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Heavily muscled cows are often more desirable because of Artificial Selection. The stronger and more healthy, ultimately the more attractive they are in whole. This is why there is an increase in healthier/stronger cows from the product of reproduction. Although, If the cow doesn't have the dominate genes and has incomplete dominance, there are often chances that the cow can be affected by a genetic mutation that can alter the genetic coding of the animal  

Explanation:

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Double-muscled cattle are those that have been genetically altered to reduce the activity of myostatin. This protein limits muscle growth and causes animals to have higher numbers of muscle fibers. These animals also tend to have lower body fat content.

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