Genetic Issues in Rats?

[MrLang]

I am about to go pick up 2.2 or 2.3 rat pups today.

I've been doing a lot of research and I have seen a few details about genetic issues but haven't been able to come across a consolidated list of things I should be aware of.

Homozygous Merle is thought to be fatal and the embryo is reabsorbed in the womb - this would naturally lead to smaller litters than if these genes weren't combined. Is this accurate?

Blue x Blue is thought to produce weak or failing pups. Is this accurate? Any more detail?

Rex x Rex makes hairless or patched babies (I realize this isn't a negative trait but I'll pass on that look...)





Any other similar issues like this these that I should be aware of before purchasing?

Thanks.

[Michelle.C]

I am about to go pick up 2.2 or 2.3 rat pups today.

I've been doing a lot of research and I have seen a few details about genetic issues but haven't been able to come across a consolidated list of things I should be aware of.

Homozygous Merle is thought to be fatal and the embryo is reabsorbed in the womb - this would naturally lead to smaller litters than if these genes weren't combined. Is this accurate?

Blue x Blue is thought to produce weak or failing pups. Is this accurate? Any more detail?

Rex x Rex makes hairless or patched babies (I realize this isn't a negative trait but I'll pass on that look...)

Any other similar issues like this these that I should be aware of before purchasing?

Thanks.

Pearl (not Merle) is lethal dominant (in utero). Merle can show on any Mink based rats, including Pearl. Most commonly, you do see Pearl Merles, but Merle can also show on Mink. Pearl also require Mink to show. That being said, I do Pearl x Pearl all of the time and usually end up with 8-10 babies. Imo, it's actually a positive. The doe doesn't have too many pups to feed and she doesn't get overloaded.

American/British/Russian Blue is no more or less healthy than any other variety. It's all about the line of rats you have. If you have a weak/sick line of American/British Blue, there health can be improved on by selective breeding.

Double Rex is ugly to me, but there aren't any health problems associated with being ugly. lol They just go through odd, patchy molts. Some stay patchy/baldish.

[Coleslaw007]

Semi related, I had a double rex that as a pup just looked like a really really curly rex and then her fur started falling out, I said it was because she was infected with I Am Legend and I named her Marley. She was mostly hairless for a little while and after a bit it all grew back and never fell out again, it was just a little thinner than most.

[sorraia]

As was stated, homozygous pearl is lethal in utero. There has not been any indication that this is necessarily a problem though, just means you might have smaller than normal litters.

There is another gene that is lethal homozygous. I want to say it is true roan, but I need to look up the sources again. Part of the confusion is there are several different varieties that are called "roan". Like pearl, it is lethal in utero.

Also as was stated, blue rats, whether American/British blue or Russian blue (two different genes) are no healthier or unhealthier than any other variety. It depends on the strain/bloodline you are using and what is present in that line. While I was breeding rats, I had a true breeding line of Russian blue, and had no issues with them. In the past American/British blue were thought to have bleeding issues (where the blood wouldn't clot even from small wounds), but that has proven to be a bloodline trait, and not a color trait. Other lines of blue have had no issues.

Rex bred to rex does have a chance to produce double rex/patchwork hairless, but quality will vary. Some will be nearly hairless, others will be patchy, still others may simply look like short-coated rex. The statistics are 25%, but of course reality can vary from the statistics, so a litter of rex x rex could have a couple double rex, lots of double rex, or no double rex.

Another issue to be aware of is megacolon. It is genetic, linked to a spotting gene commonly (and inaccurately) called "high white". If you know anything about horse genetics, megacolon in rats resembles overo lethal white syndrome (OLWS) in horses. In horses the foal is born completely white and dies within 72 hours (if not euthanised before then) because their colon is unable to function properly. In rats, megacolon also affects the function of the colon, but usually does not become apparent until around 3 weeks of age when the young rats start to eat solid foods. Before then, they may show signs of failure to thrive. When megacolon starts to affect them, their stomachs may become bloated and they may be unable to pass feces normally. Eventually they do die, although some people have successfully kept these rats alive on special diets. These rats are often called “high white”, but that is a misnomer. Rats affected with megacolon may be nearly completely white, have only a little white, or may have almost no white or even be solid (self) colored, and these rats may come from parents who have very little or a lot of white. The problem is not the coloration necessarily, but the genes behind it. The current scientific thought is there are actually two genes responsible. One is thought to be a dominant white spotting gene (note: this is NOT the same gene responsible for more common recessive white spotting, such as hooded, Berkshire, or even variegated or capped), the other is thought to be the gene directly responsible for megacolon when combined with the dominant white spotting gene.

[snakesRkewl]

The high white gene is either dominant or recessive.
Recessive trait high white has no ill effects on the babies while dominant trait high white causes mega colon.

Awww, who doesn't like a good double rex patchwork

[sorraia]

The high white gene is either dominant or recessive.
Recessive trait high white has no ill effects on the babies while dominant trait high white causes mega colon.

Depends on what you are calling "high white". Using it as a descriptor, yes there is a dominant "high white" and a recessive "high white". In the rat hobby "high white" is used to refer specifically to the dominant spotting that can result in megacolon. Unfortunately that is misleading (and the reason I hate the term) because the recessive hooded allele (as it is called in the scientific world and rat hobby) can also create a lot of white ("high white"). Personally I think the term should be thrown out completely and the animals should be described as what they are genetically... either they have the hooded allele, or they have dominant spotting. Dominant spotting is associated with megacolon, where the hooded allele is not.

Unfortunately these genes can also be combined, and that's where it gets tricky.

[Michelle.C]

There is another gene that is lethal homozygous. I want to say it is true roan, but I need to look up the sources again. Part of the confusion is there are several different varieties that are called "roan". Like pearl, it is lethal in utero.

Roan is not homozygous lethal in rats. You are probably thinking of Essex.

[SlitherinSisters]

The only two that I know personally to stay away from, high white x high white and hairless. High white breedings don't always result in mega colon, but it does happen and it's very disheartening. Some hairless moms are a good, some suck. They tend to have small litters in my experience. They are fine to breed as pets, but they make really awful feeder breeders.

[sorraia]

Roan is not homozygous lethal in rats. You are probably thinking of Essex.

I think that's what I'm thinking of. Like I said, different varieties get called "roan" and that lends to confusion.

[satomi325]

90% of my colony are high whites(bareback, capped, blaze, masked, patched) I haven't seen megacolon pop up yet. *knock on wood*



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