72 HEREDITY 



fairly close to expectation. We may conclude that 

 segregation probably followed the usual rule, but the 

 reason for the irregular dominance remains unknown. 

 It is interesting to note that the degree of dominance 

 has been found to vary in different families, and to be 

 inherited. 



We do not know how frequent such cases are. It 

 is interesting enough to know that they do not form 

 exceptions to the law of segregation, but such irregu- 

 larities must necessarily interfere with the practical 

 application of Mendelism. It is not impossible, however, 

 that some fuller explanation of these phenomena may 

 yet be found. 



It is interesting to note the harmony that exists 

 between Mendelian theory and the facts of " pure line " 

 inheritance. In a Mendelian crossing, all the indivi- 

 duals of the first generation are hybrid. In the second 

 generation, half are hybrid and half are pure with 

 respect to any particular character. It is easy to see 

 that if all these individuals reproduce at the same rate, 

 by self -fertilisation, the number of hybrids will be 

 reduced to one-quarter in the next generation. The 

 fraction of the whole population that are hybrid is 

 thus halved in each successive generation, and the 

 proportion would soon become negligible. A self- 

 fertilising population would therefore be expected to 

 consist of a number of perfectly pure types, showing 

 no variations of hereditary value. This is exactly the 

 state of affairs, as discovered by the pure line experi- 

 menters. It appears fairly certain, then, that segrega- 

 tion occurs with regard to the special characters of 

 slightly differing pure lines. We have no means of 

 knowing whether segregation is according to the Men- 

 delian scheme, but there is evidently a presumption 

 that it is so. 



In many cases it is impossible to carry out 

 actual Mendelian analyses of inheritance, owing to 

 the fact that the distinctions to be dealt with are 

 too vague and irregular to be readily classified. Most 

 of the differences between pure lines are of this nature. 



