384 INTRODUCTION TO CYTOLOGY 



chromosome pair therefore seems to be responsible for a certain group 

 of characters. It has been shown above that one of these groups, 

 the sex- and sex-linked characters, can be definitely assigned to the pair 

 of sex-chromosomes; and Morgan further believes that the factors 

 for the two characters of the small linkage group are located in the 

 m-chromosomes. The two remaining linkage groups, which contain 

 many characters each, are assigned to the large euchromosome pairs. 

 Each chromosome is accordingly regarded as a body containing the fac- 

 tors or genes for a considerable number of characters; and on the basis 

 of the evidence to be presented below it is concluded that these genes, 

 differing thus in their hereditary potencies, are arranged in the chromo- 

 some in a linear series as suggested by Roux many years ago. 



In plants the best known cases of linkage are in Zea Mays, in which 

 Emerson and his students at Cornell have identified six linkage groups, 

 and Pisum, which has so far shown four linkage groups (White 1917). 

 Since maize has 10 pairs of chromosomes, four more groups may be 

 expected, while in Pisum, which has seven pairs, three more groups will 

 probably be established; in fact seven independently inherited characters 

 are known. It is an interesting fact that Mendel, in his famous researches 

 on Pisum, happened to select for study seven pairs of characters belonging 

 to the seven different groups, and so did not detect the phenomenon of 

 linkage. 



From the foregoing considerations there arises an interesting and 

 very important question. If two homologous chromosomes, each carry- 

 ing factors for a certain group of characters (those of one group allelo- 

 morphic to those of the other group), separate into different gametes 

 (or spores) at the time of reduction, how does it happen that occasionally 

 there appears an individual with some of the characters of each group? 

 And if a single chromosome carries a series of factors for a certain group 

 of characters, how shall we account for the occasional individual with 

 some of these characters but not the rest? To state the problem in the 

 terms of linkage, if each group of linked characters is represented by a 

 series of genes in a given chromosome, how is the linkage broken in 

 a certain percentage of cases, with the resulting formation of new link- 

 age groups, as shown by the exceptional red-yellow and white-gray flies 

 in the experiment described at page 379? A solution to this problem has 

 been offered in the Chiasmatype Theory. 



The Chiasmatype Theory. — In our discussion of chromosome con- 

 jugation it. was pointed out (p. 257) that various opinions have been 

 entertained regarding the nature of the association between the members 

 of the synaptic pair. Some workers have held that the chromosomes 

 fuse completely and lose their identity, and that the two chromosomes 

 appearing on the first maturation spindle are not to be looked upon as 

 identical with those which entered into conjugation. On the contrary, 



