240 A TEXTBOOK OF GENERAL BOTANY 
for round and yellow can fertilize any of the four kinds of eggs, 
and according to chance will fertilize the different kinds in equal 
numbers. Likewise the three other kinds of male nuclei will fer- 
tilize all four kinds of eggs in equal numbers. The different 
combinations produced are shown in rectangles in the lower part 
of the diagram. Such a set of rectangles may be very simply 
constructed by writing the factors for one of the kinds of eggs 
in each square of the first vertical row of squares, and the fac- 
tors for each of the other kinds of eggs in a separate vertical 
row of squares. The characters for male nuclei should be simi- — 
larly written in horizontal rows. If we examine the squares in 
the lower part of the diagram, we shall see that there are nine 
which contain both R and Y, and so represent plants with round 
yellow seed ; three which contain R but not Y, and so represent 
plants with round green seed; three which contain Y but not 
R, and so represent plants with wrinkled yellow seed; and one 
square which contains neither & nor Y, and so represents a plant 
with wrinkled green seed. The last-mentioned rectangle is the 
only one of the sixteen which represents a plant that is pure for 
both recessive. characters. Likewise, there is only one of the 
sixteen rectangles which represents a plant that is pure for both 
dominant characters. | 
A line drawn diagonally from the upper left-hand corner to 
the lower right-hand corner of the large square in the lower 
part of Fig. 234 passes through four small squares, each of 
which represents a different kind of individual that is homo- 
zygous for one of each of the two pairs of characters. Simi- 
larly, a line drawn from the lower left-hand corner to the 
upper right-hand corner of the large square passes through four 
small squares, each of which is heterozygous for both pairs of 
characters. 
The F, generation of a cross between peas having round yel- 
low seed and peas having wrinkled green seed shows very clearly 
that in this case the pairs of characters are independent of each 
other. In other words, round and yellow, and likewise wrinkled 
and green, are inherited independently of each other. — 
