THE PHENOMENA OF MUTATION 171 



new dominant character. A recessive character is 

 the absence of some positive character, and if in the 

 cell-divisions of gametogenesis the factor for the 

 positive character passes wholly into one cell, the 

 other will be without it, will not ' carry ' that factor. 

 If such a gamete is fertilised by a normal gamete the 

 organism developed from the zygote will be hetero- 

 zygous, and segregation will take place in its gametes 

 between the chromosome carrying the factor and the 

 other without it, so that there will now be many 

 gametes destitute of the factor in question. When 

 two such gametes unite in fertilisation the resulting 

 organism will be a homozygous recessive, and the 

 corresponding character will be absent. In this way 

 we can conceive the origin of albino individuals from 

 a coloured race, supposing the colour was due to a 

 single factor. 



In Bateson's opinion the origin of a new dominant 

 is a much more difficult problem. In 1913 he dis- 

 cussed the question in his Silliman Lectures. 1 He 

 considers the difficulty is equally hopeless whether 

 we imagine the dominants to be due to some change 

 internal to the organism or to the assumption of 

 something from without. Accounts of the origin of 

 new dominants under observation in plants usually 

 prove to be open to the suspicion that the plant was 

 introduced by some accident, or that it arose from 

 a previous cross, or that it was due to the meeting 

 of complementary factors. In medical literature, 

 however, there are numerous records of the spon- 

 taneous origin of various abnormalities which 

 behave as dominants, such as brachydactyly, and 

 Bateson considers the authenticity of some of 



1 Problems of Genetics, Oxford Univ. Press, 1913. 



