14 Segregation [CH. 



HflU 



and the heterozygous type is intermediate in character 



between the two pure types. 



To the detection of the genetic system of any given case 

 it is however necessary that the results of combinations 

 should be sensibly regular. When, as occasionally happens, 

 a character may sometimes behave as a dominant and 

 sometimes not, we have as yet no satisfactory means of 

 further analysis. These irregularities in dominance may 

 confidently be attributed to the disturbing effects of other 

 factors or of conditions, but the detection of such unknown 

 factors must be a long and perhaps impossible task. 



Mendel applied his method to the following seven 

 distinct pairs of characters in peas, and found that in each 

 the inheritance was similar. The dominant character is 

 put first. 



1. Height : whether tall or short. 



2. Distribution of flowers on the stem : whether 



arranged along the axis of the plant, or bunched 

 together at the top so as to form a false umbel *. 



3. Colour of unripe pod : whether a shade of green 



or bright yellow. 



4. Shape of pod : whether simply inflated, or deeply 



constricted between the seeds, i.e. as in "sugar- 

 peas " or " Pois sans parchemin." 



5. Colour of seed-skin : whether various shades of 



grey or brown, with or without violet spotting, 

 or white. The "grey" skins are always asso- 

 ciated with coloured flowers and almost always 

 with a purple or red mark in the axils. 



6. Colour of cotyledons : whether yellow or green. 



7. Shape of seeds : whether rounded or wrinkled. 



It will be observed that the first five are plant-characters. 

 In order to see the result of crossing, the seeds must be 

 sown and allowed to grow into plants. The last two 

 characters belong to the seeds themselves. The seeds of 

 course are members of a generation later than that of the 

 plant which bears them. Thus when a cross is made the 



* This is a fasciated and semi-monstrous form. 



