46 AN INTRODUCTION TO MODERN GENETICS 



contained two chromosomes which failed to pair; the obvious explana- 

 tion is that these two were not of the same kind. 



General evidence of the parallelism between the phenomena of 

 segregation of genes and chromosome pairing and reduction is found in 

 the genetic behaviour of organisms with chromosome cycles different 

 from the normal, in polypoids, and in cases of abnormal meiosis 

 (Chap. 2.) 



In some organisms pairs of chromosomes have been found in which 

 the two members, although homologous, are yet recognizably different, 



[ 



I 



Fig. 12. "Mendelizing" Chro- 

 mosomes. — Crosses between 

 races of Circotettix verruculatus 

 (Orthoptera) with heteromor- 

 phic pairs of chromosomes. On 

 the left is a cross between two 

 animals each with an IJ pair; it 

 gave offspring with JJ, IJ, and // 

 pairs. On the right is a cross 

 between JJ and IJ animals, which 

 gave JJ (two upper figures) and 

 IJ (two lower figures) pairs In 

 the offspring. The numbers of 

 the different kinds of offspring 

 are indicated. 

 (From Stern, after Carothers.) 



one being larger than the other; they are probably cases of translocation 

 or deletion (Chap. 4) such as are well known genetically in Droso- 

 phila. Thus in the Orthopteran Circotettix^^ in crosses between a race 

 with an imequal pair and a race with an equal pair, the unlike chromo- 

 somes were inherited exactly like a Mendelian factor in a cross between 

 a heterozygote and a homozygote. This provides general evidence for 

 the continuity of the chromosomes, their segregation, and their random 

 recombination in fertilization. 



7. Independent Assortment and Linkage 



The independence which is sometimes found between different pairs 

 of genes, and is shown by the formation of equal nimibers of new and 

 old combinations in the gametes of a double heterozygote, is explained 

 as a consequence of the independent assortment of chromosomes to the 



^ Rev. Carothers 1926. 



