The effects of mutations - Rex

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The effects of mutations - Rex

Post by Fancy Rats Admin » Sat Aug 06, 2011 2:26 pm

The effects of the rex mutation

The dominant rexing gene (Re) is responsible for most of the rexes in the UK. It creates a rex when heterozygous (one copy of the gene) and a double rex when homozygous (two copies of the gene). This article only considers the standard heterozygous rex.

As far as is known, the rexing gene only affects hair production in the rat, so in order to understand its effects we need to begin by looking at normal hair growth.

The rat has a number of different types of hair:
  • Whiskers (vibrissae)
  • Guard hairs
  • Undercoat (vellus)
  • Eyelashes (cilia)
Each hair grows out of a follicle in the lower layer of the skin (dermis). The part of the hair below the surface is called the root, while the part that is seen above the skin is the shaft. Each follicle grows one hair.

The activity of the hair follicle is cyclical. There is a period of activity when the cells at the root of the hair divide continually – during this period the hair grows. After the growing period the cells at the root die back and the hair separates from the follicle. Then there is a short resting period. After this a new cycle begins and a new hair is formed. This pushes the old hair out from below and the hair is shed (moulting). The length of a hair is determined by how long the active growing period is. This varies from species to species and within one animal may vary between groups of hairs. This is why humans can have long hairs on their heads and short hairs on their arms.

The rex mutation causes the rate of hair growth to be slower, and the guard hairs are affected to a greater degree than the under coat. This means that a rex’s guard hairs are generally as short as the under coat or only slightly longer. Individual hairs also tend to be thinner and do not form properly, resulting in a kinked or curved hair.

Whiskers are sinus hairs, each one growing from an individual follicle, which is almost completely encased in a blood sinus and is supplied with up to 2,000 separate nerve endings. Many of these are pressure sensors. When a whisker touches something it bends (sometimes almost unnoticeably). This movement is amplified by the liquid blood in the sinus around the hair root, which stimulates the pressure sensors and sends messages to the brain. Using this technique the rat can easily
determine in which direction and to what degree each whisker was moved, which helps them to build a picture of the object that their whiskers touched. Whisking allows the rat to develop a 3D image of the objects, textures and spaces around it. The whole system is extremely sensitive and efficient. The rex mutation affects the whiskers too, with more short hairs and a deranged pattern as the hairs often curl in a variety of directions.

Whiskers have many other uses to a rat beyond simple navigation. They can help a rat to:
  • Swim (helping a rat detect the surface thus enabling it to keep it’s head above water).
  • Defend itself (whisker to whisker contact is a useful tool in a stand off).
  • Balance.
  • Perceive depth at short distances.
  • Find and identify food.
  • Detect airflow (helps locate tunnel entrances).
  • Identify objects by size and texture.
  • ‘See’ in the dark.
  • ‘Hear’ sound through vibration.
So we see that rats interpret their world through the sense of touch, relying primarily on their whiskers (vibrissae), and using information from millions of nerves endings throughout their skin, particularly surrounding sinus hairs (like whiskers), guard hairs and many vellus (normal body) hairs. These nerve endings are often specialised and can sense pressure, velocity and tension. Rats can ‘see’ their world through touch, which enables them to make accurate judgements without relying on sight. This is a particularly useful skill to have when it is dark.

A rat’s coat has a variety of other functions . These include:
  • Insulation.
  • Waterproofing.
  • Display (e.g. increasing physical ‘size’).
  • Colour patterning (e.g. camouflage).
So what difference does rexing make?

If a rat has curly whiskers or short whiskers its brain will compensate to make the best possible use of the information available. These whiskers are still functional, just not as useful as long, straight whiskers. Whilst such differences would reduce efficiency in a wild rat it is unlikely that a domesticated rat with curly or short whiskers will be disadvantaged in any meaningful way, though his tactile world will be more limited. Unruly curled whiskers can also cause irritation to the eyes and may need to be trimmed occasionally.

Guard hairs:

These are generally very short, thin and curled in a rex rat, which can limit the information available to the rat through touch and pressure. Erecting the guard hairs is a useful tool for a smooth coated rat to increase insulation (by trapping air) when cold/ill, and to posture and communicate with other rats. To a large degree these tools are lost to a rex rat. Their coats are generally long and thick enough to afford a good degree of insulation, but they cannot improve on this greatly by trapping air. Rat to rat communication is a very complex subject and coat display is only a small tool in a large tool box available to a rat when communicating with its own kind. However, lack of ability to ‘foof’ up their coat when sick or annoyed may make it harder for us humans to read their signals.

This is probably the type of hair that is least affected by the rex gene. However, rex coats are generally thinner and may bald with age. This can reduce powers of insulation at a time when they are more vulnerable.

Eyelashes are usually normal enough in a rex rat not to cause problems, though occasionally they can be in-growing.

Sensory Innervation of the Hairy Skin (Light and Electronmicroscopic Study) Zdenek Halata - Journal of Investigative Dermatology (1993) 101, 75S–81S; doi:10.1111/1523-1747.ep12362877
Encyclopedia Britannica – Hair
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