GENETICS AND EVOLUTION 651 
ef e 
SsVYyXSsVy 
Male gametes— SY Sy sf sy 
S| SSTY- | ESV | SEX 1- SeXy 
ila Sy | SSYy | SSpy | SsYy | Ssyy 
—— Eh! MYX Ss Vy ssY¥Y ss Vy 
SsYVy 
Ssyy ssYy ssp 
Fr, Generation (Showing progeny with a 9-3—3-1 ratio) 
In the checkerboard diagram there are evident homozygous 
(like) characters and heterozygous (unlike) characters. Any 
zygote with one or both characters for dominant (SS, YY or Ss, 
Yy) will have the appearance of smooth surface and yellow color. 
Zygotes with one dominant character in the dihybrid but with 
2 or 3 other recessives (Ssyy or SSyy) will appear with smooth 
surface and green color. Likewise, zygotes homozygous for one 
contrasting character, if that character is recessive, will appear 
recessive for that particular character regardless of how the 
second contrasting character appears. For example, if a zygote 
has the factors for ssYY, the character ss is homozygous for 
wrinkled surface, a recessive character, and homozygous for _ 
yellow color, a dominant character; the dihybrid is, then, yellow 
and wrinkled. If the dihybrid appears with characters ssYy, the 
character is still homozygous for wrinkled surface but heterozy- 
gous for yellow color; it will then appear also yellow and 
wrinkled. Although the result seems to be the same as far as 
external appearance is concerned, the second contrasting char- 
acter of Yy, being heterozygous, will not breed true for yellow in 
succeeding generations. In the like manner, if the first pair of 
contrasting characters were Ss instead of ss, again there would be 
a heterozygous character which would not breed true should that 
zygote be self-fertilized. 
With this fact as a basis, the F: generation of the cross between 
varieties of Pea plants having yellow color, wrinkled-surface 
seeds and plants having green color, smooth-surface seeds bear 
zygotes in the ratio of 9 plants with dominant.characters for both _ 
