The individuals of the second generation which contain the potentialities of both characters of the pair, if self-fertilized or bred with similar indi viduals containing the potentialities of both char acters, exhibit in the third generation exactly the same nature that first-generation hybrids exhibit in the second generation. The two potentialities are commingled in their cells, and to all intents and purposes they are exactly the same as first generation hybrids. When such self-fertilized hy brids are grown they give again, in the third gene ration, the regular Mendelian proportion of 1BB to 2Bb to lbb. Here the individuals containing only potentialities of one character, that is, BB and bb, would come true to these characters in succeeding generations, while those individuals containing the potentialities of both characters, Bb, would be ex pected to appear again in the fourth generation in similar proportions.
When we deal with more than one character pair the matter becomes complicated, but will become clearer on careful study. If we combine with the above characters the character of hairy (H) and smooth (s) chaff in the head, and remember that the potentialities of these two characters in the hybrids segregate exactly as in the case of bald and bearded heads, we can foretell what will occur. In this case, the hairy chaff is the strong dominant character, as in the first- generation hybrids of hairy with smooth sorts the chaff is always or very generally hairy. We would thus represent these characters by H, for the hairy or dominant character, and s for the smooth or reces sive character. In this character-pair we would expect a splitting and segregation to have occurred in the formation of the germ-cells of the first-gen eration hybrids, so that the hybrid plants of the second generation would exhibit these characters in Mendelian proportions, as in the characters described above. The progeny in the second gen erat ion would thus exhibit these characters in the following combinations and proportions : 1HH to 2Hs to lss. This probable proportion should hold rather constantly, either in small or large numbers of hybrids, though in large numbers it would prob ably be more accurately realized. The potentiali ties of the four characters, or two character-pairs, are commingled in the cells of the first-generation hybrid. When the egg-cells or pollen-grains are formed, however, a segregation of the potentiali ties of the two character-pairs occurs, but inde pendent of each other. Each egg-cell or pollen grain will receive only the potentiality of one character of a certain character-pair, but will, at the same time, receive potentialities of other char acters belonging to other character-pairs. Consid ering the two character-pairs described, an egg cell receiving the potentiality of the bald head (B) might contain the potentiality of either H or s, representing the characters of hairy or smooth chaff. These two character-pairs would thus give
us egg-cells of four combinations, namely, BH, Bs, bH and bs.
In the formation of the pollen-grains the same combination occurs, so that with reference to the two character-pairs described, the pollen-grains that would be formed have the same combination of potentialities as the egg-cells, namely, BH, Bs, bH and bs. We thus have four kinds of egg-cells and four kinds of pollen-grains, so far as these two character-pairs are concerned. If these are brought together, sixteen combinations are possible as follows : Examining these combinations carefully, and cut ting out the letters that occur twice, as the occur rence of the same potentiality in both egg-cell and and pollen-grain serves only to reproduce the same character, we have the following nine combi nations, all of which are different: 1BH, llls, lbH, lbs, 2BHs, 2BbH, 2Bbs, 2bHs and 4BbHs. In the illustration taken of the character-pair of bald and bearded heads, and the probable ratio of unions in second-generation hybrids, it was shown that out of 100 unions we should expect, by the law of chance, the ratio 25B to 50Bb to 25b. Now, con sidering the second character-pair, that is, the hairy and the smooth chaff, in connection with these same 100 unions, we would have the follow ing as the probable combinations, according to the same law of chance : These nine combinations are the same as the nine given above, only multiplied by Gi in each case. In each of the nine combinations when only one of the potentialities of the character is present, the progeny from such an individual from self fertilized seed will come true to this character in all succeeding generations, as the potentiality of the opposed character has been eliminated. Thus, in the first combination, BH, representing the potentialities of the bald head and hairy chaff, if such a hybrid is fertilized with its own pollen, it will produce only progeny with bald head and hairy chaff. In the second combination, BHs, we have present the potentialities of the bald head of one character-pair and both the hairy and smooth chaff of the other character-pair. Self-fertilized progeny of this hybrid should all come bald, but some should have hairy chaff and some smooth chaff. In the third combination, Bs, we have simply the potentialities of the bald head and smooth chaff, and such a combination should give plants that will come true to type in later genera tions when self-fertilized. Similar conditions of purity or hybridity of the germ-cells can be figured out for each of the other six combinations.