Plantbreeding at Svalöf - continued...

From the artificial crossing of pedigrees 0353 and 0462, four individuals were raised in 1904 on one parcel, for which the field book entries (ff 376 recto, ff 376 verso) look similar, if only differentiated for each individual ('376a-d'). As expressly stated in the entries c and d ('lik 376a'), and as to be expected by the Mendelian rules for the first hybrid generation, the four plants were similar to each other, except for 376b, which was said to possess slightly shorter seeds. The seeds from these four individuals were then sown out on separate fields in the following year. Now, however, the field book entries (ff 233 recto) exhibited a completely different structure: A botanical description of the plants growing on each of the parcels was lacking. Instead a single category – the colour of the seed – was observed in regard to the absolute frequency with which its alternatives appear on the parcels (small column in the right hand middle of the page): '8 grey (gråa)', '4 white (hvita)', '207 black (svarta)', in various shadings ('mörkare', 'ljusare'). Contrary to Mendelian expectations, colour did not segregate discretely, and not at all in a 3:1 proportion.

It is these absolute frequencies, tabulated in a little diagram for the descendants of each of the four plants raised in 1904 ('a, b, c, d'), and their strong deviation from what should be expected from Mendelian rules, to which an essay refers that Nilsson-Ehle published in 1907 and in which he maintained – in open opposition to his superior Nils Hjalmar Nilsson's morphological point of view – 'that it can happen, that what appears to us as one property, may in fact be a composite of several properties' (Nilsson-Ehle 1907, 214-215). Why this should be so, however, is not explicitly discussed in this essay. Only a year later, in a contribution to a professional botanical journal, did Nilsson-Ehle interpret his findings under the assumption, that the 'black colour consists of two independent units (unabhängige Einheiten)' and that its hereditary pattern therefore follows the 'dihybrid scheme' of Gregor Mendel (1822-1884) (Nilsson-Ehle 1908, 266). In maintaining this, the table presented in 1907 was extended in three respects. First, the absolute frequencies were transformed into relative frequencies; second, the sum (the mean) was drawn from the absolute (relative) frequencies; and finally, a symbolic representation was added for the dihybrid scheme. Through this series of mathematical and symbolical operations Nilsson-Ehle brought about an abstraction from the concrete distribution of individuals and pedigrees on fields and parcels that allowed him to present his data as in accordance with the Mendelian rules.