IIKREDITY AND VARIATION 217 



The denotype. — The relation between this fact of segregation 

 and the actual results which are obtained in crosses is perhaps 

 best explained if we represent the factorial make-up of the plants' 

 and of their gametes by simple letters or formulas (Fig. 124) 

 in somewhat the same way that Mendel did in his original work. 

 Every individual is in a sense a double structure, since it arises 

 from the union of two gametes and draws half of its inheritance 

 from one and half from the other. If we let P represent the factor 

 for purple flower color and W the factor for white flower color, our 

 pure purple-flowered parent, which received the factor P from 

 both of its parents, we might therefore represent by the formula 

 PP; and our pure white-flowered parent in the same way by WW. 

 This formula applies to every body cell of the plant. Of course 

 it should be borne in mind that we are here representing only one 

 of the great number of factor-pairs which are present in the con- 

 stitution of the plant. In the cell divisions just preceding the 

 formation of the gametes, there is a reduction by half in the 

 amount of hereditary material contained in the nucleus* and 

 every gamete produced now carries just /la// of each of the factor- 

 pairs which composed the parent plant. The gametes of the 

 purple-flowered parent in our illustration would therefore all be 

 represented by the formula P and those of the white-flowered one 

 by W. When these two plants are crossed and an egg, P, is 

 fertilized by a male cell, W, (or vice versa), the genetic formula 

 of the resulting hybrid plant is obviously PW. Since purple is 

 almost completely dominant here, this plant appeals purple- 

 flowered, but in its factorial make-up (technically known as its 

 genotype) there is a recessive factor for white. If dominance were 

 absent and the hybrid were intermediate in appearance — pink, 

 perhaps — we should of course still represent it by exactly the 

 same genotype. When the two members of a given factor-pair are 

 alike, (as in each of the parent plants between which this cross 

 was made), the individual is said to be homozygous for the factor 

 in question; when the two members are different (as in this 

 hybrid) it is said to be heterozygous. Now the essence of the 

 phenomenon of segregation lies in the fact that when this heter- 

 ozygous individual produces gametes, these are not hybrid or 



* The chromosomes of the nucleus arc in all probability the actual bodies 

 in which the genetic factors arc carried, and we have shown (p. 187) that in 

 the "reduction division" just preceding the production of gametes, the 

 number of chromosomes in the nucleus is halved. 



