12 BREEDING CROP PLANTS 



bud sports or somatic mutations each of which may form the 

 basis for a new race. Such bud sports in some plants apparently 

 occur frequently enough to be of economic importance. 



Mendel's Law of Heredity. Mendel's experiments, pub- 

 lished in 1866, remained unnoticed until the facts were redis- 

 covered in 1900 by De Vries, by Correns, and by Tschermak. 

 This law furnished the starting point from which the modern 

 study of genetics has developed. Many students will have taken 

 a course in genetics before studying plant breeding. For such 

 students it is sufficient here briefly to review Mendel 's law in its 

 application to crop improvement. 



Mendel's law can best be understood in relation to cytology. It 

 is well known that the chromosomes are the bearers of the herit- 

 able factors. The number of chromosomes for each species 

 is constant and the form and individuality is characteristic. 

 Each chromosome is supposed to be composed of chromomeres 

 and each chromomere may be the seat of a particular inherit- 

 able factor. According to Morgan 's hypothesis, the factors are 

 located in particular regions of the chromosome. The chromo- 

 somes are considered to be in pairs and the two parts of each pair 

 are in such a relation to each other that at reduction division, 

 i.e., at the formation of gametes, the parts of each pair separate 

 and the gamete contains only half as much chromatin as the 

 somatic cell. The gamete then contains one member of each 

 chromosome pair. Exceptions sometimes occur to the above 

 rule when unusual cytologic divisions take place. 



A rather recent development of genetics is of primary impor- 

 tance. At some time in preparation for reduction division there 

 is a doubling of the spireme. Morgan supposes that at this time 

 homologous parts of chromosome pairs lie next to each other. 

 These spireme threads wind about each other and in some cases 

 breaks occur. It is then supposed that the chromosomes may 

 reunite in such a manner that a new chromosome is formed which 

 contains parts of each of the homologous chromosomes that make 

 up a pair. If factors are in particular loci this would allow for a 

 different combination of factors in a chromosome containing 

 parts of each chromosome pair. 



Most of the previous investigations show that many factors 

 are inherited independently. This allows for numerous combina- 

 tions when crosses are made. If there is a break, i.e., a cross- 

 over or some other means by which factors which are usually 



