460 



Allopolyploids 



diploids had some configurations with more than four chromo- 

 somes. 



Genetic ratios in allopolyploids are usually different from those 

 in autopolyploids. In our theoretical "pure" allotetraploid, in 

 which the genome from the one parental species is entirely dif- 

 ferent from that of the other species, no locus should be common 



/ 



Fig. 132. First metaphase in the intergeneric wheat-rye hybrid, Triticale. 

 Left, no univalents and only bivalents present ; right, another cell from the 

 same plant with eighteen univalents. These two cells indicate the great 

 irregularity in this hybrid. (Redrawn from Miintzing in Hereditas.) 



to the two genomes, so that all inheritance should be monogenic, 

 giving simple mendelian ratios. However, since most allotetra- 

 ploids are not so "pure," the one genome frequently has a num- 

 ber of small segments identical with segments of the other 

 genome. It is conceivable that some loci should therefore be 

 common to the two genomes, and such results have been found. 

 If two segments carrying identical loci are so small that they 

 rarely, if ever, pair, each locus will behave in transmission inde- 

 pendently of the other; and if each is heterozygous, a 15 : 1 

 ratio will be obtained upon selfing. For example, if the identical 

 segment of the amphidiploid just discussed contains the locus of 

 gene a, and if each parent is Aa, the amphidiploid hybrid might 

 well be AaAa, Using the conventional method of designating 



