Genetics Part II: Defining and Calculating Traits in Chinese Crested Dogs

Building on the basic inheritance principles in Part I, this article looks at how breeders actually define and calculate the traits that matter most in Chinese Crested breeding decisions: coat type and genetic diversity.

Defining the hairless trait

The hairless coat in Chinese Crested is controlled by a single dominant mutation, historically referred to in breed literature as the "Hr" (hairless) gene and now identified at the molecular level as a mutation in FOXI3, a gene involved in ectodermal development — the same developmental pathway responsible for hair follicles, teeth, and sweat glands. This shared pathway is also why hairless Chinese Crested typically have reduced or missing premolar teeth, a well-documented, expected trait rather than a separate disorder.

Using standard genetic notation, a hairless dog's genotype is Hh (heterozygous: one mutated copy, one normal copy). A powderpuff dog's genotype is hh (no copies of the mutation). There is no such thing as a homozygous hairless dog (HH) among living puppies — that combination does not survive to birth. This makes the hairless gene homozygous lethal, one of relatively few such traits documented in domestic dog breeds.

Calculating offspring ratios

Using a simple Punnett square, the expected outcomes of different pairings are:

• Hairless × Powderpuff (Hh × hh): approximately 50% hairless (Hh), 50% powderpuff (hh). All puppies are viable.
• Hairless × Hairless (Hh × Hh): the expected ratio before accounting for lethality is 25% HH, 50% Hh, 25% hh. Because HH embryos do not survive, the litter that is actually born skews toward roughly 2:1 hairless-to-powderpuff among live puppies, but with a smaller average litter size than a hairless × powderpuff pairing, since a portion of conceptions do not result in a live puppy.
• Powderpuff × Powderpuff (hh × hh): 100% powderpuff. No hairless puppies are possible from this pairing, since neither parent carries a copy of the mutation.

This is why experienced breeders favor hairless-to-powderpuff pairings: they avoid the embryonic loss associated with hairless-to-hairless pairings while still producing both coat types.

Coefficient of inbreeding (COI)

Separate from coat genetics, breeders track a dog's coefficient of inbreeding (COI) — a statistical estimate of the probability that two copies of a gene in an individual are identical because they were inherited from a shared ancestor, calculated from a pedigree across a defined number of generations (commonly 5 or 10). A higher COI increases the statistical likelihood that a dog is homozygous for recessive conditions such as PLL or PRA-rcd3, simply because both gene copies are more likely to trace back to the same ancestral carrier.

COI is not, by itself, a measure of a specific dog's health — a dog with a high COI that has tested clear for all known recessive conditions is not automatically less healthy. But across a breeding population, keeping average COI low over time preserves genetic diversity and reduces the population-wide risk of currently unknown recessive conditions surfacing.

Putting it together

A responsible breeding decision for this breed typically weighs three inputs together: coat-type genetics (to avoid unnecessary embryonic loss), DNA test results for known recessive conditions (PLL, PRA-rcd3), and COI (to protect long-term genetic diversity). None of these factors alone is sufficient — a low-COI pairing between two untested carriers can still produce affected puppies, just as a well-tested pairing with a very high COI still narrows the breed's genetic base over time.

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See Genetics Part I for the underlying inheritance concepts, or the health overview for the specific conditions referenced here.