CHAPTER 14 



Animal and Plant Breeding^ 



Genetics, as we saw in the Introduction, grew out of the practical 

 problem of the improvement of agricultural crops and stock. The 

 development of the subject which followed Mendel's discoveries has 

 not, however, been primarily concerned with practice, but has been 

 towards the formulation of a comprehensive theory and the elucidation 

 of the fundamental biological problems which were raised. This 

 scientific advance has only fairly recently begun to have much effect on 

 breeding practice, and we are still a long way from being able to apply 

 the whole of the theoretical knowledge at our disposal. In the pecuUar 

 economic situation in which man has found himself during the last 

 few decades, production, even with Uttle help from science, has been 

 so far ahead of consumption that the application of genetics to this field 

 has, in most countries, not been investigated whole-heartedly and on a 

 large scale. The results which have been obtained are, however, already 

 quite considerable, and we can envisage the possibility of quite startling 

 changes in the world's agricultural economy, particularly in connection 

 with crop plants. 



I . The Methods of Breeding 



The purpose of breeding is to produce animals and plants with 

 useful characteristics. The methods of genetics are only useful in so far 

 as these characteristics are hereditable. Crop and stock improvement 

 has an aspect concerned with methods of husbandry as well as one 

 concerned with genetics. The two methods of approach continually 

 overlap in practice; the results which are obtained with a particular 

 genotype depend on the conditions in which the zygote develops, and 

 we often find that one genetic type is most suitable in one set of con- 

 ditions, but that in other circumstances some other genotype gives a 

 better result. 



We shall be mainly concerned here with genetic improvement, and 

 in so far as the characters involved can be treated from this point of 



^ General references: Babcock and Clausen 1927, Hudson 1937, Hunter and 

 Leake 1933, Rice 1934. 



