What is a chimera cat?
A chimera (pronounced ky-meer-uh) cat is a cat that is made up of two distinct sets of DNA which occurs when one fetus reabsorbs its fraternal twin while in the mother’s womb. Interest in feline chimeras has recently grown due to the popularity Venus, Quimera and Narnia de la grace who have different colours on either side of their faces.
Types of chimerism
- Tetragametic chimerism: The tetragametic chimera occurs when early embryos fuse together to create a single organism. In most cases, one cell line will dominate the other, with only small populations of the second DNA within the body.
- Fetal microchimerism: This form of chimerism is known to occur in female humans and possibly mammals. During pregnancy, cells from the fetus pass through the umbilical cord and into the mother where they migrate to the bloodstream and organs. One study found male DNA was found to be in the autopsied brains of 63% of women aged between 32 and 101. Interestingly, of the 59 women autopsied, 33% had Alzheimer’s disease, and what was found was less male DNA was found in the brains of the women with AD, which suggests a possible protective effect.
- Artificial chimerism: Individuals who have received a blood transfusion are chimeras while the donor blood is in their system, however, organ or bone marrow recipients will remain chimeras for life. The donated organ (heart, cornea, kidney, lungs) is made up of the DNA of the donor. Blood cells are produced by the bone marrow, which means the blood cells of a person who has had a bone marrow transplant to treat leukemia or thrombocytopenia will contain the DNA of the donor. Blood transfusions are a part of veterinary medicine, but organ transplants are extremely rare.
The focus of this article is tetragametic chimerism, the rarest form.
What are the signs of chimerism in cats?
Tetragametic chimerism is hard to diagnose in cats. In order to confirm the cat is a chimera, two distinct populations of cell lines would need to be identified from different parts of the body. Most pet owners will never have the need to DNA their cats and if they do, most DNA tests only take samples from one location (usually a cheek swab). The less dominant set of DNA may only be confined to a minute part of the body, evading diagnosis.
There are only 100 or so known cases of tetragametic chimerism in humans, so it is safe to assume that tetragametic chimerism in cats is also rare.
Signs a cat may be a tetragametic chimera include:
Heterochromia (different coloured eyes):
Heterochromia is a condition in which the cat has done blue eye and one green eye. The most common cause of heterochromia in cats is due to the white masking or the white spotting gene which inhibits the production of melanin in certain areas of the body. Mosaicism and chimerism can also potentially cause heterochromia, but rare.
The cat has both male and female genitalia or ambiguous genitalia.
Male calico or male tortoiseshell cats:
Tortoiseshell coats are a mix of black, brown or blue and orange or cream and the calico coat consists of black, brown or blue and orange or cream with the addition of white patches. The majority of tortoiseshell and calico cats are female because the orange gene is located on the x chromosome. Male (xy) cats can only inherit one copy of the O gene, and will therefore be orange or non-orange (usually black). Female (xx) cats can inherit one copy of the O gene and one copy of non-orange (o). Because she has two x chromosomes, each cell in the body turns off one copy of the x chromosome (known as lyonisation or x inactivation). Therefore her coat pattern will express both O (orange) and o (non-orange) at random. The only way a male cat can be a calico is if he is a chimera, or more likely an extra X chromosome (xxy), which is a condition known as Klinefelter syndrome.
Areas of skin pigmentation:
This is more apparent in people who tend to have fairly monotone skin and hair colour. More than one coat colour in cats is common and more often caused by coat genetics such as tabbies who have areas of dark fur with a striped base colour, white spotting gene, partial albinism (pointed cats), and lyonisation in female cats.
Famous chimera cats…or are they?
Venus, Quimera and Narnia de la grace have reached celebrity status on social media. These popular cats are also known as ‘two-faced or split face‘ cats due to their unique faces which are made up of two colours split equally down each side. Apricot, an orphaned kitten rescued by Nashville Cat Rescue is also reported to be a chimera.
Venus and Quimena are calico cats, and while their coat patterns are certainly unique, it is unlikely due to chimerism. The heterochromia is likely due to the presence of the white spotting gene.
Narnia is a little more interesting because he is a male cat with a face that contains one half black fur and one half blue fur. Black is dominant over dilute blue, and therefore these two colours should not appear together. A likely explanation is somatic mosaicism. In the mosaic cat, cells originate from one original embryo, but a mutation or abnormality occurs in a cell of the developing embryo resulting in a block or blocks of cells genetically different from the rest of the embryo. The embryo develops into a kitten with two distinct cell lines. So, the cat may be genetically blue, but the distinct cell line may affect the hair and produce spots of black.
At the time of writing, genetic testing hasn’t been performed on any of these famous cats to confirm they have two distinct sets of DNA, and it is unlikely that these cats are a true chimera cat. What is more probable is these unique facial markings are caused by the randomness of X inactivation.
Frequently asked questions
Do chimera cats have chimera kittens?
It is unlikely that a tetragametic chimerism cat would produce chimera kittens. Chimerism occurs when two zygotes fuse and form one individual.
How common are chimera cats?
We don’t know how common chimera cats are because most domestic cats will never be tested for chimerism.
Are calico and tortoiseshell cats chimera cats?
No, calico and tortoiseshell cats are not chimeras.
Can humans be chimeras?
Yes, humans can be chimeras. There have been several high profile cases of human chimerism. During her third pregnancy, Lydia Fairchild separated from her partner Jamie Townsend and applied for government assistance. Genetic testing showed that Jamie was the father but Lydia was not. Lydia was accused of fraud and threatened with the loss of her children. The case was solved when Lydia’s lawyer, Alan Tyndall heard about a case published in The New England Journal of Medicine in which Karen Keegan was also found to have different DNA to her children. Karen was later found to be a chimera. DNA in cells from a cervical smear matched those of her children.
What is the difference between a chimera and a Janus cat?
Medically termed craniofacial duplication, a Janus cat has a face that is fully or partially duplicated. This is due to abnormal activity by the sonic hedgehog (SHH) protein. SHH and its equivalent gene play an important role in signalling craniofacial patterning during early development. The Janus cat has one set of DNA while the chimera cat contains the DNA of two different individuals.
How much does a chimera cat cost?
It isn’t possible to speculate on the cost of a tetragametic chimera cat because they are so rare, and it almost always goes undiagnosed. The only way to determine if a cat is a chimera would be DNA testing from multiple tissue samples which would be invasive, expensive and unethical to do unless there was a medical reason to do so.
Do chimera cats have health problems?
As there very little information or research on chimerism and its impact on the health of affected cats. Specific health issues could potentially be a problem for a cat that carries two sets of DNA. Singer Taylor Muhl is a chimera who has spoken out about her autoimmune issues and allergies that are the result of her body’s immune system reacting to the DNA of her absorbed fraternal sister.
How do I know if my cat is a chimera?
The only way to determine if a cat is a chimera is through genetic testing. Multiple tissue samples would need to be evaluated to look for evidence of different populations of DNA.
Chimeras in mythology
In Greek mythology, the chimera was a fire-breathing monster with a lion’s head, a goat’s body and a serpent’s tail.
Feature image: Eric Parker, Flickr