114 GENETICS 



mature germ-cells, unite, they form a zygote having the 

 proper number of determiners normal to the species 

 in question instead of double that number. Symbols for 

 dominant characters in the diagram are placed on the 

 outside of the somatic arcs, because these are the char- 

 acters that are visible or phenotypic, while the non- 

 apparent recessives are placed on the inside out of 

 sight. 



11. THE CASE OF THE TRIHYBRID 



Mendel went even further and computed the possi- 

 bilities which would result when two parents were 

 crossed differing from each other with respect to three 

 unit characters. He found that the results actually 

 obtained by breeding closely approximated the theo- 

 retical expectation. 



This expectation in the case of a trihybrid cross is 

 that the cross-breds resulting will all exhibit the three 

 dominant characters, while their genotypic constitution 

 will include six factors, namely, these three dominant 

 characters plus their corresponding recessives or "ab- 

 sences." 



Cross-breds of the first generation will, therefore, 

 have eight possible kinds of triple gametes and when 

 interbred may form a possible range of sixty-four 

 (8X8) different zygotes, which corresponds to a 

 monohybrid raised to the third power (3-f-l) 3 . These 

 sixty-four zygotes group together in eight different 

 phenotypes and twenty-seven different genotypes as 

 shown on page 116. 



The trihybrid cross with its resulting combinations 



