Only hh males can be cock-feathered, all other genotypes are hen feathered. Therefore all F1 Hh birds are hen-feathered. Therefore, in a cross of AA x AA, although the allele from each parent is identical in DNA sequence, it may not be identical in methylation pattern and may behave differently. This is a violation of the very basic tenet in genetics that genes are preserved unchanged in the germ-line cells. It is also very rare.
Anticipation also violates the tenet.
The longer alleles cause increasingly severe phenotypes. Discontinuous traits cannot be treated with the same mathematics. They must be analyzed with the stochastic equations of finite math and chi square. Because each gene has 2 alleles, the methods are identical. AAbb x aabb so F1 is all Aabb or light red.