Scientists believe that more than 13,000 people in the UK have DNA which indicates they are the result of 'extreme inbreeding'.

Analysis of the UK Biobank data-bank by researchers at the University of Queensland uncovered evidence of people with whose parents are considered to be first- or second-degree relatives.

This includes children created when parents and their offspring (first degree) have a child.

It also assessed children born from the intercourse of half-siblings (second degree).

The researchers say scaling up the research is difficult due to the limitations of the data-set, but claim the real number may be even higher than the extrapolated 13,200 figure from the paper.

People born out of such extreme inbreeding often suffer myriad health concerns, the researchers confirm.

This includes reduced lung function, fertility, cognitive function and a 44 per cent higher risk of all diseases.

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Analysis of the UK Biobank data-bank by researchers at the University of Queensland found uncovered evidence of people with parents who are considered first- or second-degree relatives (stock)

WHAT IS EXTREME INBREEDNG? According to the guidelines from the American College of Medical Genetics, extreme inbreeding can be suspected if over 10 per cent of the DNA sequence in an individual has runs of homozygosity. Homozygosity is the term given to stretches of the genetic code that are identical, and therefore must have been inherited from both the mother, and the father. If this is significantly higher than normal, it can indicate a person's parents are closely related. Extreme inbreeding (EI) is often defined as genome-wide homozygosity consistent with mating between first or second degree relatives. This includes children created when parents and their offspring (first degree) have a child. It also considers that of children born from the intercourse of half-siblings (second degree). In many countries, mating between close relatives is forbidden by law. For example, mating between first- or second-degree relatives is explicitly prohibited by the Sexual Offences Act (2003) in the UK. Nevertheless, law enforcement records in the UK and other countries show that EI does occur. These few cases are likely under-reported because of the social stigma attached to them, experts say. Advertisement

The analysis into inbreeding and its potential effects on health are published in the journal Nature Communications.

Dr Loic Yengo led the research and estimated the prevalence of extreme inbreeding using anonymous data from 456,414 individuals in the UK Biobank.

It looked for an unusually high amount of homozygosity, the term given to stretches of the genetic code that are identical and therefore must have been inherited from both the mother, and the father.

If this is significantly higher than normal, it can indicate a person's parents are closely related.

'These runs of homozygosity are identical sections of DNA and can be used by geneticists to study the association between the proportion of a person's genome that is homozygous and measurements on that individual.

'This is the first time that ten per cent or more homozygosity in the genome has been quantified in a sample of nearly half a million people,' said Dr Yengo.

The authors chose the threshold of ten per cent because, according to the guidelines from the American College of Medical Genetics, extreme inbreeding can be suspected if over ten per cent of the DNA sequence in an individual has runs of homozygosity.

They then looked at whether this was associated with a number of health outcomes.

Among the participants included in the study, the authors found 125 individuals whose genetic data suggested that they were offspring of first- or second-degree relatives.

Researchers looked for an unusually high amount of homozygosity — the term given to stretches of the genetic code that are identical, and therefore must have been inherited from both the mother, and the father (stock)

However, the researchers warn that this percentage of approximately 0.02 per cent may not be comparable to the entire population, and could well be higher.

The researchers were unavailable for interview, but said in a statement: 'The extent to which our estimate reflects the true prevalence of [extreme inbreeding] in the entire UK population is a difficult question.

'The UK Biobank is known to have over-representation from healthy and highly educated individuals which likely biases our estimates.

'Highly inbred individuals who suffer severe health consequences may be less likely to participate in a study such as the UK Biobank.

'Therefore, our estimate of the prevalence might be too low.'

It also found that these people have rather a bleak health outlook, and the study confirms previous research on the impact of incest.

The authors write: 'We confirmed previous findings suggesting that inbreeding leads to reduced stature, cognitive ability, lung function and fertility.

'Moreover, we found that these 125 participants were approximately 44 per cent more at risk of any kind of disease, as compared to the rest of the study participants.

'We also found that the effect of EI on these traits can be predicted using observations from less extreme forms of inbreeding (e.g. between first-cousins).'