Popular Sire Syndrome in Chihuahuas: How One Great Dog Can Shape a Whole Breed

By Rev. T. Stringer, Lead Researcher, SIPGS

A male Chihuahua starts winning big shows.

Breeders notice him.

Many of them want a litter by him.

Each breeder has a good reason.

But when dozens of breeders choose the same male, his genes start to fill the breed.

That is called popular sire syndrome.

Why one male can change a breed

A female can only have a few litters in her life.

A male can sire dozens of litters a year.

So one male can spread his genes far faster than any female can.

He may be an outstanding dog.

The problem is not the dog. The problem is the numbers.

Frozen semen makes it last longer

In the past, a sire's influence ended when he did.

Today, his semen can be frozen and used for decades.

A male born in 2010 can still sire litters in 2040.

That is a great tool for saving rare lines.

It also means one popular male can be used for much longer.

The real size of a breed is smaller than it looks

A breed can have thousands of dogs and still be genetically small.

Geneticists measure this with effective population size, called Ne.

Ne asks: how many dogs are really passing on genes?

It depends heavily on how many males are used.

The formula is:

Ne = 4 × males × females ÷ (males + females)

Here are two breeding groups of 100 dogs each:

• 50 males and 50 females used: Ne = 100

• 5 males and 95 females used: Ne = 19

Both groups have 100 dogs.

But the second group behaves, genetically, like a group of only 19 dogs.

That is what popular sire use does to a breed.

In a study of 10 breeds, the effective number of sires was only 33% to 70% of the males actually used (Leroy and Baumung, 2011).

Why genetic diversity matters

Genetic diversity is the variety of genes in a breed.

Think of it like a toolbox. More tools means the breed can handle more problems.

Low diversity is linked to:

• more inherited disease

• smaller litters and lower fertility

• shorter lives

A 2021 study of 227 breeds found the average inbreeding level was about:

25%

That is about the same as breeding a full brother to a full sister.

Less inbred breeds had fewer health claims (Bannasch et al., 2021).

How a hidden mutation spreads

Most harmful genes are recessive.

A dog with one copy looks healthy. He is a carrier.

A puppy needs two copies to be affected.

So a carrier sire can win, look perfect, and still pass the gene to half of his puppies.

This example assumes the sire carries one copy and his mates do not:

• Years 1 to 2: He is bred to 20 females. That gives about 120 puppies, and about 60 are carriers.

• Years 3 to 5: 30 of those carriers are bred. That gives about 300 puppies, and about 150 are carriers.

• Years 6 to 8: 75 carriers are bred. That gives about 750 puppies, and about 375 are carriers.

Total in about 8 years:

1,170+ descendants

Nobody sees the problem until two carriers are bred together.

Across 10 breeds, heavy popular sire use spread genetic disorders about:

4.4 times faster

than random mating (Leroy and Baumung, 2011).

A UK Kennel Club pedigree study found that in some breeds, one stud sired over 2,500 puppies.

Many breeds lost over 90% of their unique gene variants within 6 generations (Calboli et al., 2008).

Three examples from other breeds

Impressive, the Quarter Horse

Impressive was a hugely popular show stallion in the 1970s.

He carried a dominant gene for a muscle disease called HYPP.

About 100,000 horses now have him in their pedigrees.

In testing from 1992 to 1996, 36% of the horses tested carried the gene.

Every confirmed case traces back to him.

Since 1999, the AQHA has required testing of foals from his line before registration.

The Cavalier King Charles Spaniel

Cavaliers went through several small-population bottlenecks.

A 2021 whole-genome study found they carry more harmful gene variants than the other breeds studied.

It also found gene variants tied to mitral valve heart disease, the breed's top cause of death (Axelsson et al., 2021).

Cavaliers also have syringomyelia (SM), a fluid-filled cavity in the spinal cord.

On MRI, the share of Cavaliers with SM rises with age:

• About 1 year old: about 20%

• 6 years and older: about 70%

(Parker et al., 2011)

SM in Cavaliers is highly heritable, about:

0.31 to 0.37

Breeders have used MRI screening to cut it.

In Dutch and Danish Cavaliers, SM fell from 38% to 27% in dogs born 2010 to 2019.

But researchers warn that stricter rules could remove up to 40% of breeding dogs.

That could create a new bottleneck (Limpens et al., 2024).

So the breed is stuck between two problems that popular sires helped create.

The Yorkshire Terrier and liver shunts

A liver shunt is a blood vessel that lets blood skip the liver.

Yorkshire Terriers get it far more than other breeds.

In a study of vet school records from 1980 to 2001, Yorkies were:

36 times more likely

to have a liver shunt than other breeds.

Yorkie cases rose from 0.6% in 1980 to 7% in 2000.

Researchers also studied the pedigrees.

One male showed up in:

• 98% of healthy Yorkies' pedigrees

• 79% of affected Yorkies' pedigrees

He was behind almost the whole breed.

That means he could not be blamed or cleared. Everyone descended from him.

When one dog fills nearly every pedigree, pedigree research can no longer pick out where a disease came from (Tobias and Rohrbach, 2003).

Liver shunts in Yorkies are likely caused by several genes, not one.

There is still no DNA test.

What Chihuahua DNA shows: one male line

Every male gets his Y chromosome from his father, and his father got it from his father.

It passes down the straight male line almost unchanged.

So the Y chromosome is a direct record of which sires were used.

In our Stringer Integrated Pedigree-Genomic Study (SIPGS), we read the Y chromosome of 53 tested Chihuahua males at 294 markers.

We found only:

2 male lines

One line was carried by 44 of the 53 males:

83%

Every male in that group was identical at all 294 markers.

Then we traced those males back through their sire lines on paper.

19 of the 21 males we could trace go back to one dog:

Little Pedro, AKC 286.620

He was registered in the 1920s.

His parents are recorded as unknown.

He sired 19 puppies from 7 mates in 1926 and 1927.

His descendants reach today's dogs through 7 separate branches.

Those dogs now live in the US, Russia, and South Korea.

That is about 25 generations of sons, all carrying his Y line.

That is what popular sire use looks like in real DNA.

One male from about 100 years ago is still the father of almost every male line we tested.

This does not mean every gene in these dogs came from him.

The Y is only one chromosome, and the rest of the genome is mixed from both parents every generation.

But it shows how few males have carried the breed forward, generation after generation.

Popular sires also lock in a look

Many health problems are not caused by one gene.

They come from body shape, which is controlled by many genes.

That is a second danger of popular sires.

They do not just spread hidden disease genes.

They also lock in the look that was winning at the time, good and bad.

Skull shape, bite, size, and leg length can all be pushed in one direction by a few popular males.

Why culling carriers backfires

When a DNA test shows a dog is a carrier, some breeders remove it from breeding.

That feels safe. It often makes things worse.

Every dog removed shrinks the gene pool.

The breed then leans even harder on the few "clear" males left, and that creates the next popular sire.

A better plan:

• Keep good carrier dogs in the program.

• Breed them only to tested clear mates.

• Test the puppies.

• Over time, keep clear puppies that are just as good.

No affected puppies are born, and the breed keeps its diversity.

What we can do

1. Test before you breed

• Embark for DNA health panels and genetic COI.

• OFA health clearances. Each breed has its own required tests, listed by OFA's CHIC program. For Chihuahuas, that is heart, patella, and eye (CAER).

• Structural measuring, so you know your dogs' shape, not just their genes.

• When DNA COI and pedigree COI disagree, trust the DNA.

2. Spread the sire use around

• Limit how many litters one male sires.

• The FCI and the Norwegian Kennel Club recommend that no dog have more than 5% of the breed's puppies registered over 5 years.

• Use good males that are not the current "it" dog.

• Bring in different lines when you can.

3. Health over ribbons

A win lasts one day.

A breeding decision lasts 50 years.

Questions puppy buyers can ask

• What health testing has been done on both parents?

• What is the DNA COI for this litter?

• How many litters has this sire produced?

• How long do dogs in your lines usually live?

Key takeaways

• Using a few males can make a breed of thousands act like a breed of dozens.

• Heavy sire use spread disorders 4.4 times faster in a 10-breed study.

• 83% of the Chihuahua males we tested share one male line, traced to Little Pedro in the 1920s.

• In Yorkies, one male is in 98% of pedigrees, so no one can trace where liver shunts came from.

• Testing and breeding carriers to clears protects diversity better than culling.

• Every dog we choose to breed is a vote for the future of the breed.

Join the study

SIPGS combines DNA, pedigrees, and structure to study the Chihuahua's history and health.

Chihuahua breeders and owners with Embark-tested dogs can take part.

Contact Rev. Tanya Stringer:

healthresearchataa@gmail.com

Sources

• Axelsson E, et al. (2021). The genetic consequences of dog breed formation: accumulation of deleterious genetic variation and fixation of mutations associated with myxomatous mitral valve disease in cavalier King Charles spaniels. PLOS Genetics, 17(9):e1009726.

• Bannasch D, et al. (2021). The effect of inbreeding, body size and morphology on health in dog breeds. Canine Medicine and Genetics, 8:12.

• Calboli FCF, et al. (2008). Population structure and inbreeding from pedigree analysis of purebred dogs. Genetics, 179(1):593-601.

• Leroy G, Baumung R. (2011). Mating practices and the dissemination of genetic disorders in domestic animals. Animal Genetics, 42(1):66-74.

• Limpens C, et al. (2024). Syringomyelia in Dutch and Danish Cavalier King Charles Spaniels. Frontiers in Veterinary Science, 11:1326621.

• Parker JE, et al. (2011). Prevalence of asymptomatic syringomyelia in Cavalier King Charles spaniels. Veterinary Record.

• Shannon LM, et al. (2015). Genetic structure in village dogs reveals a Central Asian domestication origin. PNAS, 112(44):13639-13644.

• Tobias KM, Rohrbach BW. (2003). Determination of inheritance of single congenital portosystemic shunts in Yorkshire Terriers. Journal of the American Animal Hospital Association, 39(4):385-389.

• UC Davis Veterinary Genetics Laboratory. Hyperkalemic Periodic Paralysis (HYPP).

• The Horse. HYPP: Hyperkalemic Periodic Paralysis.

• FCI breeding strategy recommendation on sire use (2010).

• Stringer, T.A. (ongoing). Stringer Integrated Pedigree-Genomic Study (SIPGS), paternal line analysis.

© 2026 A & A's Chihuahuas and Rev. T. Stringer. All rights reserved.

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