111 



Homologous (as applied to chromosomes) : Containing the 

 same genes in the same sequence. 



Inert Chromosome : A chromosome all or most of whose genes 

 are physiologically inactive. 



Interference : The process by which the occurrence of one 

 cross-over or chiasma reduces the probability of another 

 taking place in its immediate neighbourhood. 



Interkinesis : The resting stage which often occurs between 

 the end of the first meiotic division and the beginning 

 of the second. 



Interphase : See Interkinesis. 



Inversion : A section of a chromosome which is reversed in 

 comparison with the usual sequence. 



Kinetochore : See Spindle Attachment. 



Leptotene : The earliest part of the prophase of the first 

 meiotic division, before pairing of the chromosomes has 

 taken place. 



Meiosis : Two modified mitoses in the course of which the 

 chromosomes only divide once. The two divisions are 

 called the first and second meiotic divisions. 



Metaphase : The stage of mitosis which follows prophase or 

 pro-metaphase and precedes anaphase ; .when the 

 spindle attachments of the chromosomes are lying in 

 approximately one plane, the equatorial plane. 



Microchromosome : A chromosome which is considerably 

 smaller than the other members of the set, e.g. the 

 IVth pair of chromosomes in Drosophila melanogaster . 



Multivalent : A group of more than two chromosomes which 

 are held together at meiosis by mutual attraction or by 

 chiasmata. 



Nuclear Sap : The substance inside the nucleus, in which the 

 chromosomes lie. 



Pachytene : The middle part of the prophase of the first 

 meiotic division, when the pairing of the chromosomes 

 is complete. Pachytene may be subdivided into 

 2 -strand pachytene (before the chromosomes have split) 

 and 4-strand pachytene (after they have split). 



Pairing : The approximation of genetically homologous 

 genes, chromomeres or chromosomes, considered either 

 statically or dynamically. Somatic pairing : the more 

 or less complete pairing of homologous chromosomes 

 which is sometimes found at mitosis. Zygotene pairing : 

 the pairing of chromosomes which takes place at the 

 zygotene stage. 



Pairing Segment (of a sex-chromosome) : A segment or short 

 portion of a chromosome which undergoes pairing with 

 a corresponding segment in another chromosome. 



Polyploid : An organism with more than two baploid sets of 

 chromosomes in its somatic cells. 



