Very interesting article. Do you suppose modifers are at play in how much black is allowed to express in the I/i heteros? Maybe the Leghorns in this cross experiment had modifiers that severely limited how much black would be allowed to express, but the Spanish outcross brought with it a group of modifiers that allowed a more liberal expression?
that is Exactly what happened, White leghorns have hordes of eumelanin inhibitors and the black spanish breed had eumelanin enhancers,
in this paper we get a chance to see what genes most production white leghorns hide under the white, so the list is as follows>
White leghorns are
E/E = Extended Black
S/S= dominant sex linked silver
Bl/Bl = Splash Blue
I/I = Dominant White
B/B = Sex linked Barring
c/c = Recessive white(not all white leghorns carry this, but on this paper this birds do)
Id/Id = Dermal Melanin inhibitor sex linked
Black Spanish(most black based birds will have this genes, including the black Erminette)
E/E = Most black birds are based on Extended black, others are based on Birchen
S/S(or s+/s+) = self black birds can hide gold or silver
Ml/Ml = Melanotic Turns hackles and saddle all black
Pg/Pg = most black birds carry the pattern gene, on itself Pg cant make any pattern on most genetic backgrounds and is also a melaizer,
i+/i+ = wildtype recessive non white
bl+/bl+ = wildtype recessive non blue
b+/b+ = wildtype recessive non barred
Id+/id+ = wildtype recessive non dermal inhibitor
C+/C+ = wildtype dominant non recessive white
so the F1s of this cross would have had a genetic make up of E/E S/? Ml/ml Pg/pg+ Bl/bl+ B/b+ C+/c id+/ and as mentioned the first F1s of the cross showed small amount of black flecks(resembling the California White egg layer)
Each Consecutive breeding back to white leghorn produced more and more Bl/Bl B/B c/c homozygotes that were also I/i+, and for the each consecutive breeding back to the black breed the melanizers got also in homozygous form and produced these Erminette colored birds, and this is what the Erminette color is about, a breed with with full set of melanizers ganging up on that Lonely I gene(i+ is also fighting back doing its part on this battle) , now two copies of I(I/I) are too strong to fight back, thats why you see white Erminette too when they are I/I
so in simple terms, the Paper shows how a bunch of black feather Diluters are ganging up a lonely i+ gene to make the I/i+ white leghorn line pure white, and how a bunch of black feather enhancers are ganging up on a lonely I gene to make the I/i+ black spanish line have an Erminette phenotype,
hope this helps
Marvin