THE MENDELIAN METHOD 



odoratus, Batcson and Punnett in 1902 observed segregation for 

 flower colour (P-p) and pollen shape (L-/). Each factor gave a 3 : i 

 ratio, three purples to one red, three long-pollen to one round- 

 pollen. But the combinations of these characters were not in the 

 9:3 : 3 : I ratio that Mendel had found. They were as follows: 



Purple Long Purple Round Red Long Red Round 



1,528 106 117 381 



Evidently the four classes of gamete were not being produced in 

 equal numbers. The double dominant and double recessive classes, 

 PL and pi, must have been about seven times as numerous as their 

 alternatives Pi and pL. Later, other cases were found in which the 

 two classes of gamete with one dominant and one recessive (such 

 as Ab and aB) were in excess. 



The reason for this behaviour was made clear by Morgan and 

 his colleagues in their experiments with Drosophila melanogaster in 

 1910 and later years. In this small fruit fly, which raises its enormous 

 progenies in a ten-day cycle, they studied the inheritance of some 

 hundreds of factors, to which they gave Johannsen's name o£ genes. 

 These, they found, fell into four groups. A gene from one group 

 would segregate independently of any gene from the other three. 

 But two genes from the same group showed linked inheritance. 

 Certain of the combinations of these genes were more frequent in 

 the gametes than their alternatives. And these favoured combinations 

 (whether dominant with dominant or dominant with recessive) 

 turned out to be the ones which had been present together in the 

 parents. 



For example, let us take the case of females produced by crossing 

 the double dominant wild or standard type B B Vg Vg, with the 

 double recessive h h vg vg, having a black body and vestigial wings. 

 These females were back-crossed to hh vg vg males and their progeny 

 were as follows : 



Bb Vg vg ^ ^ ^S ^S ^^ ^S ^S ^ ^ ^'S ^S 



586 106 III 465 



Thus the old or parental types of gamete are about five times as 

 common as the new or recombinant types. To be precise, in this 

 experiment, 17 •! per cent of gametes show recombination. 



When, however, the doubly heterozygous females came from 



42 



