Aficionados of the hairless breeds are motivated to perpetuate and conserve these very historical and special dogs. The Xoloitzcuintles (“show-low-itz-QUEENT-ly.” or just Xolo “SHOW-low”) and the Chinese Crested dog are the more common hairless dogs but are no means the only hairless breeds. Some controversy surrounds the hairless breeds due to the nature of the hairless gene variant and its inheritance from one generation to the next. Looking at some basic genetics and how they apply to the inheritance of the mutation responsible for hairlessness will benefit the breeder trying to determine the best pairing for these unique dogs.
All genes are inherited from the parents. Half of the genetic material, or alleles, come from the mother, the other set of alleles come from the father. Two alleles, one from the dam and one from the sire, make up the genotype. The genotype will then determine the phenotype, or what trait or condition we observe in the dog. Further, we define genes as either being dominant or recessive. A dominant gene requires only one allele, passed from one parent, to express its phenotype. Whereas, a recessive gene requires two alleles, one passed from each parent, for its phenotype to be on display.
The phenotype of a hairless dogs is a condition called canine ectodermal dysplasia (CED). Not only do these dogs demonstrate a level of alopecia, or hairlessness, CED dogs often have missing teeth, or dental hypoplasia. CED is inherited as a dominant trait. This means these dogs only need one copy of the CED mutation to present with some or all the clinical signs of CED, including hairlessness. The location of the CED mutation within the DNA was initially mapped to canine chromosome 17 in 2005. This mutation was given the label FOXI3 and is specifically expressed in developing hair and teeth and is a regulator of skin development. The FOXI3 mutation has been identified as the causal gene for hairlessness in the Chinese Crested, the Xolo and the Peruvian Hairless dog. Interestingly, Chinese Crested dogs can still show a varying degree of hair on the head, legs, and tail. This suggests that there are more genes involved in the production of hair than has been recognized. Further studies of the FOXI3 mutation show that this mutation plays a role in post-natal hair growth and hair regeneration. In addition, hairless Chinese Crested dogs have only primary hair follicles, while their haired counterparts, the powder puff, have compound hair follicles like other haired breeds.
Dogs with two identical alleles are classified as “homozygous”. If these alleles are both dominant, we would call the genotype homozygous dominant. If the dog has two copies of the recessive alleles, then we would refer to them as homozygous recessive. When a dog carries one copy of both the dominant and recessive alleles then we classify them as “heterozygous”. All hairless Chinese Crested are heterozygous. They have one copy of the recessive haired allele, and one copy of the dominant hairless allele. When a Chinese Crested is homozygous recessive, meaning they have two of the recessive haired genes, then they have fur all over their bodies and are referred to as “powder puffs”.
The hairless gene variant is lethal when the mutation is homozygous (i.e. the dog has two copies of the mutation). Therefore, again, all hairless dogs are heterozygous with only one copy of the mutation. When two copies of the hairless gene are inherited by an offspring, the offspring tends to die in utero and is resorbed by the female. When two hairless dogs are mated, the resulting litter size is reduced because any embryo that gets both copies of the FOXI3 mutation usually does not survive to term.
Looking closer at this pairing, a hairless dog is heterozygous or ‘hr/HR’. They have one copy of the haired gene ‘hr’, and one copy of the hairless gene ‘HR’. If we breed two hairless dogs, each with the genotype of ‘hr/HR’, the following probabilities result. There is a 25% chance a pup would be ‘hr/hr’ and would present as a powder puff. A 50% chance a pup would be ‘hr/HR’ and would demonstrate a degree of hairlessness. Finally, there is a 25% chance a pup would be ‘HR/HR’ and, as previously mentioned, would probably not survive to term.
Some use this phenomenon as a reason to advocate for only breeding a hairless dog ‘hr/HR’ with a powderpuff ‘hr/hr’. This pairing has a 50% chance of producing a hairless dog (hr/HR), and a 50% chance of producing a powderpuff (hr/hr), depending on what allele is inherited from the hairless parent. There is no chance of producing the non-viable ‘HR/HR’ pup in this scenario. The concern with this breeding scenario would be the likelihood of producing more powder puffs in the litter.
Breeding two powder puffs, ‘hr/hr’, would result in the entire litter presenting with full haired coats. Because powder puffs can only pass on the haired alleles, there are no hairless alleles to pass on to the offspring. Is there a way to guarantee all hairless dogs? No, that would require breeding two ‘HR/HR’ dogs which cannot occur since embryos with this genotype typically die in utero.
In some dogs, ‘hairy hairless’ phenotypes are seen in which it is difficult to determine whether the dog is a hairless with some body hair or is a powderpuff with minimal coat. Genetic testing these dogs through Paw Print Genetics® will determine if the dogs carry a copy of the hairless allele. In our laboratory, we have identified hairy hairless Chinese Crested dogs as either carriers (heterozygous) or clear (homozygous recessive) for the hairless allele. Knowing the genotype allows the breeder to make informed breeding decisions, as using the phenotype alone, may lead to litters with unexpected presentations. If you have any questions about the FOXI3 gene or the hairless genetic variant, please contact us for more information at AskUs@pawprintgenetics.com or give us a call 509-483-5950 (Mon. to Fri., 8 am to 5 pm Pacific time) to receive a personal consult with one of our veterinarians or geneticists.