MENDELISM 109 



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

 ing into the expected grouping. 



The sixteen possible zygotes resulting from a dihybrid 

 cross will give rise to sixteen possible kinds of indi- 

 viduals which in turn, as will be demonstrated directly, 

 present four kinds of phenotypic and nine kinds _o 

 genotypic constitutions. 



A dihybrid mating, using the same symbols em- 

 ployed in the case just described, would be expressed 

 algebraically as follows:-*** 



SG+ WY+ /SF+ WG = all the possible egg gametes 

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

 80SG+ 80WY+ SGSY+ SGWG 



SGWY + WYWY + WYSY+ WYWG 



SGSY -f WYSY +SYSY+ SYWG 



SO WO + WYWG + SYWG+WGWQ 



SGSG+2SGWY+28G8Y+2SGWG+WYWY+2WYSY-\-2WYWG+SYSY+28YWG-}-WGWG 

 \+~* V~ 



The second and the ninth items in this result are 

 alike; by combining them the revised result reads: 



>gOSG + 4SGWf+ 2SGSY+ 2SGWG+WTWY ^T 



There are then these nine different combinations 

 of germinal characters or nine different genotypes 

 in any dihybrid cross. By placing the recessive char- 

 acters in parentheses whenever the corresponding 

 dominant is present, to indicate that the dominant 

 causes the former to recede from view, these nine geno- 

 types may be combined into four phenotypes as shown 

 in the table at the top of page 110. 



From this analysis it may be said that the Mendelian 

 ratio for a typical dihybrid is phenotypically 

 that for a moriohybrid, as we have 



