Recent Advances tn the Study of Heredity. 241 
A zygote which is the result of the union of dissimilar gametes is 
termed a heterozygote. 
The number and nature of zygotes formed from the random 
union of gametes of known contents can also be—and I think most 
conveniently—represented in tabular form as follows :— 
<? Y G 
YY 
YG 
GY 
GG 
The figures at the top of the table relate to the two kinds of 
male gametes; those to the left of it, the two types of female 
gametes; and the four zygotes produced are given in the four 
squares of the table. It is customary to limit the use of such 
a table to the representation of the result of matings in which more 
than two characters are concerned ; but it is advisable to be familiar 
with its use for a single pair of characters, because the application 
of Mendelian analysis to the interpretation of certain phenomena 
of the inheritance of sex can be much more clearly illustrated by 
means of such a four-square table, than by any other means. 
We may now proceed to consider the results of the unions 
between the various zygotic types ; and, in considering these, it 
will be convenient to designate the dominant members of such a 
pair of characters as yellow and green by D and the recessive by 
R. There are thus three zygotic types, DD, DR and RR. The 
following is a list of the various unions in which these types may 
take part, together with their results. 
Percentage Numbers of :—• 
a. 
DD 
X 
DD = 
DD 
100 ... 
DR 
RR 
b. 
DD 
X 
RR = 
— 
100 ... 
— 
c. 
DD 
X 
DR = 
50 ... 
50 ... 
— ' 
d. 
RR 
X 
RR = 
— 
—- 
100 
e. 
RR 
X 
DR = 
— 
50 ... 
50 
f. 
DR 
X 
DR = 
25 ... 
50 ... 
25 
We have already considered all of these with the exception of 
c and e, i.e., the union of the heterozygote with the dominant and with 
recessive homozygote respectively. As will be seen from the table 
the results in both cases are equal numbers of the two types mated. 
I wish to lay stress on this type of mating, because of the important 
