ii6 INTkODlK"! ION TO SKXUAl. IM lYSIOl.OGY 



ji^(», iiiiiy Ix' Siiid ill one icsix-cl- fo he nil cxlciisioii ol Mic llicory 

 (tf \\'('i.siii;iiiii, lor it likewise, jissimies 1 liiit a,c(jiiire(l cluiriielcrs Jinj 

 not- 1 riinsMiillcd, vvliile ils Fiiodcni (levelopnieiiis jifc, siis('('j)iil)l(; 

 ol" a, coiirniniiiory iiitcrpn't jiI ion (lerivcd Iroiii I lie study ol clironio- 

 soMie iiilieritaiKH;. It. irinrk'S ;i very deliiiitc, and iinportaiit/ 

 advance n|)on VVeisinaiiirs tlieory in t liai it. eiia,bJes one, to diseuss 

 variability in terms ol" t/lie ('onju<^a.t.in^ eclls themselves, and 

 not merely in terms of t lie resnltin<i; zygotes. 



'IMie. original e,\|)(;riment-s ol" Mendel were; upon liyhridisat ion 

 in \)ViiH, the. two parent, varieties initially seleeted dilTerin;^^ from 

 eaeli other in one particular character. The hybrids produced 

 by crossin<^ wen; all similar superlicia.lly, and resembled one 

 of the parents in the charaeter in (piestioii, \vhi(;li was therefore 

 called the (loniiiian/ charaelei', the other character bein^ known 

 as iccrsslvc. When the hybrids wr.w. crossed amon^ themselves, 

 approximately one-half of t he olTsprin^ were found to be identical 

 with their hybrid parents (dominant, liybrids), oiie-(piarter 

 i-esenil)le(l one of the ori<^ina.l varic^ties (the ^ra,ndpa,r(!nt< with 

 the dominant chanicter), while the remaininj^ (piarter W(;re like 

 the other pure variety (the; »^ra.nd parent, with the recessive 

 chaiactei). ( onsiujuently the pure dominants and the dominant 

 hybrids resembled one another outwardly, but they dilTered in 

 their capacity to transmit the characteristics in (piestion, since 

 the pure dominants alone invariably bred true. The recessives 

 also always bred true. Meiuh't drew the conclusion that in the 

 h\d>rid the nrainctes (both male and female) were of two kinds, 

 which were jcspect ively identical with the two kinds icpresentetl 

 by the gametes of the ori<^inal pure varieties. Tin; dilTerentiatiou 

 uf gametes carrying dilTerent characters is the essential priiicipie 

 in Menders theory, the existence of dominant and recessive 

 characters, though often observable, being by Jio means 

 universal. 



Another example^, taken irom the work of liateson and 

 l*unnett, will be sullicient to elucidate fuither the Mendelian 

 conception of gametic difl'erentiation. Breeders of blue Anda- 

 lusian fowls have always recognised the practical impossibility 

 of obtaining a pure strain of this breed. However carefully 

 the birds are selected they invariably produce two sorts of 

 " wast.ers,'' some l)eing pure black, and some white with irregular 

 black marks ur splashes. Bateson and Punnett were the first 



