324 EVOLUTION AND GENETICS 



occur in pairs except in cells which have only the haploid number 

 of chromosomes. Here a single gene may represent a character. 

 Characters are said to be linked when their genes occur in the 

 same chromosome and therefore usually remain together. Linkage 

 relations may be modified by interchange of material between 

 two chromosomes of a kind during the reduction division; this 

 phenomenon is called crossing over, and gives evidence of definite 

 localization of the genes. When genes are located in the sex 

 chromosomes the characters which they control are said to be 

 sex-linked. In addition to characters determined by a single pair 

 of factors many are known which involve two or more pairs. 

 These multiple factors behave in the normal Mendelian manner, 

 but the phenotypical ratios differ from those produced by the 

 same number of independent factors. The difference depends 

 upon the number of factors and the nature of their interaction. 

 The way in which genes act to produce characters in the individual 

 has been the subject of some speculation but is still unknown. 



REFERENCES 



Castle, W. E., Heredity of Coat Characters in Guinea-Pigs and Rabbits, Pub. 



Carnegie Inst., No. 23, 1905. 

 Castle, W. E. and Wright, S., Studies of Inheritance in Guinea-Pigs and 



Rats, Pub. Carnegie Inst., No. 241, 1916. 

 Morgan, T. H. and Bridges, C. B., Sex-Linked Inheritance in Drosophila, 



Pub. Carnegie Inst., No. 2.37, 1916. 

 Babcock, E. B. and Clausen, R. E., Genetics in Relation to Agriculture, 



2nd edition., 1927. 

 Morgan, T. H., Sturtevant, A. H., Muller, H. J. and Bridges, C. B., 



The Mechanism of Mendelian Heredity, revised edition, 1922. 

 Morgan, T. H., Cowdry's General Cytology, Section XI, 1924. 



