166 Plant-Breeding 



be dominant in any species we cannot determine until we 

 perform the experiment; that is, there is no mark or 

 attribute which distinguishes to us a priori a dominant or 

 a recessive character. However, the mere fact as to 

 whether the one or the other character is dominant is 

 relatively unimportant, for constant dominance is no 

 more a regular behavior than recessiveness is. In various 

 subsequent experiments it has been found that even 

 when marked dominance is not shown in the first product, 

 the hybridization may follow the law in essential numeri- 

 cal results. The really important points are : (1) That 

 the characters typically remain pure or do not blend, 

 and (2) that their reappearance follows a numerical 

 order. 



Explanation of mendelian results. After finding such 

 surprising results as these, Mendel naturally endeavored 

 to discover the reasons why. The product of his specu- 

 lations is the theory of gametic purity (to use our present- 

 day terminology), which is a partial theory of heredity. 

 Every plant is the product of the egg, or female, cell 

 fertilized by the sperm, or male, cell. When constant 

 progeny is produced, it must be because the two cells, or 

 gametes, are of like character. When inconstant progeny 

 is produced, it must be because the sperm-cell is of one 

 character and the egg-cell of another. When these un- 

 like gametes come together, they will unite according to 

 the law of mathematical probabilities, one-fourth of those 

 of each kind coming together and one-half of those of 

 both kinds coming together. If A and B represent the 

 contrasting parental characteristics, they would combine 

 as: 



