248 CHROMOSOMES IN HEREDITY : MECHANICAL 



partially or completely suppressed between chromosomes of opposite 

 parental species. Thus in Raphanns-Brassica, where no segregation 

 occurs, no pairing of Raphanns and Brassica chromosomes occurs — 

 simple bivalents are always formed in the tetraploid (and no 

 bivalents are formed in the diploid). In Primula kewensis, on the 

 other hand, there is segregation in the tetraploid ; quadrivalents 

 are formed. 



5. Where four, six or more chromosomes are associated in a ring 

 in a diploid, each of these chromosomes must have a specifically 

 different combination of materials from any of the others, and only 

 two types of viable gametes should be produced— the opposite types 

 arising from separation of each chromosome in the ring from the 

 other two with which it is " paired " at opposite ends. This 

 is verified both by genetical and by cytological tests of the progeny. 

 It is found that two such gametic types are produced by wild 

 species with chromosome rings [v. Ch. IX), and these differ 

 genetically. Similarly the inheritance of the pairing properties of 

 their chromosomes agrees with prediction. Each form with rings 

 produces two kinds of gametes which give specific and different 

 kinds of pairing or ring-formation with other kinds of gametes. 



6. The direct effect of segregation in giving equal numbers of 

 products bearing opposite allelomorphs can be seen in the haploid 

 generation produced by a heterozygous diploid. This direct segre- 

 gation is seen in organisms with an important haploid cycle, e.g., 

 Bryophyta (Wettstein, 1924 ; Allen, 1926, 1935 a and h) ; Asco- 

 mycetes (Dodge, 1936 ; Lindegren, 1936) ; Basidiomycetes (Buller,' 

 1931) ; Pteridophyta (Andersson-Kotto, 1931). 



7. It is even possible when the spores produced at meiosis remain 

 together or when one of the divisions is suppressed, to show that 

 segregation has occurred sometimes at the first and sometimes at 

 the second division, and with special proportions for particular 

 factors, as is the case with particular unequal bivalents (v. infra). 

 Similarly in the flowering plants segregation of two types of pollen 

 has been shown in CEnothera (Renner, 1919 h), Oryza (Parnell, 1921) 

 and Zea (Demerec, 1924 et alii). The special type of reproduction 

 of the Hymenoptera by which diploid females produce haploid 

 male offspring without fertilisation has made it possible to study 



