igoS] 



SHU LL— MEN DELI AN INHERITANCE 



113 



carry 



tive qualities making up a pair of Mendelian units. These different 

 germ cells unite according to the laws of chance, and we should mean 

 therefore by the expression "Mendelian expectation" that our obser- 

 vations if seriated in the form of a curve will present a normal proba- 



A^ithin the limits of orobable error. Not until the number 



curve 



of observations becomes infinite have we a right to expect absolute 



r 



agreement with the theoretical ratios. 



20 



Anal 



heterozygote families of Lychnis dioica, as 

 represented in the diagram, shows that it 

 approaches closely to the normal probability 

 curve, and that therefore every proper ex- 



30% 



lOOVc 



Fk3. ^.--Lychnis dioica L. : variation in the percentage of purple-flowered offsprxt^ 

 in hybrid families of the third generation ha\-ing the form (D + aDR-f R)XR; 1^- 

 hand group represents the extracted recessives (RXR), the right-hand group the 

 extracted donunants (DXR), and the middle group the heterozygotes a>^><*^2; 

 superposed upon the heterozygote group is the normal cur^-e ha.'lng the same standard 

 deviation, showing the close ^eement with a perfect chance distribution. 



pectation is fulhUed. As the departure of \-ariates from the mean 

 is a fundamental part of the law of chance, the fulfilment of expecta- 

 tion is just as complete in the case of the one DRXR family that had 

 onlv 35 per cent, of purple-flowered individuals, or the tliree similar 

 fan^ilies that had about 65 per cent, of purine flowers, as in the twelve 

 families which cons^ted of about 50 per cent, purple. This is an 

 important fact that needs to be taken into account by both t!^ student 

 and the critic of Mendelian inheritance. 



