VI CROSSING OVER 177 



general Mendelian scheme and the theory of its cytological basis, and 

 therefore invalidates their generality. On the contrary, the departure 

 from the ordinary equality of different classes of gametes is quite orderly 

 and, as we shall see, intelligible. Thus if ^x is the total number of gametes 

 produced we find that they occur in the following proportions : 



In the first case {AB x ah) the gametes are : 



{x +y) AB + {x-y) Ab + {x -y) aB + {x -\-y) ah. 



And in the second case {Ahx aB) : 



{x -y) AB + {x +y) Ah + {x +3') aB + {x -y) ab. 



This will be apparent from the following experiment with the sweet 

 pea {Lathy rus) described by Bateson and Punnett (191 1) and by Punnett 

 (1913) 1 : the characters concerned are the colour of the flower, blue {B) 

 and red (h), and the shape of the pollen grain, long (L) or round (/). 

 Blue is dominant over red and long pollen is dominant over round. 

 Now the Fg from the cross BL x hi is different from the F2 from the cross 

 Bl X bL, as the following parallel experiments show : 



Generations : 

 Blue, long x red, round P Blue, round x red, long 



The Fi is of course the same in both cases (blue, long), since blue is 

 dominant over red, and long over round. The number to be expected 

 in Fo, if it were a simple Mendelian case with no connection between 

 colour and pollen shape, would be the same for both experiments, and 

 would be in the proportion 9 blue, long : 3 blue, round : 3 red, long : 

 I red, round. These proportions are what would be obtained supposing 

 the four kinds of gametes BL, Bl, hL and hi were produced in equal 

 numbers. The actual Fo's obtained - show that this cannot have been 

 the case in this instance. Supposing, however, that the gametes had been 

 produced in the first experiment (blue, long x red, round) in the pro- 

 portion 



7 BL : 1 Bl : 1 hL : 7 hi, 



^ The hypothesis of reduplication advanced by these authors in explanation is funda- 

 mentally different from Morgan's view of " crossing over," which is here adopted. 

 * Included in F^ are a number of Fg families descended from heterozygous 1^., plants. 



N 



