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English Mastiff — BetterBred Breed Page – BetterBred.com
BetterBred Breed Profile

English Mastiff

Temperament: Calm, dignified, and affectionate with family. English Mastiffs are low-energy for their size and prefer being near their people to anything else. They can be protective but are rarely aggressive, and their size means early socialization and training are essential.
Height: Males: minimum 30 inches (76 cm), Females: minimum 27.5 inches (70 cm)
Weight: Males: 160-230 pounds (73-104 kg), Females: 120-170 pounds (54-77 kg)
Life Span: 7-10 years
Outlier Index ?
0.48
Born before 2016: 0.36 Born after 2024: —
Avg Genetic Rel. ?
-0.05
Born before 2016: -0.03 Born after 2024: —
Internal Relatedness ?
-0.05
Born before 2016: -0.08 Born after 2024: —
#NameGender OIAGRIR
1 Ouachita's Hungarian Angel F 0.34 -0.06 0.00
2 Joseph's As You Wish F 0.30 0.01 0.01
3 Joseph's Brian M 0.44 0.01 -0.25
4 Lonestar Texaspete Boogie Bear M 0.29 -0.04 -0.04
5 Summer Princess Madelyn F 1.13 -0.19 0.00
6 Nya Rae F 0.37 -0.04 -0.03
The English Mastiff is one of the oldest molosser-type breeds, with ancestors traceable to war and guard dogs of the ancient world and a documented presence in Britain going back to Roman times. Large estates in medieval and early modern England kept mastiff-type dogs as guards and hunting partners for large game. Numbers collapsed during and after World War II - the breed came within a few dogs of disappearing in the UK - and was rebuilt largely from North American stock in the post-war decades, with contributions from Bullmastiff and a small number of other breeds used along the way. The modern English Mastiff descends from that narrow post-war founder base, which shows up clearly in the breed's current genetic profile.
The breed's short lifespan reflects a serious burden of disease: cancer (especially osteosarcoma and lymphoma), cardiomyopathy, hip and elbow dysplasia, and gastric dilatation-volvulus (bloat) are all common. Progressive retinal atrophy (a breed-specific form, DNA test available) and cystinuria also occur. The very large size itself is a risk factor for most of these.
VGL has reported English Mastiffs carrying an average of around 5.91 alleles across the 33 STR loci, with an effective allele count near 2.89. Both numbers are on the lower end, consistent with the breed's narrow post-war founder base. Preserving rare alleles and deliberately seeking less common ones in breeding decisions is especially important in a breed this genetically constrained. VGL testing has found English Mastiffs with observed heterozygosity slightly above expected, giving a small negative inbreeding coefficient. Given the severe post-World War II bottleneck the breed came through, that is a reasonably encouraging signal at the population level. Individual IR values still range widely, and breeders should continue prioritizing less related pairings.

Average metrics by birth year for dogs with recorded birthdates in the BetterBred database.

A 3D genetic map of enrolled English Mastiff dogs in the BetterBred database, based on allele-sharing distance across 33 STR loci. This is not a complete picture of the breed — it reflects only dogs whose owners have enrolled them. Drag to rotate · scroll to zoom · hover for dog names (public profiles only).

Historical founders — oldest 25% of enrolled dogs     Current gene pool — most recent 50% of enrolled dogs
Building plot… this may take a minute for larger breeds.

The Dog Leukocyte Antigen (DLA) region controls immune function and is the most polymorphic portion of the canine genome. Every dog carries two sets of DLA haplotypes — one from each parent — which almost never recombine across generations. Frequency percentages reflect how often each haplotype appears across all allele copies in the breed, not the percentage of dogs carrying it.

Class I Haplotypes

HaplotypeFrequency
1066
50.0%
1016
16.7%
1030
16.7%
1056
16.7%

Class II Haplotypes

HaplotypeFrequency
2046
50.0%
2014
16.7%
2023
16.7%
2017
8.3%
2128
8.3%

Class I & II Combinations

Class IClass IIFrequency
1066 2046
50.0%
1016 2014
16.7%
1030 2023
16.7%
1056 2017
8.3%
1056 2128
8.3%

The Canine Diversity Test from UC Davis Veterinary Genetics Laboratory is the foundation of BetterBred’s breed management tools. Testing your dog adds to the breed database and gives you access to the full suite of breeding analysis tools.

Order from UC Davis VGL →
OI
Outlier Index

The Outlier Index (OI) measures how unusual a dog’s allele combination is relative to the tested population of its breed. A higher OI means the dog carries alleles at lower frequencies across the breed — genetic variants that are underrepresented in the population as a whole.

Dogs with high OI are genetically valuable because they carry diversity that most other tested dogs don’t share. Breeding them thoughtfully into the broader population helps redistribute lower-frequency alleles rather than allowing them to become even rarer over generations.

The breed average shown here reflects all dogs tested in the BetterBred database. The senior and youngster comparisons show whether diversity is trending up or down in recent generations.
AGR
Average Genetic Relatedness

Average Genetic Relatedness (AGR) measures how similar a dog’s DNA is, on average, to every other tested dog in the breed. It reflects the dog’s mean kinship — how closely related it is to the population as a whole.

A lower AGR means the dog is more genetically distinct from the majority of the breed. Dogs with low AGR carry alleles at lower frequencies across the tested population, making them higher-priority candidates for inclusion in a well-managed breeding program.

AGR is conceptually related to mean kinship used in conservation genetics for managed species populations. A declining AGR average in younger dogs suggests the breed is successfully redistributing genetic diversity across generations.
IR
Internal Relatedness

Internal Relatedness (IR) measures how much identical genetic material an individual dog inherited from both parents. IR is a useful indicator of inbreeding: lower IR generally reflects lower inbreeding, and it is widely used and reported in the scientific literature.

A high IR is not the cause for alarm it might appear. Elevated IR can often be resolved in a single generation by pairing with a dog whose ancestry is less represented in the breed. It is a signpost, not a sentence.

BetterBred reports IR for reference, but it is not our primary breeding guidance tool. OI and AGR together give a more complete and actionable picture of a dog’s genetic value to the population.

AGR tells you which dogs are genetically unusual relative to the population. OI tells you which dogs are most valuable to preserve. OI specifically weights mid-frequency alleles, identifying dogs whose ancestry is underrepresented in a way most likely to benefit the breed.