158 



STRUCTURAL HYBRIDS 



This gives all the expected small configurations (like those in the 

 tertiary trisomic Datura), but none with the maximum association 

 (including all 21 chromosomes), since this would involve the 

 apparently unlikely chance of 14 of the chromosomes forming two 

 chiasmata in each arm (Catcheside, 1931 a). 



Several tetraploid species show by their pairing that they are 

 interchange heterozygotes, but for the reasons stated above an exact 

 constitutional formula cannot be given with certainty. In these 



Fig. 55. — First metaphase in Aucnha japonica pollen mother-cells 

 {4X = 32). a, two sixes, four fours, and two pairs, b, one eight, 

 three fours, one three, four pairs, and one unpaired chromosome. 

 X 2400 (from Meurman, 1929). 



tetraploids an association of more than four chromosomes indicates 

 that the plant is heterozygous for one interchange, and an association 

 of more than eight, or two of more than four but not more than 

 eight, indicates that the plant is heterozygous for two interchanges, 

 and so on. The following table shows the percentages of 

 configurations with different numbers of chromosomes in three 

 such tetraploid species. It will be noticed that occasional unpaired 

 chromosomes are characteristic of the polyploid as of the diploid 

 interchange heterozygote. 



