70 



GENETICS IN RELATION TO AGRICULTURE 



experiments it so happened that one member of each of the seven pairs 



of characters displayed complete dominance. 



The explanation for the appearance of the recessive character in 



the second generation and in subsequent generations rests on the fact 



that the contrasted characters are 

 represented in the germ cells by units 

 or factors. The factor for tallness 

 may be represented by T and the 

 factor for the contrasted character 

 dwarfness by t. The relations which 

 exist when plants bearing these dif- 

 ferent factors are crossed are shown 

 in Fig. 34. In the tall race of plants 

 the gametes all bear the factor T, 

 so that since any individual of this 

 race arises from the union of two 

 germ cells its genetic constitution 

 with respect to this particular factor 

 is TT. Similarly the genetic con- 

 stitution of any plant of the dwarf 

 race is represented by tt and it pro- 

 duces germ cells each of which bears 

 the factor t. When tall and dwarf 

 plants are crossed, the hybrid receives 

 a factor T from one germ cell and a 

 factor t from the other, so that the 

 tall hybrids which are produced are of 

 the genetic constitution Tt. 



In the production of germ cells 

 and in the union of these germ cells 

 to produce the individuals of the 

 second generation is seen the opera- 

 tion of Mendelian principles. The 

 contrasted units T and t separate in 

 the germ cells of the offspring so that 



a particular germ cell receives only one of these factors, either T or t. 



Half the germ cells consequently bear the factor T and half bear the 



factor t, and this is true of both pollen grains and ovules. 

 When a tall hybrid plant is self-fertilized, therefore: 



a T ovule may be fertilized by a T pollen grain producing a TT plant, tall, 

 a T ovule may be fertilized by a i pollen grain producing a Tt plant, tall hybrid, 

 a t ovule may be fertilized by a T pollen grain producing a Tt plant, tall hybrid, 

 a t ovule may be fertilized by a t pollen grain producing a it plant, dwarf. 



FIQ. 34. Diagram showing factor history 

 in a cross between tall and dwarf peas. 



