200 MENDELISM 



until afterwards. There is, therefore, clearly some 

 coupling between the presence of white blossoms and 

 early flowering on the one hand, and between lateness 

 and purple flowers on the other. Two characters 

 more diverse than colour of the flowers and time of 

 flowering could at first sight scarcely be imagined. 



In this last example the two characters concerned 

 do not appear to be completely, but only partially 

 coupled. This phenomenon of partial gametic coupling 

 was discovered by Bateson and Punnett in the Sweet 

 Pea, and has since been observed in a number of plants 

 and in several animals. [In animals, by far the most 

 completely known case is that of the Fruit -fly (Droso- 

 phila), investigated by T. H. Morgan and his school, 

 see p. 270.] In the Sweet Pea, one of the first cases 

 to be investigated was concerned with the shape of the 

 pollen grains, whether oval or round, and the colour 

 of the flowers, whether blue or red. 



When blue-oval is crossed with red-round, the 

 F! plants are all blue-oval. But in F 2> instead of 

 getting 9 blue-oval, 3 blue-round, 3 red-oval, and 

 i red-round, the numbers of the different kinds of 

 plants obtained closely approached the following 

 proportion 177 : 15 : 15 : 49. 



Such a series would be produced if the allelomorphs 

 concerned were associated in the gametes in the 

 following proportion: 7 blue-oval, i blue-round, 

 i red-oval, 7 red-round, as may easily be verified by 

 multiplication. Enormous numbers of plants must 

 naturally be examined before it can be asserted that 

 the series actually chosen is really the correct one ; in 

 fact, mere statistics are hardly capable of proving so 



