36 MEXDELISM chap. 



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. Xow when such a bird comes to form gametes a 

 separation takes place between the part of the zygotic cell 

 containing R and the part which does not contain it (r). 

 Half of its gametes, therefore, will contain R and the other 

 half will be without it (r). Similarly half of its gametes 

 will contain P and the other half will be without it (p). 

 It is obvious that the chances of R being distributed to a 

 gamete with or without P are equal. Hence the gametes 

 containing R will be of two sorts, PR and Rp, and these 

 will be produced in equal numbers. Similarly the gam- 

 etes without R will also be of two sorts, rP and rp, and 

 these, again, will be produced in equal numbers. Each of 

 the hybrid walnut-combed birds, therefore, gives rise to a 

 series consisting of equal numbers of gametes of the four 

 different types RP, Rp, rP, and rp ; and the breeding to- 



