262 



AN INTRODUCTION TO MODERN GENETICS 



bivalents and four groups of tv^^o, indicating that the haploid set is 

 made up as AAA,BBB,CCC,DD,EE,FF,GG. (Fig. 30 p. 74). 



Species in which a chromosome has been lost also occur, again in 



Fig. 116, Crepis artificialis. — A. Mitotic metaphase of C. setosa (2n = 8). 



B. C. biennis (2n = UQ). C. F1 hybrid (2n = 24). D. The true breeding hybrid 



C. artificialis {2n = 24) with 10 pairs of biennis chromosomes and 2 pairs of setosa 

 ones, which are marked 1 and 4- as in A, 



(From Hurst, after Collins, Hollingshead, and Avery.) 



conjunction with polyploidy which minimizes the resulting unbalance. 

 The artificial Crepis artificialis^ is an example. It was derived from a 

 cross between C. biennis (« = 20, probably a tetraploid) and C. setosa 

 (« ^ 4, a diploid). In the Fi the 20 biennis chromosomes formed 10 

 pairs by autosyndesis, but the four setosa chromosomes were unpaired 

 and segregated at random. After continued selling, a plant segregated 

 ^ Collins, Hollingshead and Avery 1929. 



