32 MENDELISM CHAP. 



the expression of the only two possible states of a 

 single factor, viz. its presence or its absence. The 

 conception will probably become clearer iC we follow 

 its application in detail to the case of the fowl's 

 combs. In this case we are concerned with the 

 transmission of the two factors, rose (R\ and pea (T 3 ), 

 the presence of each of which is alternative to its 

 absence. The rose-combed bird contains the factor 

 for rose but not that for pea, and the pea -combed 

 bird contains the factor for pea but not that for rose. 

 When both factors are present in a bird, as in the 

 hybrid made by crossing rose with pea, the result is 

 a walnut. For convenience of argument we may 

 denote the presence of a given factor by a capital 

 letter and its absence by the corresponding small 

 letter. The use of the small letter is merely a 

 symbolic way of intimating that a particular factor 

 is absent in a gamete or zygote. Represented thus 

 the zygotic constitution of a pure rose-combed bird 

 is RRpp ; for it has been formed by the union of 

 two gametes both of which contained R but not P. 

 Similarly we may denote the pure pea-combed bird 

 .as rrPP. On crossing the rose with the pea, union 

 occurs between a gamete Rp and a gamete rP, 

 resulting in the formation of a heterozygote of the 

 constitution RrPp. The use of the small letters 

 here informs us that such a zygote contains only 

 a single dose of each of the factors R and P, although, 

 of course, it is possible for a zygote, if made in a 

 suitable way, to have a double dose of any factor. 

 Now when such a bird comes to form gametes a 

 separation takes place between the par.t of the 

 zygotic cell containing R and the part which <3oes 



