The inheritance of quantitative traits in Brassica napus ssp. rapifera(swedes): augmented triple test cross analysis of yield

Abstract

Two augmented triple test crosses were produced from inbred lines of swede (Brassica napus ssp. rapifera L.) and assessed in field trials at Dundee in 1988 and 1989, respectively. The first cross was between lines derived from cvs. Criffel and Marian and the second between the same Criffel line and one from Bangholm Wilby. The genetic architectures of the two components of dry weight yield were clearly different. Dry-matter percentage was mainly under the control of additive genetical variation whereas fresh weight yield was under the control of additive and dominance variation. Although there was unidirectional dominance (positive [h]) for high yield, the dominance ratio was less than unity in both crosses. Dry weight yield was also under the control of genes showing both additive and dominance variation. In both crosses [h] was larger than [d] and the F₁ outyielded the better parent by 14 and 12 per cent, respectively. The dominance ratio was less than unity, being 0.50 and 0.94 respectively. There was no evidence of overdominance. Whilst epistasis was not a major feature of the genetic architecture of dry matter yield and its components, there were large and unexpected reciprocal differences, particularly for fresh and dry weight yield in all TTC generations and the F₃ of the first cross. These require further investigation. Finally it was concluded that it should be possible to produce inbred cultivars of swede which outyield F₁, hybrids in a modest sized breeding programme because 12 and 2 per cent, respectively, of the recombinant inbred lines from the two crosses were predicted to outyield the F₁.