The white tigers of Rewa and Clifton: Mendelism and Bristol Zoo

On 20-21 July we will be welcoming people to Bristol and online around the world to our Mendel at 200 conference. For those who will be joining us face-to-face, there is an opportunity to visit the historic Bristol Zoo Gardens. George Davey Smith shares the story of some of the early zoo residents and how they relate to Mendel’s discoveries.

The 20th July is the bicentenary of the birth of Gregor Mendel – the so-called “father of genetics”. We are marking the occasion by holding a two-day (20th-21st July) meeting at Bristol Zoo – with a free online attendance option, with a stellar cast of internationally recognised experts in the history, ethical discussions and latest research building on Mendel’s work.

The meeting is being held at Bristol Zoo, the world’s oldest provincial zoo, which opened in 1836, when Gregor Mendel was only 14. It provides a glorious setting for such a meeting, and also played a part in an international Mendelian drama.

In 1963 the zoo acquired two white tigers – Champak and Chameli – bought from the Maharaja of Rewa for more than £100,000 in today’s money. These extraordinary animals – the only ones of their type in Europe – increased the attendance figures, and by 1970 the zoo held one third of the captive world white tiger population. At this time each tiger was valued at around £200,000 in today’s money. But where had the tigers come from, and what is the link with Mendelism?

The Bristol Zoo white tigers came from a lineage that was established in captivity by the Maharaja of Rewa, then a princely state – and now part of Madhya Pradesh – in India. They had been seen in the wild around Rewa for half a century, but were not true albino tigers, having brownish stripes on an off-white background and blue eyes (true albinos lack any pigment, have pink eyes and are snowy white all over). The Maharaja named the white male tiger cub he first obtained Mohan. He kept Mohan with an orange (i.e. normally-coloured) tigress, Begum, who, in three litters, gave birth to 10 normally-coloured cubs. Mohan mated with one of his daughters, Radha, resulting in 14 offspring, some white and some orange.

Mendel famously crossed peas with different features – including having yellow or green seeds – to demonstrate the regularities in heredity that now bear his name. Using contemporary terminology and interpretation, white tigers can be used to illustrate these regularities. Mohan must have received a copy each of the same allele (labelled w in the figure below) from his mum and his dad. At each location in the genome, individuals have two alleles. When germ cells (sperm and ova) are formed, it is a matter of chance which allele ends up in each germ cell. Begum had two orange (W) alleles; she and Mohan can be referred to as homozygous for the coat colour trait. All of their offspring would thus have a w from Mohan and a W from Begum, be designated Ww and referred to as heterozygous at this locus. W is the dominant allele, thus all the offspring were orange coloured, as in the first row of the figure below. When Mohan mated with his daughter Radha (second row of the figure), each offspring will have received one allele by chance from both parents, and (with large numbers of offspring) the ratio of colours would be 1 orange to 1 white. When two orange heterozygous Ww tigers are mated – such as two of the grandchildren of Mohan and Begum, Ramana and Ramani, were – their offspring will be in a 1-2-1 genetic ratio (WW, 2Ww, ww), and a 3 to 1 orange to white colour ratio.


(Image source)

As with Mendel’s extensive pea crosses, the theoretical expectations based on a model that the white colour is recessive to orange and that alleles segregate equally can be tested against data. As the table below shows, this expectation is close to what was observed.

Indeed, more recently the causal genetic variant has been identified as one leading to a single amino acid change in the transporter protein SLC45A2 (although, as with everything in genetics, there is greater complexity hidden in the spectrum of coat colour variation within white tigers).

It is clear there was considerable interest in both the genetics of the captive white tigers and in their financial value. When Champak (who was male) and Chameli (who was female) arrived in Bristol in 1963 thoughts turned quickly to their potential offspring. Chameli produced three litters – all white, of course – of 14 cubs in all. All four of the first litter died soon after birth and one of the second litter also died young and was devoured by Chameli. The third litter (see the photograph below, with growth monitoring perhaps being illustrated) also prefigured an unhappy story, of generally short lives. Indeed, one of the five was already dead by the time this early photograph was taken. Something was clearly amiss, and it was not anything specific to Bristol.


The reason for the poor health of the white tigers is not to do with the single amino acid change, but rather with the extreme inbreeding which was employed to extend the highly profitable white tiger lineage. The first cubs – including Champak and Chameli in Bristol – were a result of a father-daughter mating, and were siblings who were subsequently mated. Ramana and Ramani (mentioned above) were also both grandchildren of the same grandparents, whilst also being siblings. This inbreeding led to high mortality and congenital facial, eye, gastrointestinal tract, cardiac, kidney and other conditions. This was recognised early, and in the Bristol Zoo genealogy (below) the reason Seeta was exchanged for the white tiger Roop from the Delhi Zoological Park (where I witnessed many white tigers when living in the city in 2009 and 2010) was both to generate a better sex balance and in an effort to reduce the extent of inbreeding.

There has also been the suggestion that inbreeding has led to white tigers being more aggressive (perhaps from perceptual difficulties and stemming from fear). High-profile stories, such as when the white tiger Jupiter killed Chuck Lizza and Joy Holliday of “Cat Dancers” (an exotic tiger entertainment act), and the even more sensational story of the white tiger Montecore (which apparently translates into “maneater”) nearly killing Roy Horn of Las Vegas megastars Siegfried and Roy certainly raised the profile of this possibility (although in the latter case it is difficult to believe anything about the story). However the apparently innocent play below might be less benign than it looks.

As the news of the poor health due to inbreeding suffered by the white tigers became more widely known, their visitor attraction fell, and by mid-1985 they were gone from the Bristol Zoo collection. The inbreeding and display of the white tigers would certainly not fit with the ethos of the zoo (advanced in 2008) to “maintain and defend biodiversity through breeding endangered species, conserving threatened species and habitats and promoting a wider understanding of the natural world”.

Sadly, the Bristol Zoo Gardens site will be closing its doors to the public at the end of this summer. All the animals and activities are moving to the much-bigger Wild Place site in South Gloucestershire. This newer site has been purpose built with the zoo’s conservation goals at its heart, but it will be sad to say goodbye to the zoo that has been in the heart of Clifton in Bristol for nearly two centuries. Those of us who will be attending the conference in person will have a chance to visit the zoo site as it starts its ‘Big Summer Shutdown’ (admission to the zoo is part of the package for all face-to-face delegates at our conference).

We will also be joined by participants and speakers from all over the world via zoom. Registration for the conference, either face-to-face or online, is available up to the day (although in-person places are limited). Find out more information and register here.

Note: Andrew Flack’s excellent book on Bristol Zoo, “The Wild Within: Histories of a Landmark British Zoo. 2018, Charlottesville: University of Virginia Press” alerted me to the story of the white tigers in Bristol Zoo.

From a love of puzzles to studies on BMI – what Mendel’s legacy means to me, and to my cat

In the first of a series of blog posts celebrating 200 years since the birth of Gregor Mendel, Lavinia Paternoster shares how learning about genetics at school shaped her future career – and introduces us to a cat called Mendel


A cat called Mendel.
A cat called Mendel

For as long as I can remember I’ve loved spotting patterns, spending hours as a child playing logic puzzles and, more recently, Sudoku. I love how just a few simple rules can be applied to break the code of seemingly complex patterns. So when I was introduced to Mendel’s pea experiments during my A-levels it was like I got to use my nerdy love of puzzles in the classroom. Compared to how hard I found languages and chemistry, I couldn’t believe that solving these little crosses to determine the genetic inheritance of pea traits counted as work. I had a very supportive biology teacher who nurtured my passion by sending me home with a jar of fruit flies over the Easter holidays to perform my own inheritance crosses (more in homage to Morgan’s drosophila crosses, but quicker and requiring less horticultural skills than growing pea plants). I was hooked and quickly signed up to study genetics at university. 

Today I still love the simplicity in the way that the laws of genetic inheritance work to influence even the most complex of human traits. Now working on human traits such as eczema, BMI and even how a disease progresses over time, most of my work involves the simplest of statistical tests (performed millions of times, in an approach called genome-wide association  studies) to identify which variants in our genomes influence these important outcomes. 

I often think about my earliest introduction to genetic inheritance and how lucky I was to find my imagination captured by those beautifully simple genetic crosses performed by Mendel. Naming my own cat in his honour, I often find myself chatting to a random passers by outside our house about Mendel and his pea experiments. Whilst glad I share some of Mendel’s (the man not the cat) love of genetic inheritance, I definitely do not also share his talent in the greenhouse, struggling to keep the most low maintenance of plants alive. But I somewhat blame Mendel’s love of digging (the cat, not the man, this time)!


  • To celebrate Mendel’s 200th birthday we are holding a two-day conference, online and in-person in Bristol on 20-21 July. For more information and to sign up, see our Mendel at 200 pages and follow #Mendel200 on social media for Mendel activities around the world.


Why siblings are interesting for genome-wide association studies

Neil Davies discusses a new paper on a genome-wide association study of almost 180,000 siblings and discusses what additional insight siblings bring to such studies.

Thousands of genome-wide association studies (GWAS) have been published, however, the vast majority have used samples of unrelated individuals. We have recently published a sibling GWAS published in Nature Genetics. In our study, we used almost 180,000 siblings across 19 studies from around the world. But why are siblings interesting for GWAS?

GWAS have already identified tens of thousands of single nucleotide polymorphisms (SNPs) related to phenotypes – using samples of unrelated individuals. However, correlation is not equal to causation. Increasing evidence suggests these associations can be driven by more than individual-level biological effects.

There can be three key sources of bias. The first potential bias is population stratification. This means the differences in the frequency of the genetic variants that relate to phenotypic differences. For example, Iron Brew consumption will associate with variants more common in Scotland. These associations are biased evidence of the causal effect of the variant on the phenotype!

The second bias is assortative mating. People don’t mate at random. For example, studies have shown that more educated people tend to have more educated and taller partners. Such trends can result in biased associations between SNPs and phenotypes in the offspring.

The third bias is indirect parental genetic effects (also known as dynastic effects).

In these, the genotype is expressed in parents, which in turn affects offspring outcomes. One example of this is that the education of parents may influence educational outcomes in the offspring, again biasing SNP-phenotype associations.

How can data from siblings help overcome these biases? Siblings inherit their genetic variants from their parents at random. They are nature’s randomized control trials. If the siblings who share the genotype have more similar trait measures, researchers can be more confident that the genotype is influencing the trait directly.

Looking at the differences between siblings controls for each of the sources of bias above.

Which phenotypes suffer most from these biases? In our Nature Genetics paper, we estimated the shrinkage from the population to sibling estimates for 25 phenotypes, to see which suffered most from these biases. We estimated this by looking at how much the associations shrunk between the population estimates (without comparing within siblings), to the within sibling estimates. The larger shrinkage in the LD-score regression plot below indicates more bias.

We found that previously reported genome-wide association study (GWAS) associations, which typically use more widely available population samples of unrelated individuals, tend to overestimate direct effects for many traits including educational attainment, cognitive ability, age when first gave birth, whether someone has ever smoked, depressive symptoms and number of children. We also found that estimates of heritability, genetic correlations and other genetic analysis methods could substantially differ when calculated using estimates from siblings.

Biases do affect genetic correlations

A major finding from our research was that these biases do affect genetic correlations. When we use sibling cohorts, the genetic correlations from LD-score regression between educational attainment and traits such as height and BMI are not detected. Note the change in power and precision in the plot below. This suggests that the correlations that are detected in population samples are unlikely to be due to a causal effect of the genetic variants in the individuals.

Are recent findings on polygenic adaption robust to these biases? Yes, height is likely to be under polygenic selection. This suggests that selective pressures in the human population have affected the number of height-associated alleles in the population. This could lead to changes in the average height of the population over multiple generations.

Are sibling samples “better” than “population” samples?

Whether sibling samples such as we use in our study are “better” than population studies depends on the question you want to look at. Large population-based samples of unrelated individuals are great if you want to discover new genetic variants associated with a disease or other outcomes, or are interested purely in prediction.

However, if you are interested in understanding why genetic variants associated with an outcome like height, BMI, or education, then family studies can provide a powerful source of evidence. In this paper, we only looked at a very small number of phenotypes, but these results suggest that these biases are more likely for social/behavioural phenotypes, and more biological ones are less likely to be biased.

What’s next? The international collaboration established for this study is continuing to work together and explore these issues further. The next steps include using bigger samples of siblings and estimating the relative contribution of these sources of bias using samples of parent-offspring trios.

A massive thanks to all our co-authors – an international group of 100 scientists were involved in this study – and many, many others. Amazing being able to work with you all!

Read the paper

Read the press release

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Webinar on 19th May honours the career of “Aspirin Man”

In the 1970s two randomised trials of aspirin led by Professor Peter Elwood from the MRC Epidemiology Unit, South Wales made the headlines for finding that a low dose of aspirin had beneficial effects for patients who had had a heart attack.

This was just one of many important discoveries from over 50 years of epidemiological research carried out in South Wales in MRC units, including the Epidemiology Unit initially directed by Archie Cochrane, and then by Peter Elwood.

Peter joined the unit in South Wales in 1963 and led it from 1974 until it closed in 1995. Over that time he led on converting the unit from one which had researched respiratory disease and other issues to one with a focus on cardiovascular disease (which had shown increasing rates since the 2nd world war, whilst pneumoconiosis and tuberculosis decreased). Since the unit closed in 1995 he has continued working, producing more than many people do who are still employed.

In recognition of Peter’s long and valuable career, IEU’s Professor George Davey-Smith and Professor John Gallagher (Director of the Dementia Platform UK, University of Oxford), who both worked with Peter in the unit, are organising a half-day meeting in Peter’s honour on 19th May, 14:00-17:30 BST, in Oxford and online.

The meeting will feature speakers from Bristol, Oxford and UCL including Professors Nishi Chaturvedi, Andy Ness, Sir Michael Marmot and Sir Richard Peto, as well as Peter himself, discussing important topics in epidemiology. These include the role of alcohol in cardiovascular disease; how diet influences disease risk; potential causal relationships between diabetes and dementia; health inequalities, productive research environments and aspirin.

See the full programme.

Register for the online event (free) on the EventBrite page.

Biography of Professor Peter Elwood

More about the history of epidemiology in South Wales.