Chapter 23 

 INBREEDING, SELECTION, AND HETEROSIS 



Inbreeding 



When offspring are produced from closely related parents, we 

 say that they are produced by inbreeding. The degree of in- 

 breeding may vary considerably in different organisms and will 

 depend in part upon the method of reproduction in the species 

 in question. Many plants and some animals reproduce entirely 

 or largely by self-fertilization. It results in the most extreme 

 degree of inbreeding. In normally cross-fertilized organisms, 

 reproduction may be carried out by father-daughter and mother- 

 son crosses, by sib crosses, by crosses between uncle and niece, 

 or aunt and nephew, or first cousins, or by crosses between more 

 distant cousins. All such matings are considered as inbreeding, 

 but the closer the relationship between the two parents, the 

 greater is the degree of inbreeding. When two unrelated persons 

 are mated, the degree of inbreeding is zero; the offspring arise 

 by outbreeding and the cross is an outcross. 



The results of inbreeding have been under discussion for many 

 years as the effect upon the organism often seems to be very 

 deleterious. One of the best examples of this is maize, a plant 

 that is normally cross-fertilized. When a vigorous, highly pro- 

 ductive strain is repeatedly inbred by self-fertilization, there is 

 a decline in quality, vigor, and yield for several generations; 

 after about half a dozen generations the resulting strains are 

 so inferior that no farmer would consider planting them (Fig. 

 100 >. Similar harmful effects noted in other organisms have led 

 to the rather popular belief that rapid deterioration is inevitable 

 if a strain is closely inbred. Because of this belief and because 

 close breeding among human beings has sometimes produced 

 very unfortunate results, it is generally considered undesirable 

 for first cousins to marry. 



Why is inbreeding often harmful, and is it always harmful? 

 First, we must understand that there is nothing mysterious about 



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