MISSOURI BOTANICAL GARDEN BULLETIN 137 
planted they  patapee some plants which bore yellow seed, 
and others which produced seed both yellow and green in 
the ratio of three to one. The latter plants were therefore 
hybrid in nature since they possessed the two types which 
were combined in the seeds of the first generation. The 
green character of the seed in this case is now spoken of 
as “recessive” and the yellow as “dominant,” because in 
hybrid plants when both characters are present in the germ, 
only one of them, the yellow, appears. This led Mendel to 
attempt to explain the three-to-one ratio, and he believed 
that in the germ cells of the first generation hybrids, as 
well as in those of later generations which contained the two 
characters, the yellow and green factors were separated so 
that each plant contained the capacity of producing one or 
the other character but not both. The differences then which 
led to the production of one character or the other must 
therefore be segregated at some time in the production of 
the elements which fuse to ultimately produce seed. 
Still another pair of factors experimented with by Mendel 
was that of size, by crossing a pea plant which was normally 
tall with one which was normally short. It is now known 
that in many plants and animals if a dwarf race is crossed 
with a tall one that the resulting progeny are all tall. This 
was true in the case of peas, but in the second generation, 
derived from self fertilizing the first generation, there was 
obtained approximately the ratio of three tall to one short. 
If we represent the dominant character, tallness, by T 
and the recessive character, shortness—which may be 
thought of as the absence of tallness—by ¢, then the first 
generation hybrid would possess both characters and be 
represented by Tt. Since tallness is dominant, however, 
none of the plants would be short. In the next generation, 
these characters would unite in the Meant four possible 
combinations: seeds containing only the tall character repre- 
sented by T'7'; seeds possessing tallness and shortness, 7't; 
possessing shortness and tallness represented by ¢7’; 
and finally those possessing only shortness represented by 
tt. Since tallness is completely dominant over shortness 
(represented by T) the first three combinations would pro- 
uce nothing but tall plants, and although the germ cells 
were differently constituted, it would be impossible to tell 
one lot of plants from the other. The plants which possess 
only shortness, however, (tf), would of course be dwarfed, 
and consequently in the second generation the ratio of tall 
to short plants would be three to one. The same principles 
apply to all cases of Mendelian inheritance, although many 
complications have, as the result of actual experience, 
