464 GENETICS 



superficially resemble one another, they are dissimilar as parents. 

 One-third of this tall group give rise in successive generations to 

 tall offspring without exception. Such tall specimens, which con- 

 stitute one-fourth of the total F2 generation, correspond, in their 

 resemblance to the pure tall pea of the Pi generation, to that 

 quarter of the group that are like the dwarf pea of the Pi. The 

 remaining two-thirds of the tall peas of the Fo generation, or one- 

 half of the offspring of the hybrid tall peas, are like their parents. 

 When interbred, they give rise to offspring in the ratio of three 

 tall to one dwarf. As shown in the diagram, this group 

 again breaks up, when analyzed, into three types that occur in the 

 ratio of 1 : 2 : 1 ; that is, one-fourth are pure tall peas, one-half 

 hybrid tall peas, and one-fourth pure dwarf peas. In the case 

 under discussion, tallness is said to be "dominant" to dwarf ness. 

 Conversely, dwarf ness is " recessive " to tallness, and is obscured 

 or suppressed by it when the two occur together in a hybrid indi- 

 vidual. However, neither characteristic is altered in any way by 

 association with the other, as is clearly shown by the " purity " 

 of individuals of the second generation for these qualities. The 

 essential contribution of Mendel's work to our knowledge of 

 heredity is this demonstration of the purity of germ-plasm with 

 respect to determiners of characters that are inherited. Some ele- 

 ments of the hereditary make-up of an individual may be tempo- 

 rarily obscured by others, but segregation and recombination will 

 occur in new generations. The analogy to a deck of cards from 

 which new hands can be dealt after shuffling is one that illustrates 

 the possibilities of segregation and recombination: the separate 

 cards of the deck remain the same. 



It has been stated that the offspring of a cross between tall and 

 dwarf peas are tall. Superficially, they cannot be distinguished 

 from their tall parent. Yet these tall hybrids when bred together 

 give some tall and some dwarf individuals. The tall peas of the 

 P2 generation are all similar in appearance; but experimental 

 breeding proves that some reproduce only tall peas, while others 

 are like their parents in giving rise to both tall and dwarf offspring. 

 In such cases of inheritance with dominance it is impossible to dis- 

 tinguish, externally, a hybrid individual from an individual that 

 will breed true for the dominant character. On the other hand, an 

 individual that appears recessive will always breed true. 



If any dominant characteristic be represented by D and a^.> 



