DEVELOPMENT AND HEREDITY 1267 



In this case the child can be regarded as representing an arithmetical mean 

 of both its parents. In certain respects however, a quality seems to be 

 transmitted from parent to offspring either completely or not at all. This 

 is specially applicable to those characters which have been rapidly produced 

 by artificial selection, characters which, if artificial selection be abandoned, 

 rapidly disappear, with reversion to the type from which the special strain 

 was ultimately produced. 



The way in which these characteristics are transmitted was first 

 studied by Mendel and has been formulated as Mendel's law. Mendel's 

 first experiments were carried out on peas. On crossing a tall plant 

 with a dwarf plant, seeds were obtained from which all the plants were 

 tall. On recrossing the plants of this generation among one another, a 

 third generation was obtained in which 25 per cent, of the plants were 

 dwarfs and 75 per cent, were tall. Crossing the dwarf plants among 

 themselves led to the production of dwarf plants through successive 

 generations. Of the 75 per cent, tall plants one- third and all their descend- 

 ants continued to produce tall plants when self-fertilised, whereas of the 

 remaining two-thirds of the tall plants 25 per cent, produced dwarfs and 

 75 per cent, produced tall plants. On continuing the process of breeding, the 

 dwarf plants when self-fertilised always produced dwarfs, whereas of the 

 tall plants 25 per cent, produced tall plants which bred true, while the 

 remaining 50 per cent, produced the same percentage of tall and dwarf as in 

 the preceding generations. Mendel explained these results by the assump- 

 tion that a character could be dominant or recessive. If both characters 

 were present together in one plant it would partake of the dominant type ; 

 the fact that this plant possessed the recessive character would be shown only 

 by the results of breeding. In the case of the peas the tall character was 

 dominant over the dwarf. Thus when the tall and dwarf pea were crossed 

 the first generation of plants would exhibit the dominant character and be 

 tall. In the second generation however, 25 per cent, of the individuals 

 would be pure dominants (D + D), 25 per cent, would be pure recessive 

 (R, + B), while 50 per cent, would be mixed (D -J-- R). The pure dominants 

 bred together would always give rise to nothing but pure dominants, the 

 recessive to recessive, while the mixed type would always, as before, give 

 rise to 25 per cent, pure dominants, 50 per cent, mixed, and 25 per cent, 

 pure recessives. These results may perhaps be made clearer by the following 



Table : 



D + R 



DR 



D 25% D 50% DR 25% R R R 



