76 AN INTRODUCTION TO MODERN GENETICS 



the chromosomes, and some of the zygotene pairings will not be pre- 

 served. Thus the metaphase associations, in an autotetraploid for 

 example, usually include univalents, bivalents and trivalents as well as 

 quadrivalents. It was in fact the discovery of incomplete association in 

 autopolyploids which caused the abandonment of the theory that the 

 metaphase association is due to a generalized attraction between homo- 

 logues, and provided the basis of Darlington's theory that it is depen- 



fv 



r 



Fig. 29. Metaphase Associations in Autopolyploids. — Aside view of metaphase 

 chromosomes in tetrapioid oats {ht^na); the 28 chromosomes have formed three 

 quadrivalents (at the left) and eight bivalents. 6 Diakinesis trivalents in Tulipa 

 (little terminalization) with a diagram of the chromatid structure below. C Meta- 

 phase trivalents in Fritillaria showing orientation of centromeres. D some types 

 of quadrivalents found in tetrapioid Primula sinensis. 



(After Darlington.) 



dent on chiasma formation. Darhngton found one of the main supports 

 for his hypothesis in the fact that in organisms whose chromosomes are 

 of different lengths, it is in the short ones that association most often 

 fails (p. 121 ). 



Incomplete metaphase association usually leads to very irregular 

 segregation and the formation of gametes with variable and unbalanced 

 chromosome sets. A similar irregularity is common even when associa- 

 tion is complete, since, e.g., quadrivalents are not always separated two 

 to one pole and two to the other. The gametes with unbalanced com- 

 plements are usually inviable, and autotetraploids tend in consequence 

 to be rather infertile. In some plants, however, the chiasma frequency 

 is just sufficient to give nearly regular association of the chromosomes 



