I 9 o8] 



SH ULL MEN DELIA N INHERIT A NCE 



numbers of both eggs and sperms carry the antagonistic or alterna- 

 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- 

 bility curve within the limits of probable error. Not until the number 

 of observations becomes infinite have we a right to expect absolute 



agreement with the theoretical ratios. 



Analysis of the curve presented by the 

 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- 



o% 30% 65% 100% 



FIG. 4. Lychnis dioica L. : variation in the percentage of purple-flowered offspring 

 in hybrid families of the third generation having the form (D + 2DR + R)XR; left- 

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

 extracted dominants (DXR), and the middle group the heterozygotes (DRXR); 

 superposed upon the heterozygote group is the normal curve having the same standard 

 deviation, showing the close agreement with a perfect chance distribution. 



pectation is fulfilled. As the departure of variates 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 

 only 35 per cent, of purple-flowered individuals, or the three similar 

 families that had about 65 per cent, of purple flowers, as in the twelve 

 families which consisted of about 50 per cent, purple. This is an 

 important fact that needs to be taken into account by both the student 

 and the critic of Mendelian inheritance. 



