Crested chickens

Crested chickens are a special ornamental group of chicken breeds, which are characterised by a tuft ("crest") of elongated feathers on the head.
History and distribution.
Excavations in England have proved the presence of crested chicken in Roman times. The first pictures of these animals can be found on Dutch rural paintings of the 17th century. After the exhibition poultry breeding developed in Europe and North-America in the 19th century, several old crested breeds became widely known. Original crested chickens originate from the Netherlands, France, Italy, Poland, Russia, and in the Western Balkans. Apart from these breeds, crested chicken are found in Africa and Iceland.
Genetics.
The development of the feather crest is based on the mutation "Crest" (Cr). Its inheritance is incomplete autosomal dominant. This means, that homozygous but also heterozygous animals show a crest, which is however more pronounced in homozygotes. 
Vaulted skull.
The extreme upright stand of the head feathers of many breeds, like the Polish, can be attributed to a skull malformation, which is known as cerebral hernia or vaulted skull. Neurobiological investigations showed that this malformation alters the brain anatomy without any functional relevance.