With their squashed noses and wrinkled faces, dogs such as pugs, French bulldogs and bulldogs have become a favourite with pet owners.
The dogs are often bred to emphasise certain 'cute' features, leaving them susceptible to a number of health problems, including difficulty breathing.
But a new study suggests that there is no way to accurately predict from visible features whether a pug or French bulldog will go on to develop breathing difficulties.
The findings have important implications for attempts to 'breed out' this potentially life-threatening condition.
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With their squashed noses and wrinkled faces, dogs such as pugs, French bulldogs and bulldogs have become a favourite with pet owners
Brachycephalic obstructive airway syndrome (BOAS) is the term given to the effects that the shortened head of these animals has on the passage of air through the upper airways.
The signs can range from mild snoring to severe breathing problems.
Animals suffering from significant BOAS can struggle to breathe during exercise and can even collapse due to lack of air.
The reliance of dogs on panting to cool themselves also makes animals suffering from BOAS very susceptible to overheating and developing potentially serious breathing difficulties in hot conditions.
While pugs and bulldogs are extremely popular pets, many are affected by a condition called Brachycephalic Obstructive Airway Syndrome (BOAS), which is related to their head structure.
Studies suggest that for over half of such dogs, BOAS may lead to health problems, causing difficulty exercising and potentially overheating.
A study in 2015 suggested that dogs whose muzzles measured less than half their head length, and dogs with thicker necks were at greater risk of BOAS.
But a new study by scientists from the University of Cambridge is challenging this theory.
In their study, the researchers took head, neck and body size measurements in just over 600 pugs, bulldogs and French bulldogs.
Each of the dogs had also been graded objectively for respiratory function.
The team found that while the external head measurements did have some predictive value for respiratory function, the relationship wasn't strong.
The measurements that showed the best predictive relationship to BOAS also differed between breeds.
The team found that while the external head measurements did have some predictive value for respiratory function, the relationship was not strong
Researchers from the University of California compared the DNA