Chapter 18 



MULTIPLE ALLELES 



That more than two alleles can be present at the same locus 

 has previously been discussed. In Chapter 2, two series of mul- 

 tiple alleles were mentioned, and the method of transmission of 

 multiple alleles that lie on the X chromosome was discussed in 

 Chapter 7. In Chapter 6 the transmission of multiple alleles 

 on an autosome was described, with the horned series of alleles 

 in sheep as an example. Multiple alleles are frequently found 

 in plants and animals, but several series are of especial interest 

 to human beings because they determine the various types of 

 human blood groups. The series of alleles that determines self- 

 sterility or self-incompatibility in many species of plants is a 

 good example of an identical series of multiple alleles in a large 

 number of organisms. 



Self-Sterility 



Self-sterility is a phenomenon found in a great many species 

 of plants and in a few of the hermaphroditic lower animals. In 

 a self-sterile plant, the eggs maj^ be perfectly good and the 

 pollen may be normal and functional, and yet, when pollen from 

 such a plant is placed upon its own stigma, seeds will not be 

 produced (Fig. 79) . This situation is different from true sterility 

 in which either the eggs or the male gametes or both will be 

 absent or nonfunctional. Self-sterility is actually an incom- 

 patibility though the gametes are functional. It is referred to as 

 "self-incompatibility" by some geneticists, but the term "self- 

 sterility" is older and is still frequently used although it is less 

 descriptive. A feature of the phenomenon of self-sterility is a 

 peculiar cross-sterility of such a nature that the various indi- 

 viduals of a self-sterile species can be grouped into cross-sterility 

 classes in which all the members of the same class will fail to 

 set seed with each other but will usually set a normal seed 

 complement with all the other individuals of all the other classes. 



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