558 STRUCTURE AND LIFE HISTORIES 
where a and b each equal i. In the plants the multi- 
plication of inheritances (produced in fertilization) was 
as follows: 
eggs (s + w) X sperms (s + w) = ss + 2siv + ww 
where w = wrinkling and s = absence of wrinkling, i.e., 
smoothness. 
479. Theory of Purity of Gametes. The above ratio 
is what we would expect if half of the egg-cells and half 
of the sperm-cells in a heterozygous plant (one of the FI 
generation), carried only character-units or determiners 1 
that make for smoothness; the other half only those 
factors that make for wrinkling, giving 5 and w egg-cells, 
and 5 and w sperm-cells in equal numbers. Therefore, in 
pollination the chances would be equal that an s-egg would 
be fertilized with either an s-sperm or a w-sperm, giving 
(s + w) X (s + w) = ss + zsw + ww. Since s is dominant 
over w the product should be written: 
ss + s(w) + s(w) -f- ww 
giving in external appearances $s + iw. Since the re- 
sult actually observed is what it would be if the gametes 
were thus "pure" for smoothness and wrinkling, Mendel 
concluded that they really are, and moreover that each 
character behaves as a unit, appearing and disappearing 
in its entirety. 
480. Character-units versus Unit-characters. As just 
stated, Mendel held that the various visible characters of 
his plants (dwarfness, for example) behaved as units, 
1 The substance or condition (protoplasmic constitution), whatever it is, 
in the germ-cells that corresponds to any given character of the plant is 
variously referred to by the terms factor, determiner, gene (= producer), 
character-unit, and others. These terms are essentially synonyms. 
