Amphidiploids 



461 



duplicate genes, we should write these Aiai A^ao. Supposedly, 

 this chromosomal segment might be so small that it practically 

 never pairs. If such a plant was selfed, the ratio of offspring 

 would be 15A {9A1A2 + 

 3 Ai ao -}- 3 ai A2) : la (ai 02) , 

 as we pointed out in Chapter 

 21. Thus amphidiploids often 

 give ratios attributed to du- 

 plicate genes, and more or less 

 remote amphidiploidy might 

 be the cause of duplicate genes 

 in a number of plants. 



If the chromosomes of an 

 allotetraploid are AA BB CC 

 LLMMPP, and if one A 

 chromosome will pair only 

 with the other A while the 

 others pair BB CC LL MM 

 and PP, we have an extreme 

 allotetraploid. This type of 

 pairing is between chromo- 

 somes from the same parent 

 only and is usually called 

 autosyndesis. On the other 

 hand, if the chromosomes of 

 the two species are sufficiently 

 homologous so that we can 

 designate them AiAi BiBi 

 CiCi A2A2 B2B2 C2C2, Ai may 

 often pair with A2 Bi with 

 B2, and Ci with C2, and fre- 

 quently quadrivalents will be 

 present. Pairing in an allo- 



FiG. 133. A fertile amphidiploid 

 intergeneric Iwbrid between Triticum 

 monococcum and Aegilops uniaristata. 

 The diploid (2n) Fi is sterile but the 

 amphidiploid (in) is fertile. The 2n 

 and parent spikes are in flower, but 

 the 4n spike is mature. The seeds 

 above the 4/i spike are some of those 

 harvested from the amphidiploid. 

 (Photograph courtesy of Dr. E. R. 

 Sears in the Journal of Heredity.) 



polyploid of chromosomes from both parents is allosyndesis. 

 Much of the earlier confusion in studies of hybrids resulted from 

 the fact that many are allopolyploid for some chromosomes and 

 autopolyploid for others and some even incorporate similar and 

 dissimilar segments in individual chromosomes. 



We showed in the last chapter that if an autotetraploid is du- 

 plex for a dominant gene (AAaa) , its gametes will be in the 



