I found a copy of Morley A Jull's Poultry Breeding, Third Edition from 1952 - it is my summer reading book (wish it were on my Kindle Fire). I am trying to learn about breeding and genes in poultry.
This is what he states...
..."The heads of Silkies, Houdans and Polish fowl are adorned with crests. There is considerable variation in size of crest, those on Polish being much larger than those on Silkies. The crest develops in the frontal region of the skull, is present in the chick, and is inherited. The crested conditions results from the fact that the frontal bones of the skull are raised into a dome-shaped protuberance, giving rise to a condition know as cerebral hernia.
The results secured by various workers, from Hurst (1905) to Dunn and Jull (1927), led to the general conclusion that the development of the crest is due to an autosomal dominant gene, Cr. Variation in crest size is apparently due to multiple genes. From the results secured by Warren and Hutt (1936), it was concluded that cerebral hernia is in reality homozygous crest, although hernia may not be evident in quite a high proportion of the Cr Cr birds, but is sometimes present in a proportion of the Cr cr birds.".....
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