Now all of these four cases present a common feature which probably has not escaped the attention of the reader. In all of them the original cross was such as to introduce one of the repelling factors with each of the two parents. If we denote our two factors by A and B, the crosses have always been of the nature AAbb × aaBB. Let us now consider what happens when both of the
factors, which in these cases repel one another, are introduced by one of the parents, and neither by the other parent. And in particular we will take the case in which we are concerned with purple and red flower colour, and with long and round pollen, i.e. with the factors B and L. When a purple long (BBLL) is crossed with a red round (bbll) the F1 (BbLl) is a purple with long pollen, identical in appearance with that produced by crossing the long pollened red with the round pollened purple. But the nature of the F2 generation is in some respects very different. The ratio of purples to reds and of longs to rounds is in each case 3 : 1, as before. But instead of an association between the red and the long pollen characters the reverse is the case. The long pollen character is now associated with purple and the round pollen with red. The association, however, is not quite complete, and the examination of a large quantity of similarly bred material shows that the purple longs are about twelve times as numerous as the purple rounds, while the red rounds are rather more than three times as many as the red longs. Now this peculiar result could be brought about if the gametic series produced by the F1 plant consisted of 7 BL + 1 Bl + 1 bL + 7 bl out of every 16 gametes. Fertilization between two such similar series of 16 gametes would result in 256 plants, of which 177 would be purple longs, 15 purple rounds, 15 red longs, and 49 red rounds—a proportion of the four different kinds very close to
that actually found by experiment. It will be noticed that in the whole family the purples are to the reds as 3 : 1, and the longs are also three times as numerous as the rounds. The peculiarity of the case lies in the distribution of these two characters with regard to one another. In some way or other the factors for blue and for long pollen become linked together in the cell divisions that give rise to the gametes, but the linking is not complete. This holds good for all the four cases in which repulsion between the factors occurs when one of the two factors is introduced by each of the parents. When both of the factors are brought into the cross by the same parent we get coupling between them instead of repulsion. The phenomena of repulsion and coupling between separate factors are intimately related, though hitherto we have not been able to suggest why this should be so.
Nor for the present can we suggest why certain factors should be linked together in the peculiar way that we have reason to suppose that they are during the process of the formation of the gametes. Nevertheless the phenomena are very definite, and it is not unlikely that a further study of them may throw important light on the architecture of the living cell.
APPENDIX TO CHAPTER IX
As it is possible that some readers may care, in spite of its complexity, to enter rather more fully into the peculiar phenomenon
of the coupling of characters, I have brought together some further data in this Appendix. In the case we have already considered, where the factors for blue colour and long pollen are concerned, we have been led to suppose that the gametes produced by the heterozygous plant are of the nature 7 BL : 1 Bl : 1 bL : 7 bl. Such a series of ovules fertilised by a similar series of pollen grains will give a generation of the following composition:—
| 49 BBLL | + 7 BBLl | + 7 BbLL | + 49 | BbLl | + BBll | + 7 Bbll | + bbLL | + 7 bbLl | + 49 bbll |
| + 7 BBLl | + 7 BbLL | + | BbLl | + 7 Bbll | + 7 bbLl | ||||
| + | BbLl | ||||||||
| + 49 | BbLl | ||||||||
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|
|
| ||||||
| 177 purple, long | 15 purple, round | 15 red, long | 49 red, round | ||||||
and as this theoretical result fits closely with the actual figures obtained by experiment we have reason for supposing that the heterozygous plant produces a series of gametes in which the factors are coupled in this way. The intensity of the coupling, however, varies in different cases. Where we are dealing with another, viz. fertility (F) and the dark axil (D), the experimental numbers accord with the view that the gametic series is here 15 FD : 1 Fd : 1 fD : 15 fd. The coupling is in this instance more intense. In the case of the erect standard (E) and blueness (B) the coupling is even more intense, and the experimental evidence available at present points to the gametic series here being 63 Eb : 1 EB : 1 eB : 63 eb. There is evidence also for supposing that the intensity of the coupling may vary in different families for the same pair of factors. The coupling between blue and long pollen is generally on the 7 : 1 : 1 : 7
basis, but in some cases it may be on the 15 : 1 : 1 : 15 basis. But though the intensity of the coupling may vary it varies in an orderly way. If A and B are the two factors concerned, the results obtained in F2 are explicable on the assumption that the ratio of the four sorts of gametes produced is a term of the series—