ALLELOMORPHISM. 



17 



tute one-fourth of the second generation. The remaining half of the 

 R ovules meet r pollen and the remaining half of the r ovules meet 

 B pollen, giving in each instance the combination Rr, which con- 

 stitutes half the second generation. If R is completely dominant 

 the types RR and Rr can not be distinguished, since the latter has 

 red flowers like those of type RR. Hence, where dominance is com- 

 plete the second generation appears to consist of only two types. 

 One of these types shows the dominant character, the other the 

 recessive character, and the dominant type is three times as numer- 

 ous as the recessive. Thus we arrive at the well-known Mendelian 

 ratio of 3:1, or three dominants to one recessive in the second gen- 

 eration of a hybrid. 



In the above second generation the two types RR and rr are seen 

 to consist of like things united, while the type Rr consists of unlike 

 things united. Types RR and rr are said to be homozygote, a term 

 which means ^^ike things united," while i?r is said to be heterozygote, 

 which means ^ ^unlike things united." An individual is said to be 

 homozygote with reference to a given character when the cells of that 

 individual contain two determiners for the presence of that character. 

 If its cells contain only one determiner for any character it is said to 

 be heterozygote for that character. Thus a bearded w^heat is homo- 

 zygote for beards, a pure race of smooth wheat is homoz3^gote for 

 absence of beards, while a cross between a bearded and a smooth race 

 is heterozygote for beards. 



ALLELOMORPHISM. 



The term ^^allelomorph" was introduced by Prof. William Bateson, 

 of Cambridge, England, one of the leading investigators of Mendelian 

 phenomena. It is derived from two Greek words, one of which 

 means ^^one another" and the other ^^form." We may say that it 

 means ''corresponding forms." What we have called a ''pair of 

 determiners" Bateson calls a "pair of allelomorphs." The term " allelo- 

 morph," however, has a wider application than "determiner;" it may 

 mean characters themselves as well as the determiners of those char- 

 acters. To say that one character is allelomorphic to another means 

 simply that the two characters when brought together in the same 

 individual form a Mendelian pair and hence fall apart when repro- 

 ductive cells are produced. Thus, a pair of allelomorphs is what we 

 have been calling a "pair of Mendelian characters." Hence, the 

 term "allelomorph" is frequently used simply to mean a Mendelian 

 character; that is, a character which obeys Mendel's law of segregating 

 from its mate in the reduction division. 



The term "gamete" is also a very convenient one which we shall 

 have occasion to use frequently. It simply means a reproductive cell, 

 such as an ovule, a pollen grain, an unfertilized egg, etc. 



165 



