Heredity — Genetics 705 



oats, etc.) and some animals normally reproduce entirely or largely by 

 self-fertilization, which is a type of inbreeding. On the other hand, such 

 a plant as corn is normally cross fertilized. When a vigorous, desirable 

 strain of corn is repeatedly inbred by self-fertilization, the quality, yield, 

 and vigor decline for several generations. It is commonly known that 

 inbreeding in certain human families results in highly undesirable con- 

 sequences. If a huyian family has recessive genes for undesirable traits 

 and if they are not expressed normally because of dominant genes, then 

 inbreeding of closely related persons will tend to produce offspring who 

 are homozygous for the undesirable recessive defects. Inbreeding in such 

 a family is undesirable. 



Outbreeding frequently results in offspring which are better than either 

 parent, a phenomenon known as hybrid vigor or heterosis. This vigor 

 may manifest itself in various ways in different animals and plants. In 

 corn increased vigor may result in larger ears, greater number of grains 

 per row, greater height of the plant, etc. Much of the corn grown in 

 the United States is a special hybrid developed by crossing four different 

 inbred strains of corn (Fig. 348). This will be considered in greater 

 detail in a later part of this chapter. Hybrid vigor is a common 

 phenomenon in many types of plants and animals. When a horse and 

 donkey are crossed, the resulting hybrid mule is strong, sturdy, and more 

 vigorous than either parent. 



16. Genetic Improvements of Plants and Animals. — Many plants and 

 animals have been produced as a result of some type or other of genetic 

 improvement. Man has merely taken advantage of the natural genetic 

 phenomena possessed by these organisms and has somewhat controlled 

 and directed them so as to result in a better type. The number of geneti- 

 cally improved organisms is so extensive and the methods employed so 

 varied that only a few examples can be given. Probably one of the more 

 valuable and recent contributions is the production of hybrid corn; the 

 method employed is given briefly. 



Corn is normally cross-pollinated. When self-pollinated (selfed) for 

 at least seven successive generations, the corn plants become progressively 

 less productive and smaller. Eventually when two such self-pollinated 

 corn plants are crossed, the resulting hybrid is more productive and 

 larger than the ancestors. The procedure, in brief, is as follows: (a) 

 Inbreeding (self-pollination) for at least seven successive generations in 

 order to produce homozygous strains; (b) Two such homozygous strains 

 which possess traits desired in commercial strains are then cross pol- 

 linated to produce the Fi hybrids known as single-cross hybrids. How- 



