CORK BREEDING 
9 
the same in either case, as may be seen from Figure 4, which illus- 
trates cliaoTammatically the segregation and recombination of a single- 
factor pair when a plant heterozygous for the given factors is 
self-fertilized. 
The Fj plants are heterozygous (/ j) for japonica, but exhibit 
only green, inasmuch as the japonica factor is recessive. The F x 
plants form two kinds of reproductive cells in equal numbers, namely, 
those carrying / and those carry- 
ing j. If such a plant is self- 
pollinated — that is, if the silks re- 
ceive pollen only from the plant 
on which they grow — the follow- 
ing four combinations will be 
produced in approximately equal 
numbers in the fertilized egg cells : 
(1) J 9 / *, (2) J 9 i a, 
(3) ; 9 / *, and (4) j 9 j 2. 
The customary symbols, 9 for the 
female parent and $ for the male 
parent, have been used to indicate 
the factors coming in from the 
egg cell and sperm cell in each 
case. Combinations 2 and 3 are 
alike in so far as their effect is con- 
cerned, as it is immaterial whether 
a given factor comes from the 
male or the female parent. Con- 
sequently three kinds of fertilized 
egg cells or plants are produced 
in the F 2 : (1) / /, (2) J j, and 
(3) j j. One-fourth of the plants 
in the F 2 , therefore, will be homo- 
zygous J J and will breed true for 
green; one-fourth will be homozy- 
gous j j and will breed true for 
japonica; and two-fourths will be 
heterozygous, / j. The heterozy- 
gous plants are entirely similar to 
the F x plants as regards the japon- 
ica factor and if self-fertilized 
again will produce green {J J), 
green {J j), and japonica (j j) plants in the F 3 generation in a 
ratio of 1 to 2 to 1. 
This ratio is of much importance in genetic studies in determining 
the method of inheritance. It also is important in connection with 
the theory of hybrid vigor to be considered later. It should be borne 
in mind that in cases of complete dominance, the individuals of the 
heterozygous (J j) class can not be distinguished in appearance from 
the homozygous dominant individuals of the first class (/ J). 
Accordingly in such cases but two kinds of individuals are apparent, 
dominants and recessives^ in a ratio of 3 to 1. If, however, the 
plants are selfed (self -pollinated), the homozygous plants will breed 
36919°— 27 2 
£ 
\ P4/?£NT 
I PLANT 
{j j 
) \ 
EGGS 
2/f/A/DS 
1/ 
0- 
[ SP£/?MS 
\ 2 KINDS 
0^ 
j 
4 
F£ffT/L/Z£D £SGS 
4 MMDS 
Fig. 4. — Diagram showing segregation 
and recombination of a single-factor 
pair when a plant heterozygous for the 
given pair is self-fertilized. The two 
types of reproductive cells, J and ;', 
may come together in four different 
ways, and the chances for each com- 
bination are equal 
