BACK CROSS 



36T 



results than mating hybrids with one another, in that the 

 recessive parent, being pure, produces only one kind of 

 gamete. Moreover, since these gametes have only reces- 

 sive genes they will not obscure any dominant genes of 

 the hybrid with which they unite. The back cross is 

 thus particularly well suited for studying linkage phe- 

 nomena. The next paragraph will show how it works 

 in crosses involving independent characters. 



Back Cross involving Independent Characters. — 

 When the hybrid tall purple peas discussed in the preced- 

 ing chapter were mated to one another or self-pollinated 

 the second hybrid generation consisted of four phenotypes 

 in the ratio of 9 :3 :3 : 1. If this same hybrid is back-crossed 

 to the double recessive, short white, the same four pheno- 

 types will be produced but in the ratio 1:1:1:1. The table 

 shows why this is so. 



Four kinds of eggs which are produced in equal numbers 

 by the tall, purple hybrid. 



1 kind of 

 sperm of 

 short white 



tp 



Fig. 104. — Punnett Square (showing results of back-crossing a tall 

 purple hybrid to a pure short white). 



Linked Characters in the First Hybrid Genera- 

 tion behave just like independent characters, in that it 

 makes no difference from which parent the dominant 

 characters come. For example, (1) if a long- winged gray-, 

 bodied fly is mated to a vestigial-winged black-bodied one 

 the hybrid offspring will all be gray-bodied and long- 

 winged because these tw^o characters are dominant (Fig. 

 105). (2) If the same two character-pairs are introduced 

 into the cross by mating a gray-bodied vestigial-winged 

 fly with a black-bodied long-winged one the result will 

 still be gray-bodied long-winged hybrids, just as would 



