SEGREGATION AND DOMINANCE 135 



monohybrid proportion of 3 to 1 in tlihybrid com- 

 binations is squared, (3 + 1)- = 16. 



It of course does not follow that the offspring in 

 dihybrid crosses will always be sixteen in nunil:>er, or 

 that they will always conform strictly to the theoreti- 

 cal expectation of (3 + 1)-. The ofispring ol)tained 

 undoubtedly obey the laws of chance, but the greater 

 the number of offspring, the nearer they come to fall- 

 ing into the expected grouping. 



The sixteen possible zygotes resulting from a 

 diliybrid cross will give rise to sixteen possible kinds 

 of indi\dduals which in turn, as will be demonstrated 

 directly, present four kinds of phenotypic and nine 

 kinds of genotypic constitutions. 



A dihybrid mating, using the same symbols em- 

 ployed in the case just described, would be expressed 

 algebraically as follows : — 



SG+ WY+ SY+ WG = all the possible egg gametes 

 SG+ WY+ SY+ WG = all the possible sperm gametes 



SGSG+ SGWY+ SGSY+ SGWG 



SGWY +WYWY+ WYSY+ WYWG 



SGSY + WYSY +SYSY+ SYWG 



SGWG + WYWG + SYWG+WGWG 



SGSG+2 SGWY+2 SGSY+2 SGWG+WYWY+2 WYSY+ 2 WYWG+SYSY+ 2SYWG+ WGIVG 



The second and the ninth items in this result are 

 alike ; by combining them the revised result reads : — 



SGSG+4 SGWY+2 SGSY+2 SGWG+WYWY+2 WYSY+2 WYWG+SYSY+WGWG 



There are then these nine different combinations 

 of germinal characters or nine difTerent genotypes 

 in any dihybrid cross. By placing the recessive char- 

 acters in parentheses, whenever the corresponding 

 dominant is present to indicate that the dominant 



