I'll need to answer this in stages so first I'll see if the pics come through....... OK, the top one expands, but just look at the second as is, because if you click on it, it fills several pages. Beyond my skills.
So, the first pic is the oldest known photo of Hebs, taken in the late 1800s at Storrs Hall in Windermere. These were sheep brought down to 'big houses' around the country, to grace the parkland.
Take a look and you will see that just about all of them have topknots. There is a two horn in there, but just about all display the multihorned version.
Another characteristic many display in this photo is the 'silver mantle', which I see as a colour pattern, akin to katmoget in Shetlands (all the Scottish northern short tailed sheep were developed from the same multi-coloured, multi horned 'land sheep'). The silver mantle is very different from the greying which is seen in older animals, and is also different from the greying which occurs with copper uptake problems.
So, nearly all Hebs used to be multihorned, and carried topknots. So it's less a question of 'where do they come from?' and more to do with 'where have they gone?'
In fact, the type of Heb we keep (see second pic which is of Gladstone Brigitte with her mighty topknot) are very like the Storrs Hall photo, which we have labelled 'The Ancient Type', as opposed to the modern Heb, which has been modified by the show ring into an all black, two horned creature, so heavily changed by human interaction.
For the actual genetics of topknots - well, no-one has done enough research into it, as until recently they were not registerable, and were as rare as hens' teeth. It was also put about that only ewes bore topknots. However, many multihorned tups have small topknots between their top horns. Our old boy Gladstone Laughing Boy is our best route to topknotted lambs, especially when bred with our Zanfara stock, or certain of our topknotted ewes.
There are more topknot pics on our website - see below for URL