W. F. V^. Wkldon 291 



of Crossing a Pca of green (recessive) cotyledoii oolcjur and wiiiiklod (recessive) 

 sliapo with one of yellow (dominant) ooionr and round (dominant) shape, and 

 denoting tiic gametic clcMncnts by tiic initial lotters of the qualities thcy rc[)resent, 

 tbe ganietes of one pea will im Mcndel's vicw contain the eluments G + W, those 

 of the other Y + R. On cross fertilisation, each dement of ouo gametc will unite 

 with the coniplementary element of the other, to form a fertilised Zygote element ; 

 the Zygote will thcrefore be of Constitution (YG + RW). Now the plant resulting 

 from this Zygote will produce each of the elements Y, G, R, and W in ecpial 

 numbers ; and one member of each pair will be included in each gamete formed ; 

 a member of each pair will enter a given gamete independently of the member of 

 the other pair; and there will result four kinds of gametes, each kind e(|ually 

 numerous, of composition {Y + R), (Y+W), (G + R), and (G+W); half the 

 gametes of each kind will be of one sex, half of the other sex, and when self- 

 fertilisation occurs, the gametes of opposite sexes will be nialed at random. It is 

 clear that the composition of the zygotes formed, and their relative frequeucy, will 

 be represeuted by 



( W W +YY), 4 ( WR + YG ), ( 11' W + GG), 



2 ( WW + YG), 2{WR+ GG), ( RR + YY), 



2{WR + YY), 2 (RR + YG), (RR + GG). 



It will be noticed that only one individual dominant in both characters, and 

 one recessive in both characters, occur in evcry IG individuals. 



Now Mendel assumes that the flower-colour of his purple boan is a combination 

 of two or niore entirely independent colour elements " which individually act like 

 any other constant character of the plant," and he proceeds to treat each of these 

 as if it were a separate unit character, like the shape or cotyledon colour of a pea. 

 Supposifig two such colour elements, A^ and A.^, in the purple flower, he regarded 

 the gamete of his Ph. multlflorus as containing these two elements (^i + A„^, while 

 the gamete of the white-flowered Ph. nanus contained two identical recessive 

 white elements (a + «). The Constitution of the hybrid Zygote he writes 

 {A^a + A„a) and as in the case of the peas referred to, he deduces nine possible 

 forms anKHig the descendants of such a zygote, occurring with the relative 

 frequcncy 



{A,A, + ^4,,.4.,), 4 {A,a + A.,a), {A^A, + aa), 

 2{A,A^^ A.a), 2{A,a + aa), (aa + A.,A„X 



2 {A 1 rt + AnA.;), 2 {an. + A.,u), {aa + aa). 



This treatnient is quite similar to that of the simple unit characters of j)eas, in 

 every respect but one. Every unit character in a pea is treated as if it were 

 represented in the gamete by oüly a Single element ; but the simple unit character, 

 whiteness, in Ph. nanus, is represented in the gamete by a number of similar unit 

 elements equal to the whole number of dissimilar unit elements present in the 

 Compound character, pui-ple, of the inultiflorus flower. Admitting this, for the 



37—2 



