This chapter focuses on the genes affecting three important tissues of the body, namely muscle, the nervous system and the skeleton. Each will be considered in turn.

Muscle

The muscles usually make up a large proportion of the body weight (c. 40%) and also about 40% of the body protein. Muscle development is very important in the production of broilers, since meat is often the main protein component of the average human diet in many countries. There are three basic types of muscle: skeletal or voluntary muscle, cardiac muscle, and smooth or involuntary muscle. Skeletal and cardiac muscle are striated in appearance, whereas smooth muscle is non-striated, reflecting its less regular structure. Smooth muscle is found particularly in the gut and the lining of blood vessels, whereas skeletal muscle is found in association with the skeleton.

Within skeletal muscle the constituent fibres may be subdivided into the two principal types, red fibres and white fibres. The difference in colour is dependent on the larger amount of myoglobin (see Chapter 10, section 10.2) and cytochromes in the red fibres. The white fibres have a poor supply of mitochondria and are often referred to as fast twitch muscles. They are able to undergo short, but not sustained bursts of activity.