+-------+-------+-------+-------+ |RP |RP |RP |RP | |RP |Rp |rP |rp | | | | | | | Walnut| Walnut| Walnut| Walnut| +-------+-------+-------+-------+ |Rp |Rp |Rp |Rp | |RP |Rp |rP |rp | | | | | | | Walnut| Rose| Walnut| Rose| +-------+-------+-------+-------+ |rP |rP |rP |rP | |RP |Rp |rP |rp | | | | | | | Walnut| Walnut| Pea| Pea| +-------+-------+-------+-------+ |rp |rp |rp |rp | |RP |Rp |rP |rp | | | | | | | Walnut| Rose| Pea| Single| +-------+-------+-------+-------+ FIG. 5. Diagram to illustrate the nature of the F_2 generation from the cross of rose comb x pea comb. Fig. 5 shows the result of applying this method to our series RP, Rp, rP, rp, and the 16 squares represent the different kinds of zygotes formed and the proportions in which they occur. As {38} the figure shows, 9 zygotes contain both R and P, having a double or a single dose of either or both of these factors. Such birds must be all walnut combed. Three out of the 16 zygotes contain R but not P, and these must be rose-combed birds. Three, again, contain P but not R and must be pea-combed birds. Finally one out of the 16 contains neither R nor P. It cannot be rose--it cannot be pea. It must, therefore, be something else. As a matter of fact it is single. Why it should be single and not something else follows from what we already know about the behaviour of these various forms of comb. For rose is dominant to single; therefore on the Presence and Absence theory a rose is a single plus a factor which turns the single into a rose. If we could remove the "rose" factor from a rose-combed bird the underlying single would come into view. Similarly a pea comb is a single plus a factor which turns the single into a pea, and a walnut is a single which possesses two additional modifying factors. Singleness, in fact, underlies all these combs, and if we write their zygotic constitution in full we must denote a walnut as RRPPSS, a rose as RRppSS, a pea as rrPPSS, and a single as rrppSS. The crossing of rose with pea results in a reshuffling of the factors concerned, and in accordance with the principle of segregation some zygotes are formed in which neither of the modifying factors R and P are present, and the single character can then become manifest. {39}