Dwarfism in chickens
Poultry breeders begin to distinguish gradually dwarfs from normal birds by their shortest shanks and smallest body size.
But in spite of the proven advantages of raising dwarf breeder hens, their use is not generalized in broiler industry.
[6][7][8][9] This type of dwarfism produced individuals which showed a general growth delay, which was recognizable since two or three weeks of age.
It was considered as the result of a dysfunction of the thyroid gland, similar to the human pathological condition known as myxoedema infantilis.
Dwarfs of this type were homozygotes for an autosomal recessive gene td (thyrogenous dwarfism).
[5] The ultimate goal of the modern genetic studies is to find out the underlying genes involved in these traits.
The Pygmy mouse phenotype arises from the inactivation of the High Mobility Group I-C (HMGCI-C).
Hutt studied in the 1940s a remarkable type of dwarfism caused by only one sex-linked recessive gene to which he assigned the symbol dw.
Some individuals can be identified as dwarfs at 8–10 weeks of age, but classification is more precise when the chickens are five months old or more.
Double dose of dwarf gene causes the dwarfism to be much more evident in males than in females.
The above picture illustrates the comparative size of two full-sib roosters born the same day: Left: Normal sibling of genotype Dw/dw.
The T3 deficiency is explained by a lower peripheral activity of T4 monodeiodination which could be related to an abnormal T4 uptake by the cell, particularly the hepatocyte.
Further studies are needed on the GH receptor and the T4 uptake in the hepatic cell to identify the common point where the dwarf gene could act.
These birds are popular as pets, but Bantam hens are also renowned for hatching and brooding because they are very protective mothers and will attack anything that gets near their young.
In spite of this, literature refers to the gene of Bantam dwarfism with the lower-case symbol dwB.
This conclusion is based on the fact that males heterozygous for dwM/dw produce female offspring which fall into two populations with respect to shank length.
In commercial broiler chicken production the use of dwarf female parents acquired a great importance.
Under current practice, normal parent poultry breeding stock potentially face welfare problems.
Intensive selection for production traits, especially growth rate, is associated with increased nutritious requirement and thus feed consumption, but also reproductive dysfunctions and decreased sexual activity in broiler breeders.
This severe feed restriction has negative effects on bird welfare as it causes chronic stress resulting from hunger.
