MULTIPLE HETEROZYGOTES 155 



(i.e., the parental types) and one kind of " double " heterozygote 

 with a ring of six {i.e., AB-BC-CF-FE-ED-DA). The proportions 

 of the cytologically recognisable types will be 1:2:1, and the 

 inheritance can be described in mendelian terms, if each interchange 

 is regarded as making an allelomorphic difference. Thus the cross 



.0' ( w 



a},0,OVl,l/fif 



Fig. 54. — Configurations observed at first metaphase in diploid struc- 

 tural hybrids of the simpler kinds with complete terminalisation. 

 a, fragmentation or fusion heterozygote {e.g., Vicia sativa x 

 amphicarpa or Hesperotettix). b, fragmentation-plus-reduplica- 

 tion heterozygote {e.g., Matthiola) ; note triple chiasma. 



c, deficiency or reduplication heterozygote {e.g., Circotettix). 



d, translocation heterozygote {e.g., Tradescantia bracteata or 

 Vicia sativa X angustifolia). e — o, simple interchange hetero- 

 zygote {e.g., Datura, Campanula) showing complete association 

 in a ring of four and incomplete association in one or more 

 chains. The chromosomes are represented as of three sizes — one 

 long, two intermediate and one short in a constant order in the 

 ring, e and h are dis junctional arrangements ; /, g, j, k and / 

 are non-disjunctional, the rest are doubtful. 



is analogous to AaBB x AABh ■-= 1 AABB : i AaBB : i AABb : 

 I AaBb where AaBB and AABb are indistinguishable unless the 

 chromosomes taking part in the rings can be distinguished (Fig. 52, 

 Plate VI). 



Where the interchanges in the two parents do not affect any 

 chromosomes in common, the " double " heterozygote has two rings 

 of four instead of one ring of six. Both these expected results have 



