oh - I'm cross. I just typed a long answer and decided to use spell check which I don' usually do and it lost my reply
Anyway - much more briefly. Birds which look like Cuckoo Marans can be anything unless you have sourced them from a
really reliable breeder. Contact The Marans Club for leads if you are serious.
The sex links only work between "brown" and "black" birds, so if either line is not pure, even at some time in its past it is not certain as to what the pullets will look like.
Genetics are pretty complicated, and I am only just getting my head round some of it. But this is a link to a genetics calculator - but again it will only work if you know exactly what you are putting into it.
http://kippenjungle.nl/Overzicht.htm There is so much interbreeding and even if they are sold as such to the first purchaser they sometimes do not let on further on down the line, and so many people purchase from auctions, and assume from the look of the bird what they are buying. Which is not a problem except when you get further on down the line of wanting to breed for certain characteristics
And if there are any spelling mistakes in this put it down to crossness at having to re-type.