382 Inbreeding , Selection, and Heterosis 



can be attached to individual records of egg production of an- 

 cestors beyond the third generation. This latter point is also 

 maintained by Steele, who suggests that for race horses and 

 saddle horses individuals beyond the third generation may be 

 ignored for all practical purposes. Even though pedigree rec- 

 ords for horses are among the oldest and most elaborate, Steele 

 considers that there is insufficient genetic significance for the 

 current use of lengthy pedigrees. 



Probably the best criterion yet devised for determining the 

 breeding value of a plant or animal is the progeny test. By this 

 method the individuals selected for breeding are not those that 

 necessarily appear the best or have the best ancestors, but those 

 that produce the most desirable offspring. For example, when 

 breeding for high egg production the hens selected for carrying 

 on the line are not those which produce the most eggs or those 

 whose ancestors produce the most eggs, but those whose daugh- 

 ters have the best records. When selecting in plants, seeds are 

 obtained from a number of individuals. A portion of each lot 

 of seeds is sown, and the offspring are raised and classified. If 

 the offspring from one or more test portions of seeds is superior 

 to those from others, the remaining seeds from the plants that 

 produced this superior test portion are then sown to produce the 

 next generation. The progeny test has been widely used in 

 a number of organisms, and in poultry some very valuable 

 results have been obtained by Hays at the Massachusetts Agri- 

 cultural Experiment Station. 



Although the individual that is used to start a breeding pro- 

 gram based on selection and inbreeding is frequently a heterozy- 

 gous representative of a certain species or variety, he very often 

 is a hybrid produced by crossing together two species or varieties 

 that differ in a number of traits. Being a hybrid, he would or- 

 dinarily be heterozygous for a large number of genes. For 

 example, crosses have been made between the usual breeds of 

 cattle raised in this country and strains that are used in very 

 hot chmates, such as the Brahman cattle of India and the Afri- 

 cander cattle of South Africa. The purpose is to select strains 

 which segregate out that possess the good beef quality of the 

 more usual strains and the ability to thrive in the warmer 

 regions of the United States. 



Yarnell and Hawthorn obtained some interesting results from 



