Pure 



dominant 



parent 



Pure 



recessive 



parent 



Mendel supposed that a certain factor, or element, in the gamete brings about the dominant 

 character; In its absence, the recessive appears 



A pure dominant produces 

 gametes with the factor 



(5) (a) 



A pure recessive produces 

 gametes lacking the factor 



© (S) 



These two kinds of gametes can combine in two ways: 



Dominant egg X recessive sperm vj/ B 



Recessive egg X dominant sperm \}—y J 



' All hybrid individuals have the factor and resemble the dominant parent 



But hybrids produce two types of gametes — with factor and without 



(S) (i) @ (S 



/ / / / 



Gametes produced by hybrids can combine in four different wayn 



(1) Dominant egg X dominant sperm v y / 



(2) Dominant egg X recessive sperm \~/ /^ 



(3) Recessive egg X dominant sperm \J— |/ j* 



(4) Recessive egg X recessive sperm vtJ/ j''^ 



From these combinations three kinds of individuals can result: 

 One lacks the factor and appears to be recessive (4) 

 Three contain the factor and appear dominant (1) (2) (3) 

 But two of these are like their hybrid parents (2) and (3) 

 and one is a pure, or breeds true, like the dominant grandparent 



HOW MENDEL EXPLAINED SEGREGATION: THE PRESENCE-AND-ABSENCE THEORY 



