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 C 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 T ). iinimals 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 factors 

 is concerned. Where the alternative factors are different, the animal 

 is said to be heterozygous. Guinea pigs of the formulas Cc d , Cc 7 , Cc, 

 c d <f, c d c a , and <?&■ 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 



