140 GENETICS 



making up the arcs within which are inclosed the 

 germ-cells after their reduction through maturation, 

 which results in giving to each germ-cell half the 

 number of determiners that are present in the soma- 

 tic cells. 



It will be remembered that when two gametes, 

 or 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 characters that are visible or pheno- 

 typic, while the non-apparent recessives are placed 

 on the inside out of sight. 



11. THE CASE OF THE TRJHYBRID 



Mendel went even further and computed the 

 possibilities 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 theoretical 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 "absences." 



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 



