18 BULLETIN 905, IT. S. DEPARTMENT OF AGRICULTURE. 
In the formation of the reproductive cells, it must be assumed that 
the double set of units is sorted into two single sets. The crossbred 
colored guinea pigs (Cc) produce two kinds of reproductive cells in 
equal numbers, those transmitting color (C) and those transmitting 
albinism (c). On crossing with an albino it is obvious that two 
classes of the young will be produced in equal numbers, depending 
on whether the reproductive cells of the latter (c) happen to unite with 
a cell transmitting color (C) or one transmitting albinism (c). The 
albino young (cc) have no more tendency to transmit color than pure 
albino stock, while the colored young (Cc), although three-quarter 
blood albino, are of the same hereditary make-up as the first cross 
and so breed like them. 
MENDELIAN INHERITANCE. 
This mode of inheritance was first worked out about half a century 
ago by an Austrian monk, Gregor Johann Mendel, who experimented 
with a number of alternative characteristics of the garden pea. The 
same principles have been found to apply to an enormous number of 
characteristics in both plants and animals and are now believed to 
be true of all heredity. Most cases appear more complicated than 
the case of the colored and white guinea pigs, because most charac- 
teristics depend on the cooperation of a large number of independently 
inherited unit factors. Occasionally there is also the complication 
that the same unit may have an influence on the development of a 
number of seemingly independent characteristics. 
There are a number of technical terms which ordinarily are used in 
discussion of heredity which it will be well to mention. Unit factors 
which are alternatives of each other in inheritance, one presumably 
being a modification of the other, are called allelomorphs of each 
other. Thus factors and c are allelomorphs. Two other modifi- 
cations or allelomorphs of factor C are known, which determine de- 
grees of intensity of color intermediate between full intensity and 
the white of albinos (c d and c r ). Animals produced by the union of 
an egg and a sperm which contain the same unit in a given set of 
allelomorphs are said to be homozygous in that particular. Guinea 
pigs of the formulas CC, c d c d , (?<?, and cc are all homozygous. Each 
produces only one kind of reproductive cell so far as this set of f actors 
is concerned. Where the alternative factors are different, the animal 
is said to be heterozygous. Guinea pigs of the formulas Cc d , Cc*, Cc, 
c d <f, c d c*, and (fc 3 - are all heterozygous. Each produces two kinds of 
reproductive cells in equal numbers. It is impossible for a guinea 
pig to transmit more than two of the grades of intensity, a conclusion 
which has been very thoroughly established by experiments. 
Factor C has been spoken of as producing colored young, but no 
particular color was mentioned, This is because it alone does not 
