The process of domestication where wild animals are tamed and used for the profit of humans have occurred for centuries. To clarify how domestication is defined in this paper it is when an animal is taken from the wild, bred in captivity, which is controlled by humans and usually used for profit. Domestication have shown to produce a consistent set of behavioural, morphological, and physiological changes in a range of different species, e.g. reduced fearfulness, reduced size and pigmentation and earlier sexual maturation and more frequent reproductive cycles (Wright et al, 2010). Reduced fearful behaviour has most likely been directly or indirectly selected for from the beginning of the domestication.Animals with lower fear response would cope better with the stress of captivity, and therefore have a higher fitness (Campler et al, 2008) and potentially higher production output.

Due to the long domestication history in chickens, 8000 years (Jensen & Anderson, 2005) the differences in behaviour between the wild breed Red Junglefowl (Rjf) and the domesticated White Leghorn (WL) have been described in numerous papers. By using the domestication process, it will be possible to investigate larger phenotypic differences between domesticated animals and their wild ancestors such as using domestication as a potential model for evolution (Wright et al, 2010). Modern genomics combined with analysis of behaviour offers a good method for understanding the interactions between behaviour and production and also to predict potential side-effects of breeding programs (Jensen, 2006).  

Previous research has shown that both mammalian TSHs and avian pituitary homogenates simulates thyroid gland activities; thyroid hormone release and gland growth. Some of the processes that the thyroid hormone control are; embryonic differentiation and maturation, embryonic postnatal growth, development and function of the central nervous system, skin and hair pigment production, behaviour correlation, stress response and daily and seasonal thyroid hormone variation. (Crockford, 2001). In addition to controlling the development of certain physical traits, thyrotropin (TSH) can affect a wide range of phenotypes related to domestication such as pigmentation and also behaviour (Crockford, 2006)

The thyroid stimulating hormone receptor (TSHR) mutation could potentially be involved in releasing the strict photoperiodic regulation and affecting development, growth and behaviour in the domestic chicken, as suggested by the finding of the TSHR sweep. Resequensing of the genome in several lines of domestic chicken has revealed a selective sweep over the thyroid stimulating hormone receptor gene (TSHR) (Rubin et al, 2010). A missense mutation in TSHR causing a glycine to arginine change is the most obvious candidate causal mutation for the sweep, since glycine is conserved in all known vertebrate TSHR sequences at this position (Rubin et al, 2010). TSHR could potentially be a domestication locus in chickens, where all individuals of domesticated species carry a mutant allele. Considering that a large range of animals are homozygote for this mutation indicates a central role, possible already in the early stages of domestication.