The swede is biannual and may cross-fertilize with wild turnips, turnips and rape and also other sorts of turnip. Even the common "snowball turnip" could cross-fertilize with the swede despite their being of different species. However, it doesn"™t cross-fertilize with the many Brassica oleracea variants. It is easy to grow seeds from swedes. Select some (5-10) healthy roots, typical of the sort, which have coped well with winter storage in the cellar. (They must have been harvested with 2-3 cm of leafage left). Set them out with the growth point exactly at the surface with 40-50 cm of mutual distance.
It is also possible to overwinter half-grown roots directly in the plot in a well-drained place. Cover them with hay, straw or leaves when it gets really cold and thin out in spring. Remove all deviant plants in early summer, ie. the ones with lemon-yellow rather than pale-yellow flowers and the ones with differently shaped leaves. These hint at earlier cross-fertilizations with wild field turnip or turnip.
Read more about seed cultivation under "Growing cabbage plants".