22 PHYSICAL BASIS OF HEREDITY 



factors for tall (SS). The chance combination, just given 

 may be represented graphically as follows : 



( Ovules Tall .^ ^ Short 



fA t >< t 



( Pollen Tall / \ Short 



( Tall-Short. 

 Tall- Tall, -f ^ +Short-Short. 



^/Tall-Short. 



In the actual experiment that Mendel carried out, plants 

 of the tall race measured from 6 to 7 feet, and those of 

 the short plants three-quarters to one foot and a half. 

 The 2^1 plants were as tall as, or even taller than the tall 

 parent. When these F^'s were self -fertilized, the seeds 

 (either from the same plant or from a random collection 

 of seeds from different F^ plants) produced 787 long 

 plants and 277 short plants — a ratio of 2.84 to 1. 



As a fair sample of each plant, ten seeds were taken 

 from each of 100 tall plants of this second (or Fg) genera- 

 tion. Out of the 100 plants so tested, 28 plants produced 

 only tall plants, while 72 of them produced some tall 

 and some short offspring. This means that 28 plants 

 were pure (homozygous) tall, whilst 72 were hybrid like 

 the Fi plants. Taking, then, all F2 plants together, the 

 results show % were short, V4 were hybrid, and % were 

 tall, i.e., they stand in a ratio of 1 : 2 : 1. 



This relation is illustrated in the scheme below, based 

 on what 16 F2 plants might give. Twelve would be tall 

 to 4 short. If the tall plants are tested, they are found 

 to consist of 4 pure tails (SS) and 8 hybrid tails {sS). 

 Altogether, then, there are 4 tails to 8 hybrid tails to 4 

 short, i.e., there are three kinds of F2 peas in the ratio 

 of 1:2:1. 



12 tall -1- 4 short 



, ■ , « , 



4SS + 8sS -f- 4ss 



1 2 1 



The process of disjunction, or separation of the mem- 

 bers of a pair of factors, is known technically as segre- 

 gation. While we sometimes also speak of the segrega- 



