Why would a mutation to a hox gene have a large impact on the organism

In order to understand the ramifications of a mutation in the Hox genes you must understand what the Hox gene does. To put in simple terms the Hox genes control the body plan so you can think of them as a blueprint. For instance, they are responsible for determining the type of vertebrae in a given segment within the human spinal chord. As well as where body parts are found in insects such as fruit flies. Therefore, a mutation in the Hox gene can lead to body parts and limbs in the incorrect place or order in the body.

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