356 MENDEL'S LAWS 



is said to be dominant to " short " and " short " recessive 

 to tall. When the tall hybrids are self-pollinated they 

 produce an Fo generation which consists of 3 tails to 1 

 short. Obviously this result is due to the fact that the 

 pure tails and the hybrid tails look alike. If each tall 

 plant is self-pollinated and its seeds sowed separately, it 

 is found that two- thirds of them are hybrids (i.e., their 

 seeds produce both tails and shorts in a 3:1 ratio) and 

 one-third are pure tails (producing only tails). The 3:1 

 ratio is therefore only a special case of the 1:2:1 ratio 

 and really consists of one-fourth pure dominants' (i.e., 

 tall): two-fourths hybrid dominants: one-fourth pure 

 recessives. 



The Law of Segregation. — Heredity in a simple 

 cross between two individuals involving a single pair of 

 characters may be summed up in the following statement 

 or law: 



The paired genes which are responsible for the charac- 

 ter difference segregate from one another in the reduction 

 divisions of hybrids and thereby give rise to equal num- 

 bers of gametes pure for one or the other gene; and these 

 gametes then combine by cliance at fertilization to pro- 

 duce the F> or second hybrid generation, which will con- 

 sist of approximately one- fourth of the individuals pure 

 and like one grandparent, one- fourth pure and like the 

 other grandparent, and one-half hybrid, which half will 

 always be like the first hybrid generation and either like 

 one grandparent or intermediate between the two grand- 

 parents. 



The Relation of Genes to Characters. — Though 

 the language used in speaking of genes and characters is 

 such that the student might readily gain the impression 

 that a particular gene is the sole cause of a corresponding 

 character, this is far from the truth. A moment's con- 

 sideration will show that a four o'clock flower cannot have 

 a red color unless many genes have first wrought their 

 cooperative effect to develop the plant and flowers. All 



