24 



MENDELISM 



coloured offspring; the Fj generation will all.be coloured 

 because the two factors meet and combine. You might 

 compare it to the action of one chemical upon another. I 

 have in these two bottles 1 two different chemicals dissolved 

 in a large proportion of water. They are both colourless, 

 or nearly so. 2 You observe that on mixing them together 

 a bright purple colour is produced. Something comparable 

 to this probably occurs in the production of purple flowers 

 from two white-flowered parents — i. e. two colourless 

 substances combine, and their union produces colour. 



Supposing this theory to be correct, the following 

 diagram (borrowed from Mr. Punne'tt's Mendelisni) will 



(i) 4 plants 

 where the 

 A factor 

 comes from 

 both pa- 

 rents. 



(3) 4 plants 

 where A 

 comes from 

 the other 

 (female) 

 parent only. 



(2) 4 plants 

 where A 

 comes from 

 one' (male) 

 parent only. 



(4) 4 plants 

 where A is 

 entirely 

 absent. 



help you to understand how it happens that in the Fij 

 generation we get 9 coloured- to 7- white-flowered plants. 



Let A represent one factor and B the other factor 

 which must combine in order to produce colour, and small 

 a and b their respective absence ; then the diagram shows 

 all the possible combinations of the A and B factors derived 

 from the two parents in the 16 resulting individuals. 



1 Shown at lecture. 



2 One contained a few drops of perchloride of iron, and the other a 

 few grains of salicylate of sodium. 



