**5. Popular sire effect**

The popular sire effect, also known as the popular sire syndrome, may occur when an animal with a certain trait or attribute becomes desirable. This sire will then be bred repeatedly with many females, causing a reduction of genetic diversity and allows for unconnected, undesirable genetic traits to spread rapidly. An example of selective breeding is akin to the Founder Effect that we attribute to humans. Dominant traits that have low reproductive fitness are typically removed from the gene. However, recessive traits are silently passed on until the two carriers of the recessive allele mate and have a homozygous recessive individual who is affected by a disorder. Carrier screening of the Clumber spaniel for the c.754C > T PDP1 founder mutation revealed an interesting pedigree shown below. The lines with multiple crossings highlight the complex level of consanguinity found in a subset of the breed (**Figure 3**). An example of a popular sire is highlighted by a gold square in **Figure 3**, where pedigree and genotyping analysis revealed that he was bred 11 times.

According to the Clumber Spaniel Club of America, the Clumber spaniel was one of the first breeds to be recognized by the American Kennel Club in 1884. In both the United States and the United Kingdom, it is estimated that approximately 200 Clumber puppies are registered per year. The dogs have an average lifespan of 10–12 years [23, 24]. Tests are easily available for PDP1 deficiency. The Canine Health Information Center of America reports that 362 PDP1 tests have been performed with 315 dogs being homozygous normal and 47 carriers identified [25]. Statistics from the 2018 Clumber spaniel breed health plan reports of 195 dogs tested, only 4 carriers have been identified. However, at least one parent of a breeding pair producing litters from 2015 to 2017 has not had their carrier status tested [23].

Screening of a second autosomal recessive disorder, referred to as Exercise Induced Collapse (EIC) began for Clumber spaniels in 2015. The mutation is in the dynamin 1 gene (*DNM1* gene) and was originally identified in Labrador retrievers by Patterson and coworkers in 2008, where linkage analysis of affected animals led to identifying the candidate gene. Sequencing of the *DNM1* gene of affected animals identified a mutation in a highly conserved region of the gene. The mutation c.767G > T alters a *Sml*I restriction site and a simple RFLP analysis can identify carriers and affected animals [26]. EIC is a neuromuscular disorder that presents as exercise intolerance in the absence of lactic acidosis. In contrast to the PDP1 disorder, EIC has a later onset of clinical features, at approximately 2 years of age and rarely results in early death, and does not appear to be completely penetrant. Clinical features include muscular weakness after 5–20 minutes of strenuous exercise, seen as the unsteady and uncoordinated gate of the hind limbs. Dogs typically recover from the episode within 15–30 minutes. In rare cases, EIC can progress to full-body weakness, confusion, loss of consciousness, and seizures [27]. Of the Clumbers 320 tested for EIC in the United Kingdom up to 2018, 40.8% were carriers and 4.7% were affected [23].

Pedigree analysis, linkage analysis, and modern techniques such as SNP arraybased analysis of pure-breed dogs can be used to determine the coefficient of inbreeding (COI). COI refers to identical genetic information by descent and is calculated between 5 and 10 generations. It is the probability that two genes at any locus in an individual are derived from the identical diploid alleles of a previous generation [28]. For example, the COI for first cousins is 6.25%, half-siblings is 12.5%, and full siblings is 25%, provided the dam and sire do not pass through the same animal twice. For an autosomal loci, the inbreeding coefficient (*F*) of one individual is as shown [29]:

$$F = \left(\bigvee\!\!/2\!\right)^{(u\_1+u\_2+1)}\tag{1}$$

where n1 and n2 are the numbers of generations that separate the individuals of the inbred population. An example of this calculation is shown in population genetics by Johnson, Keats, and Sherman, 2019, for first cousins related through two grandparents is *F* = (1/2)<sup>5</sup> + (1/2)5 = (1/2)4 = 1/16 or 6.25% [29].

In 2013, the COI for the Clumber spaniels in the United Kingdom was calculated to be 18.2%, which suggests low genetic diversity [30]. Repeated matings of the same dam to the same sire with complex consanguinity loops (**Figure 3**) are likely responsible for the loss of genetic diversity in this breed. In 2003, the UK Kennel Club recognized that the Clumber spaniel was a vulnerable British breed and was actively trying to decrease the COI by limiting the use of stud dogs and publishing the COI of breeding animals. A popular dam has a more limited effect on the COI, due to the constraints of the female reproductive cycle and litter count. With concerted effort and diligence, the COI for this breed had been reduced to 16.2% by 2017. The UK Kennel Club has been encouraging *PDP1* testing results to be included on its Mate Select pages [23].

In a small founder population, such as the Clumber spaniel, random genetic drift will increase relative to selection, where beneficial alleles can become lost and harmful alleles can increase in frequency in reference to larger populations. Current technologies allow investigators to study and identify harmful alleles in founder populations. In 2023, Donner and coworkers performed SNP array analysis for over one million dogs, with 242,665 purebred dogs and 811,628 mixed-breed dogs [31]. Their data, using an unparalleled population size and following over 250 disease-associated genetic variants, show that purebred dogs indeed have a lower mean heterozygosity level than mixed-breed dogs as mathematically predicted over 100 years ago by the pioneers of population genetics. This exhaustive study reinforces the need to maintain diversity by outcrossing programs to increase heterosis (hybrid vigor introduced by mating two unrelated purebred lines that each have the desirable trait) and reducing both the use of popular sires and repeated mating with the same dam.

The genotypes of the two known heritable autosomal recessive disorders affecting the Clumber spaniel can now be determined for all breeding animals. If all carriers are restricted from mating, one would assume that the disorders would be eradicated from the breed. However, many of the dogs being bred in both the United Kingdom and United States are not being genotyped and are referred to as "Heredity Clear" meaning that the disorders were not seen in the pedigree nor within the family history of the dog. In his 1967 edition of Introduction to Quantitative Genetics, Falconer calculates the number of generations required to effect a specific change in gene frequency in an unwanted recessive allele breeding program. For example, as calculated on page 36, if the incidence of a rare autosomal recessive disorder is 1/40,000, and simply restricting the mating of homozygous affected individuals, it would take 59 generations to reduce the frequency by half [32]. Therefore, if breeders of the Clumber spaniel fail to restrict breeding carriers, it is unlikely that the diseasecausing alleles will be eradicated from the breed.

To address this concern, concerted efforts of kennel clubs around the world are encouraging the publication of carrier status for breeding Clumber spaniels, which is apparently reducing the reported incidence of both PDP1 deficiency and *Founder Effect: Breeding a Dog for the Elderly Gentleman Reveals an Animal Model… DOI: http://dx.doi.org/10.5772/intechopen.113912*

EIC. However, due to the popular sire effect, the breed is approaching extinction if intervention is not taken. It concerns the effective population size (*N*e) of the dogs. *N*e is the number of breeding males and females in the population, according to the equation:

$$N\_{\epsilon} = \frac{\left(4 \times N\_{\omega} \times N\_{f}\right)}{\left(N\_{m} + N\_{f}\right)}\tag{2}$$

where *N*m is the number of breeding males and *N*f is the number of breeding females [12]. In an effective population, *N*e is 100 with a sex ratio of 1 (equal amount of breeding males to females). Due to the extreme usage of a popular sire, the Clumber spaniel has very restricted inbreeding, and the Effective Population Size (*N*e) is currently measured at 24.5. When *N*e is lower than 100, the population size is considered critical and below 50 suggests that a breed is close to extinction [23].

### **6. Conclusions**

The concept of purebred dogs is thought to have been created in Victorian Britain, between 1837 and 1901. However, the origin of the Clumber spaniel can be traced back to the mid-1700s, when the 4th Duc de Noailles kept a unique kennel of dogs, he kept for hunting which he shared with the 2nd Duke of Newcastle. Historical documents describe how the French Revolution caused a bottleneck event for the breed, and the Clumbers we see today are ancestors from the kennel belonging to the Duke of Newcastle at Clumber Park and carefully selected for by his gamekeeper. The Clumber was rarely seen outside grand estates until 1859 when it became fashionable to own the breed as a show dog. In the 1900s, show breeders began to favor an increased weight for the breed, and between 1908 and 2008, the weight of the show dog doubled in size [2].

Radical selection to exaggerate any features is not in the best interests of any species, as first described in the theoretical works of pioneers in the field of Population Genetics, Bottleneck, and Founder Effects such as Darwin, Mendel, Garrod, and Mayr. Recent advances in genetics and genomics allow for the identification of genetic variations. Biochemical assays allow for the correlation between mutations and how they lead to enzyme deficiencies which can then be linked to the pathogenesis of disease. Once a disease-causing mutation is known, techniques can be developed that allow for genotyping of individuals and follow the heritability of the variant within a pedigree. These technologies now allow us to prove the mathematical theories of the past. With this knowledge comes reproductive choices. Luckily, the owners and breeders of the Clumber spaniel family of canines remain lovingly invested in preserving this wonderful and royal breed and have been choosing genetic testing to reduce the coefficient of inbreeding, and through genotype analysis to identify and discourage breeding of known carriers of *PDP1* and *DNM1* disorders. Giving context to well-established mathematical theories, through the odyssey of the Clumber spaniel, allows for a greater understanding of population genetics (**Figure 4** and **Table 1**).

#### **Figure 4.**

*The above 7-month-old puppy was rescued from a puppy mill at 8 weeks old. He was the runt of the litter and has now been genotyped, testing positive for PDP1 deficiency. This picture is used with permission of Christy Edwards of North Carolina, USA, who is devoted to Clumber spaniel rescues.*


**Table 1.**

*Proportion of alleles that are shared by related individuals, adapted from Population Genetics by Johnson et al. [29].*
