Plantbreeding at Svalöf - continued...

How Nilsson-Ehle reached his conclusions regarding the 'dihybrid scheme' determining seed colour in 1907 is made evident by a leaflet he inserted into the field-book of 1905.

It developed the dihybrid scheme for seed colour by assuming two factors —one for black ('svart'), one for grey ('grå') colour— which in turn might either be present or absent ('un-black [osvart]' and 'un-grey [ogrå]' in the latter case). The possible combinations of these four alternatives in the zygote were listed under a hybridization scheme similar to the one Nilsson-Ehle had used in his annotations to the Mendel paper and should later also use in his printed presentation in 1908. The resultant colour of the seed was derived from these combinations under the assumption that absence of both the factor for black and for grey yields white colour and that black 'covers' grey, i.e. yields the same black colour independent of the presence or absence of grey factors. The resultant relative frequencies to be expected for black, grey, and white colour – that is, 12:3:1 – were noted to the left. The article of 1908 interprets the data under a different assumption to bring the observed absolute frequencies into better accord with the theoretically developed frequencies. It starts from the assumption that black colour results from two factors for black which severally and jointly effect black colour, only their complete absence resulting in white and greyish colours.