PLANT HYBRIDIZATION BEFORE MENDEL 301 



We have here 9 yellow round, 3 yellow wrinkled, 3 green 

 ROUND, and 1 green wrinkled. This is, to be sure, a table of 

 appearances only, or what are known as the "phenotypes." This, 

 then, is the way in which the plants or zygotes, formed by the 

 gametic union of one AaBb hybrid with another AaBb hybrid, 

 actually look. What they actually are is expressed in Table II. 

 We have in Table II a zygote AA.BB, produced by the combina- 

 tion of a gamete bearing the combination AB with a gamete bear- 

 ing the character-combination AB. Such an organism has been 

 called homozygous, the gametes forming it being alike for both 

 characters. At the end of the same row we find a zygote whose 

 constitution is AaBb, produced by a combination of a gamete AB 

 with a gamete ab, which combination is called heterozygous, the 

 gametes forming it being unlike for both characters of the two 

 pairs. We have also in the same rows, zygotes AABb, and AaBB, 

 which are heterozygous for color (Bb), in the first case, and for 

 form (Aa), in the second case. We may then have organisms that 

 are homozygous (i.e., alike) for both pairs of characters (DDrr) ; 

 homozygous for a single pair of characters (DD) and hetero- 

 zygous (i.e., unlike) for another pair (Dr) ; or heterozygous for 

 both pairs (DrDr). If the combinations in Table III, representing 

 the behavior of two character-pairs in fertilization, be compared 

 with Mendel's way of stating the combinations, using A and a 

 for round and wrinkled, and B and b for yellow and green, re- 

 spectively, differences will appear which should be explained. 

 According to Mendel's form of statement for example, wrinkled 

 yellow in the zygote is represented by Mendel as aBb. Plainly 

 this comes about as the result of the combination of a wrinkled 

 yellow gamete (aB), with a wrinkled green gamete (ab) using the 

 letter "a" but once for the character represented. Such a combina- 

 tion at the end of the second row in Table II as is now represented 

 by aa.Bb Mendel represented by aBb,. because, since the "a" char- 

 acters in the two gametes were alike, he felt no need of represent- 

 ing the character in the zygote by double letters. But since the 

 "B" character, uniting with the "b" character, gave a zygote char- 

 acter of double composition, he represents it by "Bb." At present, 

 it is of course the practice to represent the actual gametic condi- 

 tion in the zygote by giving the letters representing the full 



