American Fancy Rat & Mouse Association

This article is from the WSSF 2008 AFRMA Rat & Mouse Tales news-magazine.

Colors & Coats

Rex and “Double Rex” Rats

By Karen Robbins

Rachel Amenta, e-mail
Q I’ve got a question about rat coat types, or more specifically Rex and “double Rex.” About a week ago I adopted four baby females from an accidental litter. The mother belonged to a girl who was going through some problems and had to leave her rats with her boyfriend for a short period of time. The boyfriend had rats of his own and allowed his male to mingle with this female, who is a Rex. The owner ended up giving her rats away; momma rat went to the woman I adopted these babies from, who had no idea the little girl was pregnant. I have no idea what the father of my new babies looks like. All we could find out is that the mother’s father was “double Rex,” if that matters at all. The babies appeared to be Rex at 3 weeks of age, and had full coats and curly whiskers. At 4 weeks they began to thin out quite a bit. When I adopted them at 5 weeks, their coats had come in quite nicely. Now at 6 weeks, they are starting to thin out again, and have some pretty bald patches. The hair that they do have is very curly, as are their whiskers which are pretty short. I’ve heard that many Rex babies can go through various stages of moulting and may very well lose a lot of hair and be patchy for a little while. If this were the case, at what age would they usually grow a full Rex coat? Now if these babies are “double Rexes,” will they continue to do this patchy balding stuff throughout their lives? At what age can you generally tell if you have a Rex or a “double Rex”??? I’ve searched all over the Internet and asked quite a few times on a rat forum I frequent, but nobody seems to be able to answer one way or another!

7-week Homozygous Rex Rat
The 7-week-old Homozygous Rex (Rex to Rex breeding) showing patchy, thin hair. Rat owned and bred by Karen Robbins.

All four little females do look very much like the rat pictured in the article on your web site with this caption: “This is a 7-week-old Homozygous Rex (Rex to Rex breeding). Note the patchy thin hair. Rat owned and bred by Karen Robbins.” So, basically I have “double Rexes” then? But like I said, I don’t know how much truth there is to this. I heard some Rexes will do the same patchy/moulting thing at this age.

A Based on your description, it does sound like you have “double Rex” or homozygous Rex rats (ReRe). You don’t say if there were any normal smooth-coated rats in the litter or if the entire litter was Rex or if any of the curly ones did not go through the patchy/bald stages.

26-month Rex male rat
An adult Agouti Rex male 26 months old showing the typical thinning of coat on the back due to age. Rat owned and bred by Karen Robbins.

Adult Agouti Satin Rex rat
An adult Agouti Satin Rex female 16 months old. Rat owned and bred by Karen Robbins.

To get homozygous Rex, you need to breed two Rex rats together. If you breed two “normal” Rexes (Rere) together, you will get one-quarter of the litter homozygous Rex (ReRe), one-half the litter “normal” Rex (Rere), and one-quarter of the litter normal smooth-coated rats (rere). Now if you breed a “normal” Rex (Rere) to a homozygous Rex (ReRe), you will get one-half the litter homozygous Rex (ReRe) and the other half “normal” Rex (Rere). And of course, if you breed two homozygous Rex together, you will get all homozygous Rex kids (ReRe). With continual breeding this way, these homozygous Rex can look like “Hairless” but will still have some hair, usually on the face and feet. One way to tell if your “Hairless” is actually a Hairless gene or if it is a homozygous Rex, is to breed it to a normal smooth-coated rat. If the entire litter is normal curly rats, the “Hairless” is actually just a homozygous Rex rat bred to have as little hair as possible.

Normal and Satin Rex rats
Normal Rex (L) and Satin Rex (R) brothers at 5 weeks. The Satin Rex has some bare areas on his neck and shoulders. He appears darker due to the Satinization of his coat. Rats owned and bred by Karen Robbins.

Normal (L) and Satin (R) Rex rats
The same Black Rex 5-week-old males showing their whiskers. (normal Rex L and Satin Rex R). The Satin Rex has tighter curl to his whiskers than his brother. He appears darker due to the Satinization of his coat. Rats owned and bred by Karen Robbins.

Since you describe all four of your rats to look like homozygous Rex, then it is more likely that the Rex mom was bred with another Rex (one of them had to be a homozygous Rex) to get that many. Normally, in a litter of “normal” Rex to “normal” Rex, you don’t get this many homozygous Rex.

As far as the mother’s father being “double” Rex, that means that if he is bred with a normal smooth-coated rat, all the babies will be Rex. If he was bred with another Rex, then the percentages are above. Since it wasn’t known what the mother’s mother was, you don’t know if the mother of your babies was a smooth coat, “normal” Rex (Rere), or possibly a homozygous Rex (ReRe) herself.

9-week Homozygous Rex Rat
The same Homozygous Rex from above at 9 weeks. He went through his life looking much like the 7-week-old photo with a little less hair unless he was moulting, then he had a very short, even coat of hair like in this photo. Rat owned and bred by Karen Robbins.

As far as being patchy/bald, my “normal” Rex never go through any stage like this until they get old. As babies, their coat may get a little short overall when they are getting ready to moult into their first adult coat, but they maintain an even coat of hair.

The only other kind of Rex that I have bred that may get patchy/bald spots when they get ready to moult for the first time are Satin Rexes. They will get bare/very short areas on their shoulders, but once their first coat comes in then they maintain a complete coat the rest of their lives. They do have curlier whiskers than normal Rex.

Since you describe your rats as looking like the homozygous rat in the photo, then your babies will probably go thorough their lives looking like this with times when they may have a very short even coat when they moult. The one in the photo maintained this fairly even look the rest of his life of little/no hair broken up with his getting an even coat of hair when he moulted. Some homozygous Rex will not look this even, but be more patchy/uneven where they have hair/no hair.

Breeding Rex to Rex is never suggested as you do get these rather mangy-looking rats. For the first-time breeder not knowing genetics and doing this type of breeding, may think their baby/babies that come out looking like this may have some kind of parasite or dreaded disease. *

Go to Rex Rat
Rex Breeding

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Updated September 21, 2018