Charles Darwin embarked on a real task when he tackled cross breeding in fancy pigeons in 1856. The various combinations of dominant, recessive and sex-linked genes is daunting even today with most of the colours now explained. Poor old Darwin was undertaking something that no one else could help him with, at least no one in Victorian times although he often mentions "always breeds true" he would not have known this for sure unless he himself had bred them for at least three generations and even then this would not necessarily mean that they would breed true. It would have been very difficult to take results as true without both the genetic knowledge and the pedigree of the birds used.
See example of breeding with two distinct breeds. Darwin would have recognised the Ice Pigeon and used one in his work.
Silver Old German Owl Hen [ dilute, shell crest, chest frill, pied white, bull eyed, shorter beak and small feet correlation ] paired to Ice Barless Cock [ double dominant ice factor, no bar pattern ].
If Darwin used this pair he would have had more questions than answers and may be fooled into the wrong conclusions.
I expect he had a lot of mini disasters until he got the results he wanted, which was reversion to rock dove blue colour with black bars, it is easy to forget the non-reversion results, it can always be put down to incorrect parentage especially in an open loft nestbox system.
The first observation would be to see a silvery effect in the chicks and assume incorrectly that this was something to do with the dilute of the hen when infact the dilute of the hen would be hidden in all the cocks produced and lost in the hens. This silvery effect comes from the partial dominant factor or single dose of the ice factor which shows itself in the F1 as slightly frosted.
The next shock, where has a dominant bronze bar appeared from ? It did not show on the silver owl hen. Although the Ice used is a barless pattern it still is a bronze factor bird, the particular bird used was possibly bred from a white barred Ice, carrying barless, which in its intermediate stage shows as bronze. You would not expect this re-appearance from a dominant factor like bronze as it normally cannot be hidden exceptions being by recessive red, red chequer and almond. However the type of bronzing in this case sits on the bar or chequer pattern and if no bar it cannot be seen but the bird is still genetically bronze barred.
Do not worry if you cannot make any sense of what I am saying, just look at the photographs [left] and you will see the F1 have no crest, pied bull eye or chest frill and as I have already said the surprise appearance of the bronze bar. The only other intermediate item is the beak length which is neither wild type as the Ice or short as the Old German Owl.
If these F1's were paired together we would get F2 birds showing various degrees of crest from simple peak through to a full shell crest, this demonstrates there is more than one factor involved in producing a shell crested bird. Now to the chest frill, this again may appear as just a few feathers to a full vertical breast frill.
Traits that would appear in varying degrees of the F2 young are as follows :-
barless; white barred; bronze barred; black barred; short beak; varying types of crest from peak through to shell; varying sizes of chest frill; full Ice factor; dilute hens, pied white on head, body and tail and most importantly wild type with no traits showing and wild type showing the odd frill, crest, short beak etc. As you can see the odds of getting a complete bird showing all the traits at their maximum strength would be low and in the same respect odds of getting a wild type with no extra traits would also be low.
If you are still reading this far down I am both pleased and surprised as probably most people will have already given up, 90% from my poor explanation and the 10% the trickiness of genetics to understand.
Further information regarding this particular pairing has confirmed Ice barless cock paired to both blue and chequer Old German owl hens produce the same slight frosting in F1 however the chequer hen then produces all chequer offspring.