172 



HEREDITARY CHARACTERS 



both dominants, are present in one parent, and two re- 

 cessives in another parent, among the descendants indi- 

 viduals with the two dominant characters pure and other 

 individuals with the two recessive characters pure will 

 appear in Mendelian proportions. If, however, each parent 

 possess a dominant and a recessive character, the two domi- 

 nants and two recessives may be segregated together. Again, 

 a particular character exhibited by one of the parent indi- 

 viduals may not be a pure character, the individual being 

 an impure dominant. Thus, if the impure dominant tall 



Q"'O 



FIG. 21. Diagram illustrating a cross between an impure dominant and a recessive. 

 The small circles represent the gametes, white dominant, black recessive. The 

 large circles represent individuals, and are tinted to show whether the dominant 

 or recessive character is apparent. It will be seen that a pure dominant cannot 

 result from this cross, and from it are produced individuals of which 50 per cent, 

 exhibit the dominant, 50 per cent, the recessive. 



pea be crossed with a pure dwarf recessive, while the pure 

 dominant has equal numbers of gametes containing the 

 dominant and the recessive character respectively, the 

 pure recessive produces only gametes containing the re- 

 cessive character. In this case an equal number of tall 

 and dwarf plants will be produced. All the tall plants, 

 however, will have a recessive character, because they 

 could only be fertilised by gametes derived from the pure 

 recessive parent ; whereas there was only a possibility of 

 half the gametes of the recessive parent fusing with gametes 



