HOMOZYGOTE AND HETEROZYGOTE 207 



to the other, the difference between them lying in the fact that 

 one is paternal and the other maternal in origin. 1 Reduction (or 

 halving of the total number of chromosomes) is accomplished by 

 the distribution of the two members of each pair to different 

 daughter cells. Hence if the paternal and maternal chromosomes 

 contain different alternative factors (or determinants), derived 

 from different parents, so also will the gametes when these are 

 formed. 



Suppose we represent the two members of any pair of factors 

 by the letters a and b, then any one gamete may contain either 

 a or b, but not both. When the gametes unite in conjugation the 

 zygote will again have both maternal and paternal chromosomes 

 and there will be three possibilities as to its constitution in 

 regard to the characters in question. If a gamete containing a 

 happens to unite with another containing a the zygote will 

 contain the pair a a, and will be termed in Mendelian phraseology 

 a " homozygote." 2 If a gamete containing b happens to unite 

 with another containing b the zygote will contain the pair b b and 

 will again be a homozygote. If, on the other hand, a gamete 

 containing a unites with another containing b the zygote will 

 contain the pair a b, and such a zygote is termed a " heterozygote " 

 or hybrid. 



In accordance with what is termed the " presence or absence 

 hypothesis " one of the two " factors" in an allelomorphic pair may 

 be merely negative in character ; i.e. it may have no real existence 

 as a material primordium or determinant. In the words of 

 Professor Bateson, "All observations point to a conclusion of great 

 importance, namely that a dominant character is the condition 

 due to the presence of a definite factor, while the corresponding 

 recessive owes its condition to the absence of the same factor. This 

 generalization, which so far as we yet see, is applicable throughout 

 the whole range of Mendelian phenomena, renders invaluable 

 assistance in the interpretation of the phenomena of Heredity. 

 The green pea, for instance, owes its recessive greenness to the 

 absence of the factor which, if present, would turn the colouring 

 matter yellow, and so forth." 3 



It is obvious that if, in our general formula, we take a to 

 represent the dominant factor and b the recessive, we might 



1 Compare Chapter X. 



2 This term has been extended to the fully developed organism which arises from 

 the zygote. 



3 Bateson, " Mendel's Principles of Heredity" (Cambridge, 190'J), pp. 53 54. 



