**Abstract**

Animal models of genetic disorders that have risen due to selective breeding can be used as a valuable model to teach the basic concepts of population genetics. The Clumber Spaniel is a breed of dog created in the mid-1700s by the 4th Duc du Noailles. He selectively bred this dog for the elderly gentleman. This sleepy-looking breed survives today, though 1% suffer from severe exercise intolerance due to an autosomal-recessive founder mutation in the pyruvate dehydrogenase phosphatase 1 (PDP1) gene. PDP1 deficiency was long suspected to be a human metabolic disorder and described at the molecular level in 2005 by Robinson and coworkers. The Robinson group later identified a founder mutation within the PDP1 gene of the Clumber spaniel. This case clearly illustrates how a detrimental mutant allele in a small population, when selecting for phenotype, can persist in the progeny of that group. In this review, we discuss the origin of the "Founder Effect" theory and present an example of how a bottleneck that occurred during the selective breeding of the Clumber spaniel over 250 years ago led to the current genetic status of the breed. Today, genotyping can help reduce the incidence of PDP1 in the Clumber breed.

**Keywords:** founder effect, population genetics, PDP1 deficiency, DNM1 deficiency, Clumber spaniel

## **1. Introduction**

Healthy genetic diversity of populations, in humans and all other animals, is subject to fluctuations in population size. In a large population, random mating will occur, and allelic changes that are not compatible with life, or of low fitness, will be removed from the gene pool. If a population becomes somehow limited by the environment, natural selection may influence allelic changes to either increase or disappear. Charles Darwin brought his idea of natural selection forth with the publication of his book "On the Origin of Species". Here he defined that natural selection

is a process whereby the population of an organism will adapt and change to suit an environment and that offspring tend to resemble their parents [1]. Notably, mutation is the source of allelic variability and beneficial mutations increase fitness, whereas deleterious mutations reduce fitness. A limited population, or founder population due to some type of bottleneck or separation from the larger parent population, may suffer a loss of genetic variation and thus be closed off to genetic diversity. In a small and limited population, deleterious mutations may accumulate. The Wright-Fisher Model, named for the early pioneers of theoretical population genetics, describes allelic frequencies of a single gene assuming random mating, nonoverlapping generations, and no selection or mutation. This model also considers variations in population sizes and how a sudden reduction in the size may distort allelic frequency. The distortion may lead to allelic variants which were found in extremely low frequency within the general population yet relatively common in isolated groups. In a population with a finite number, a deleterious allele can fix, and thus reduce its fitness. In the absence of outcrossing, the fitness of the group may continue to reduce, up to and including extinction, under the load of the deleterious alleles. There are many examples of inherited human diseases where a pathogenic variant is found at a higher frequency in particular groups or in populations that have an elevated level of consanguinity.

In animals, a phenomenon called the "popular sire" describes the reproductive dominance of specific animals, increasing the likelihood of later matings between related animals. Applying selective pressure based on the phenotypic attributes of the popular sire within a small population of animals markedly restricts genetic diversity. This becomes problematic when there is an increase in the incidence of harmless recessive alleles, or carriers, leading to an increased potential for progeny to inherit the recessive allele in a homozygous fashion, thus resulting in an autosomal recessive disorder. Corrective measures are currently being put in place by kennel clubs around the world such as pedigree analysis, coefficient of inbreeding calculations, and the development of single nucleotide polymorphism (SNP) arrays are currently being used to manage the diversity of breeds and control heritable disorders. Autosomal recessive disorders, which are known to a specific breed can be tested to identify carriers and reduce the numbers of affected pups born. The Clumber spaniel is one of the oldest of all British spaniel breeds, with over a 250-year-old history of joining the hunt with many of Europe's crowned heads and aristocracy beginning in the Georgian era. It is not a common breed, but better known to sporting gundog enthusiasts, being bred originally for work during the hunt rather than for appearance at show. The breed was named after Clumber Park, originally part of the Sherwood Forest and home to the Dukes of Newcastle in Nottinghamshire, England. First arriving in England between 1763 and 1768, the dog was selectively bred in the kennel of the 2nd Duke of Newcastle to work the undergrowth of the Sherwood Forest during the hunt, to be strong, slow, and steady during the gunfire, and have an exceptional nose for the game [2].

When a large population is suddenly reduced, for a variety of reasons, the resulting population will experience a reduction in the genetic variability known as Bottleneck and Founder Effects. Selective crossbreeding of purebred animals will increase the frequency of rare mutations, or alleles, which may cause diseases similar to the Founder Effect. Events that cause a group of humans to become isolated from a larger population, in time, will similarly increase the frequency of rare mutations. For canines and humans alike, autosomal recessive disorders require progeny to inherit two mutated alleles before the disease is expressed (homozygosity). Here,

*Founder Effect: Breeding a Dog for the Elderly Gentleman Reveals an Animal Model… DOI: http://dx.doi.org/10.5772/intechopen.113912*

we review the bottleneck events that took place during the development of the Clumber spaniel breed. We describe how selective breeding for traits important for gun hunting, such as slow and steady with a specific nose, has led to the increased frequency of mutations leading to extreme exercise intolerance in the homozygous state. The particular mutation of the autosomal recessive disorder pyruvate dehydrogenase deficiency 1 (PDP1) was first identified by us in humans and determined to be disease-causing through protein complementation assay [3]. Using the same assay conditions on cell cultures obtained from Clumber spaniels, we were able to restore pyruvate dehydrogenase activity. Molecular analysis identified the diseasecausing mutation in the PDP1 gene. A genotyping test was developed, and over 100 Clumber spaniel dogs were analyzed. It was determined that the carrier frequency of the disease-causing allele was 20% of all dogs tested [4]. Our historical investigation showing over 250 years of selective breeding of a hunting dog for the elderly gentleman led us to the current molecular diagnosis, as an example of a Founder Effect, similar to that seen in humans. What was once theorized by the pioneers of population genetics can now be proven by modern-day investigation. This review combines historical descriptions of the Founder Effect as it can currently be seen by genotype analysis.

## **2. Mendel's law and the founder effect**

In 1865, Gregor Johann Mendel presented his results of artificial fertilization to achieve color variation with ornamental plants and this work that proposed the laws of inheritance was published the next year [5]. A generation later, when scientists had a better understanding of cells and genetic material, Mendel's work was independently rediscovered by three botanists. In the early years of 1900, Hugo DeVries, Carl Correns, and Erich von Tschermak each confirmed Mendel's laws of inheritance, as reviewed by Wilkie [6]. The first human disorder identified to follow Mendelian inheritance was published in 1902 by Archibald Garrod who described alkaptonuria as being inherited according to Mendelian rules. Garrod wrote of his investigation of a family, where two of five children were secreting homogentisic acid in their urine, which is pathognomonic for a diagnosis of alkaptonuria. Alkaptonuria was thought to be a peculiar disorder at the time for two reasons. First, homogentisic acid could be observed in patient's urine from approximately 60 hours after birth and could be followed for life. Homogentisic acid causes urine to darken after exposure to air. Second, alkaptonuria did not appear to cause any detriment to the health of those with the condition but was thought to be an alternative mode of metabolism. It is now known that while the onset of clinical phenotype can be delayed in life, oxidative forms of homogentisate deposited in sclerae, cartilage, and skin can lead to early osteoarthritis and cardiac valve disease. Garrod made inquiries to several other investigators who were following alkaptonuric patients at the time, and they kindly shared their data for publication. After compiling the data for 11 families, it was found that 60% of families of children with alkaptonuria were born to first-cousin parents [7]. Garrod surmised that alkaptonuria was a congenital disorder that followed Mendelian recessive inheritance, where the rare recessive trait passed through many generations before the union of two recessive traits manifest in one zygote, as was described by Bateson in the same year [8]. The peculiar nature of alkaptonuria and the level of consanguinity in most affected families studied, allowed these early investigators to calculate the incidence of disease and postulate

the mode of inheritance at a most opportunistic time when Mendel's laws of inheritance were rediscovered.

At the time when Garrod published his findings on the incidence of alkaptonuria, and into the 1920s, the statistical study of Mendelian laws in a large population was gathering interest. Working independently, Hardy and Weinburg described similar theories that in a sexually reproducing population, the frequency of two alleles will come to equilibrium in one generation and remain unchanged provided that mating is random, the population is large and no additions to the population from outside occurs [9, 10]. The Hardy-Weinberg principle describes the relationship between allelic and genotypic frequencies in a diploid organism from one generation to another. Evolution can alter this equilibrium to include the influence of mutation, recombination, selection, and isolation. The relative frequencies of alleles in large populations can be influenced by factors that disturb equilibrium such as mutation, migration, and Darwinian selection was also considered, along with the concept of the effective population size (*Ne*). *Ne* is a mathematical representation of genetic diversity within an idealized population in comparison to a population of interest [11, 12]. In 1954, Ernst Mayr theorized that if a group of persons, which he classified as "founders", are somehow isolated from their original large, or ancestral, population, they would abruptly undergo reduced levels of genetic variability leading to the accumulation of inbreeding within this new founder population [13]. Population geneticists have developed mathematical models to estimate how allele frequencies in a large, ancestral population can be expected to change with a sudden reduction in the population and lead to a founder effect [13]. The alleles of the founder population, with the accumulation of inbreeding, will level off to a new stable allele frequency [14]. If an unfavorable allele is in the first generation of a founder population, and this unfavorable allele has a fitness between 0.5 and 1, the allele will increase in numbers while the population doubles in size [14, 15]. Experiments have been performed in the laboratory to demonstrate rapid speciation employing an insect model to demonstrate, as recently reviewed by Haudry, Laurent, and Kapun [16].

Human population genetics typically defines the founder effect as a loss in genetic variation that can arise when a new population is derived from a small number of individuals that were segregated from a larger group. The average heterozygosity seen per locus will initially decline in the smaller, reduced group initially. Over time, new mutations will increase as the population rises. Scientists have used this theory to explain why some inherited disorders can be found at a higher frequency in a defined population than is seen in the general population. There is empirical evidence that suggests an individual in a small population that has been isolated by geography, religion, or culture, can carry an alteration to their DNA. This person is referred to as the common ancestor. After many generations, as the population expands, this variation will be seen at high frequency in the group. If the DNA variant is not disease-causing, it can be considered a polymorphism that is found at high frequency in this segregated group. If the variant is disease-causing for an autosomal recessive disorder, we will see an increase in the number of persons expressing the disorder with each successive generation, and this supports the concept of the founder effect. The founder effect has also been seen to occur naturally in animal populations. However, in many cases, it is due to humans who wish to enrich specific traits in a species of animals. Founder effect is one of the causes of the concentration of genes for specific diseases in purebred (genetic isolates) dogs [17]. Breeders of animal stock actively participate in the phenomena of heredity in real-time.

*Founder Effect: Breeding a Dog for the Elderly Gentleman Reveals an Animal Model… DOI: http://dx.doi.org/10.5772/intechopen.113912*
