352 MENDEL'S LAWS 



classes: one-fourth are red, like one of the grandparents; 

 one-half pink, like the hybrid Fj generation; and one- 

 fourth white, like the other grandparent. The reds and 

 whites breed true in successive self-pollinated generations 

 but the pinks are always hybrids and always produce 1 

 red; 2 pink; 1 white when self-pollinated. 



The Purity of Gametes. — At the time Mendel 

 worked with his peas no one knew the structure of cells 

 so he was compelled to formulate a purely hypothetical 

 explanation in order to understand why the grand-par- 

 ental types appear in a definite ratio in the Fj and later 

 generations along with the hybrids. His explanation ap- 

 plied to the four o'clocks would be as follows: (1) The red 

 parent and the white one each transmits to the pink hy- 

 brid through their respective germ cells some sort of a de- 

 terminer for flower color, whose interaction produces a 

 pink color; (2) When the pink hybrid forms its germ cells 

 or gametes the two determiners are separated and any 

 particular gamete may receive either the " red " deter- 

 miner or the " white " one but not both. (3) Hence some 

 gametes are pure for red and an equal number are pure 

 for white. (4) The chance combinations of equal numbers 

 of " red " and " white " gametes will produce one-fourth 

 red X red, (pure red), one-half red x white, (hybrid pink), 

 and one-fourth white x white (pure white) combinations. 



The Recombination Ratio, 1:2:1 can be readily 

 understood by examining the accompaning diagram. 



Red XT -jrRed 



SPERMS< 



^EGGS 



LWhite^i:^ ^ White J 



4 



Fio. 98 is a diagram showing possible combinations of red and 

 white sperms with red and white eggs to produce the 1:2:1 ratio. 



The possible combinations are four. Combination 1 

 occurs one time in four and produces pure red. Combina- 



