Morphological variation provides examples of phenotypes which mode of inheritance was characterized by Mendelian genetics since many years. This chapter describes seventeen phenotypes affecting skeleton, skin appendages, feather distribution, growth rate or structure, for which the underlying genetic mechanism has been identified since 1990. In most cases, the genes involved play a central role in development and their expression is modified by a change in DNA sequence. Thus, most of these mutations involve regulatory changes. Molecular diagnostic tests can often be proposed to distinguish heterozygous from homozygous carriers. Not all phenotypes previously described have been identified, and, in some instance, carriers of these defects may have been lost.