72 AN INTRODUCTION TO MODERN GENETICS 



similar sets, as in the example above, leads to a condition which is 

 genetically indistinguishable from that in a diploid. Balanced gametes 

 of the type AB are formed regularly, and the tetraploid is compara- 

 tively, sometimes very, fertile. This fertility depends on there being a 

 considerable difference between the "homologous" chromosomes of the 

 two original species from which the tetraploid was derived; this same 

 differentiation will tend to cause a failure of pairing in the diploid 

 hybrid AB, and we therefore find an inverse correlation between the 

 fertilities of the diploid and tetraploid hybrids.^ 



Association of the chromosomes derived from one of the original 

 ancestors may be called homogenetic association, while the association 

 of A with B chromosomes may be called heterogenetic association. 

 Homogenetic association in an allotetraploid hybrid may lead to the 

 complete disappearance of some of the characters of the ancestors. Thus 

 if a character of one species is determined by a factor which is recessive 

 to the homologous factor in the other species, it will never appear in 

 the progeny of the hybrid, since the recessive factor, e.g. w from A, 

 will always be accompanied by the homologous dominant W from B. 

 This phenomenon is very noticeable in wheat crosses, where some of 

 the forms dependent on the more extreme members of polymeric series 

 of faaors can never be recovered; it has been spoken of as "shift." 



Heterogenetic association also occurs in allotetraploids, and can be 

 detected cytologically by the appearance of tetravalents. An example is 

 Primula kewensis, a tetraploid derived from doubling in a hybrid 

 between P. floribunda and P. sinensis; metaphase association is nor- 

 mally in twos, but occasional quadrivalents can also be found. Similar 

 association occurs in derivatives of allopolyploids, for instance, in the 

 "haploids" (really diploids as regards the basic number) which can be 

 derived by parthenogenesis in an allotetraploid. Nicotiana tahacum is 

 itself an allotetraploid AABB, and from it a "haploid" AB has been 

 formed; in this some bivalents are formed, which must be the result of 

 pairing between A and B chromosomes.^ Similarly, cases are known, e.g. 

 in Prunus, in which a hybrid AABC between a diploid AA and a 

 hexaploid AABBCC has complete association of the BC sets or even 

 of all four sets together to give quadrivalents. 



Regular heterogenetic association, to the exclusion of homogenetic, 

 is not to be expected. If it were found, it would give a gametic output, 

 from a tetraploid AABB, of i AA : lAB : i BB. It probably usually 

 occurs only in individual chromosomes, associated with more or less 



^ Darlington 1932a. - Lammerts 1934. 



