INTRODUCTION 13 



correlated may be recombined, a greater degree of segregation is 

 possible than when factor correlation is absolute. 



In general we may say that the number of groups of correlated 

 or partially linked factors is not greater than the number of 

 chromosome pairs. Whether the above explanation is correct, 

 partly so, or entirely wrong, it is a convenient theory with which 

 to account for a large body of facts. It allows for classification 

 of facts in such a way that correct breeding methods may be used. 



Mendel's law may then be summarized from the standpoint 

 of the plant breeder as follows : 



1. Plants breed true for certain characters when all factors 

 necessary for the development of the character are in a homozy- 

 gous condition. There is a relative stability of factors. Changes 

 in factors or " mutations" are far too infrequent to furnish a basis 

 for a system of breeding. 



2. There is independent segregation of certain factors. 



3. Partial coupling of certain determiners sometimes is found. 

 The degree of linkage in transmission is quite constant. 



4. Perfect coupling of certain factors occurs, i.e., constant 

 association of characters in inheritance. 



As a possible exception to the usual behavior we may mention 

 apparent segregation in the somatic cells of some hybrids. In 

 some forms these changes apparently occur frequently enough to 

 be of practical selective value. 



We may summarize Mendel's law in another way by saying 

 that the first generation cross between stable forms may resemble 

 one parent in one character, the other parent in another character 

 or may be intermediate in the character in question. All mem- 

 bers 1 of FI are of uniform habit. Segregation occurs in F 2 and 

 " segregation of potential characters in the germ cells of hybrids 

 arid their chance recombination" (East and Hayes, 1911) may be 

 considered as a general law. In F s and later generations some 

 forms breed true, others segregate. 



Homozygous forms may be obtained which contain the de- 

 sirable characters of both parents. Such forms are as stable 

 as races which ha\e been bred by straight selection. 



Hybridization as a Means of Producing Variations. A quite 

 recent explanation for the cause of germinal variation and 

 therefore the main cause of evolution is that of Lotsy (1916), 



1 The meaning of FI, Ft, etc., and other genetic terms not explained in the 

 text is given in the glossary. 



