Lethal Factor

[Darren Lim]

We hear about the lethal factor when breeding chinchillas.

Do not breed a white chinchilla to any other type of white chinchilla.
Do not breed a velvet chinchilla to any other type of velvet chinchilla.

What does it mean?

A lethal is the result of a defective gene that causes a disturbance in the development of the growing organism that then causes it to die. There are many different kinds of lethals. They occur in any species, even humans (ex. cystic fibrosis). Lethals occur in either a heterozygous (single gene) or homozygous (double gene) state. A heterozygous lethal only requires one copy of the gene to cause a problem and that problem rarely allows the organism to reach sexual maturity, so the gene cannot be passed and dies out in the first generation. It is the homozygous lethal that we have to monitor because these genes can function in a heterozygous or "carrier" state.

Which chinchillas have a known lethal factor?

Of the mutation colors associated with chinchillas, only dominant white and domianant black (velvet) chinchillas have a lethal factor associated with their genotypes or genetic makeup. These lethals only occur in the homozygous state, so there are no homozygous whites or velvets. All whites and velvets are then heterozygous for those genes. They are unaffected by the lethal because the affected genes are paired with normal genes that act as backups for those functions that the affected genes cannot perform. Keep in mind that the white and velvet lethals are two different kinds of lethals and perform separately from each other. If you breed a white chinchilla to a velvet chinchilla, there is no lethal. It is when we breed two whites or two velvets that two of the same genes may combine in the offspring and cause the lethal.

Are we even sure these lethals exist in chinchillas?

During the early development of both the white and velvet chinchilla, breeders attempted to breed animals that were homozygous for these genes. A chinchilla that is homozygous for a gene will always pass that gene to its offspring and so has a lot of value in a breeding program, but all chinchillas proved to be heterozygous by producing normal-colored offspring. It was clear that no homozygous animals were being born. Reviewing the productivity of those animals also indicated that production went down when white x white or velvet x velvet were bred. When those same animals were switched to dissimilar partners, production went up and more babies were born. It was then suggested that the white and velvet genes were linked to a lethal factor.

So what happens when a lethal is present?

Most lethals are known to cause death in the early embryonic (or pre-embryonic) stage, resulting in a lack of production because fewer babies are born. The embryos are reabsorbed by the mother. Scientific evidence in both mice and rats supports this theory. No studies have been done on chinchillas (none that the author has been able to find and verify), but this is what breeders assume happens in the case with both the white and velvet chinchillas.

-cont-

[Darren Lim]

Is it dangerous to breed lethals?

In the author’s opinion, the information about the dangers of breeding lethals as found in chinchilla species is inconclusive (whereas the breeding of lethals in other species is better documented and no argument can be made for the breeding of). Some speculate that the process of reabsorbing dead embryos may increase the risk of infection to the mother depending upon the stage of development during which the embryo actually dies. Others feel it poses no threat. It is also suggested that breeding white x white leads to more complications than velvet x velvet (even suggesting that velvet x velvet is acceptable). It is difficult to know if reports of post-birth complications are actually associated with the lethal or other genetic factors.

Keep in mind that not all organisms affected by a lethal will die in the womb. An example of this is the Overo Lethal White syndrome in horses. A foal that is homozygous for the OLW gene will die within a few days after birth because its intestinal system has not properly formed and it cannot function properly when it eats. Some lethals may allow for viable organisms part of the time. These are referred to as semi-lethal. The merle gene, commonly found in species like dogs and rats, causes a splotchy, spotted coat in the heterozygous state and a predominantly white coat in the homozygous state. The double-merle or homo-merle may also be affected by health problems associated with organ development (eyes, ears, digestive, etc.).

As such, or until more research is done, the breeding of lethals should not be attempted by pet breeders. If you still wish to breed lethals, the first thing you have to ask yourself is why you need to breed them in the first place (since the author cannot think of a very good reason why a pet breeder would need to do so). Some people feel that breeding lethals will result in more mutant offspring, but this is a misconception on their part.