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Bannasch D., Young A., Myers J., Truv K., Dickinson P., et al., 2010. However, as the cost of targeted sequence capture and next-generation sequencing continue to decrease, fine-mapping regions of extensive LD should become less problematic, enabling unprecedented haplotype resolution and prioritization of genetic variation for further investigation. For a typical binary GWAS (e.g., a case-control study), the allele frequency differences for one group with a trait are compared to another group without it. Accessibility This latter approach demonstrated that principal component 1 (PC1) explained 76% of skull variance among skulls that were measured and described the continuum of morphological changes extending between brachycephalic and dolichocephalic dog breeds. Fine mapping on CFA1 defined a 296-kb critical interval that included throbospondin2 (THSB2), next to sparc-related modular calcium binding 2 (SMOC2). Finally, we thank the many pet owners, veterinarians, and breeders who have supplied us with DNA and information about their dogs for this and other studies. To date, we know little of the genetic underpinnings and developmental mechanisms that make dog skulls so morphologically plastic. A 1-Mb resolution radiation hybrid map of the canine genome. Based on this approach, Bannasch et al. Skull traits profiled by Boyko et al. Learn more This observation raises the possibility that mixing and matching genetic variants that independently regulate development of either structure enrich canine skull diversity. 2000) and physical (Guyon et al. While comparing multiple breeds helps reduce the extent of LD at a locus of interest, typically the resulting critical interval is still too large and prohibitively expensive to exhaustively Sanger sequence. Aside from boosting statistical power, crossbreed comparisons benefit fine mapping, as interbreed-associated haplotypes are inevitably smaller than intrabreed-associated haplotypes. 2004). Because linkage disequilibrium (LD) is extensive in dogs (Sutter et al. Popular proxies of overall size such as the skulls centroid or cranial base length do not linearly correlate with size across all breeds. Future studies will need to address the origins of the genetic variation that underlies traits like brachycephaly to determine whether variants sprung forth following domestication or were consolidated from wild canids (Wayne and Vonholdt 2012). Together, the morphological variation among these breeds is so diverse and readily discernible that, for many, skull shape is breed-defining (Figure 1). Multiple and ancient origins of the domestic dog, Short interspersed elements (SINEs) are a major source of canine genomic diversity, Cranial morphology of domestic and wild canids: the influence of development on morphological change, Evolutionary genomics of dog domestication, Zeder M. A., 2012. Using geometric morphometric analysis, Drake and Klingenberg (2010) found evidence of modularity between the rostrum and the neurocranium of dogs, such that changes in rostrum shape are not strictly correlated with shape changes in the neurocranium. Figure is adapted with permission (Schoenebeck et al. Our experiments revealed that Bmp3 plays an ancient role in craniofacial development, and overexpression indicated that functional differences were encoded in the variants that we identified in dogs (Schoenebeck et al. FOIA The reference assembly of the dog genome and development of single nucleotide polymorphism (SNP) chips enabled geneticists to undertake mapping studies of all types in the dog. Also note the angle of the palate relative to the cranial base. 2006). Mellersh C. S., Hitte C., Richman M., Vignaux F., Priat C., et al., 2000. 2009). Variation at FGFR3 was examined for its role in canine chondrodysplasia; however, no variants were found when sequences were compared to the Boxer (a brachycephalic breed) reference genome (Smith et al. Hunemeier T., Salzano F. M., Bortolini M. C., 2009. Thus, the rostrum angle of the bull terrier and the boxer represent opposing extremes of an interbreed continuum (Figure 3A; Nussbaumer 1982). First, geneticists recognized that different dog breeds were characterized by an enormous variety of genetically fixed morphologic traits whose genetic underpinnings were likely to enlighten our understanding of mammalian developmental biology. Canine fibroblast growth factor receptor 3 sequence is conserved across dogs of divergent skeletal size, The Genetic and Endocrinic Basis for Differences in Form and Behavior. In addition, for traits like brachycephaly, we are unable to determine what percentage of the trait is accounted for by the loci that we discovered. Also, owing to their more recent common ancestry, mice are arguably better suited for modeling the mechanistic impact that genetic variants exert on craniofacial morphology. Both of our studies relied on the CanMap dataset that entails 62,000 SNP profiles from 915 dogs, representing 80 breeds. To date, our skull trait GWAS has relied on use of breed averages as quantitative traits. Canine skull length is a complex trait. Evaluation and management of nonsyndromic craniosynostosis. Parker H. G., Kukekova A. V., Akey D. T., Goldstein O., Kirkness E. F., et al., 2007. Although the morphological variation of canine skulls has been described extensively, the identification of underlying causal genetic variants has only recently become possible. 2012). Levi B., Wan D. C., Wong V. W., Nelson E., Hyun J., et al., 2012. The dog model is young in human years, yet the remarkable insights gleaned from the eight years since its genomes public debut make it an old soul. To date, most efforts to identify the genetic underpinnings of canine skull shape have focused on brachycephaly. TCOF1 T/Ser variant and brachycephaly in dogs. A potentially telling clue is the observation that other asymmetrically chondrodysplastic (short-legged) breeds like Scottish terriers are also carriers of the BMP3 mutation, although the allele frequency of the mutation among these breeds was found to be unfixed (Schoenebeck et al. Just as important, numerous other associations with snout length and cranium shape were described, including QTL on CFA -5, -24, -30, and -32 and the X chromosome, as well as other QTL that were unique to each studys design (Figure 2B). 2004; Valdez et al. In humans, brachycephaly occurs as a result of growth zone defects within the developing skull. 2010; Schoenebeck et al. The Treacher Collins syndrome (TCOF1) gene product is involved in ribosomal DNA gene transcription by interacting with upstream binding factor. (A) The continuum of airorhynchic and klinorhynchic dog breeds, arranged in order of severity. Identification of mutations in TCOF1: use of molecular analysis in the pre- and postnatal diagnosis of Treacher Collins syndrome, Dispelling dog dogma: an investigation of heterochrony in dogs using 3D geometric morphometric analysis of skull shape, Large-scale diversification of skull shape in domestic dogs: disparity and modularity, Molecular origins of rapid and continuous morphological evolution. For example, prehistoric dog skulls excavated in Russia were from massive animals that had shortened snouts and widened palates (Sablin and Khlopachev 2002). Given its prominence as a hallmark for domestication and its indication of breed identity, we devote the remainder of this review to a discussion of our current understanding of canine skull diversity and its mechanistic underpinnings as they relate to domestication, genetics, and disease. (B) Bull terrier skulls demonstrate the continual morphological evolution in breed dogs. Some canine skull conformations are named after their resemblance to human craniosynostoses, such as brachycephaly and dolichocephaly (Figure 2). was also supported by a National Institute of General Medical Sciences Pharmacology Research Associate fellowship. Before Moreover, for the QTL that we do find, we have no way to determine the rank order of their contribution to the trait. 2007). 2005), with alternating stretches of near homozygosity separated by regions of high heterozygosity, comparably fewer SNPs should be needed to identify associative loci in dogs than in humans, where high levels of heterozygosity are the norm. Vil C. C., Savolainen P. P., Maldonado J. E. J., Amorim I. R. I., Rice J. E. J., et al., 1997. sharing sensitive information, make sure youre on a federal Goldstein O., Zangerl B., Pearce-Kelling S., Sidjanin D. J., Kijas J. W., et al., 2006. It is also important to note that morphometric approaches to quantifying shape variation have a strong bearing on what skull traits can be QTL-mapped. Cendekiawan T., Wong R. W., Rabie A. This suggests that regulation of growth at the synchondroses plays a role in the genesis of brachycephalic and dolichocephalic skull conformations. (B) GWAS of skull length demonstrates that multiple QTL are highly associated with face length. Equally dramatic to humans effect on scale, is the effect on the dogs facial features, particularly the skull (Figure 1). 2004; Lindblad-Toh et al. Curiously, Scottish terriers were also fixed for the mutation, despite their dolichocephalic skull conformation. In its most basic form, allele frequencies are tested for statistically significant differences using a chi-square test on a marker-by-marker basis. B. M., 2010. By the 19th century, a new fad had swept across Europe that the British not only embraced, but also actively promoted. There is particular deviation among extreme brachycephalic, dolichocephalic, and some chondrodysplastic breeds (Lps 1974; Nussbaumer 1976). Moreover, the authors suggest that slippage events that result in contraction and expansion of tandem repeats represent a novel mechanism of rapid evolution. A montage of canine craniofacial shape demonstrates the incredible morphologic diversity of Canis familiaris. A strong correlation exists between the distribution of retinal ganglion cells and nose length in the dog. As previously noted, PCA tends to bury subtle dog phenotypes by spreading variation across components and can lump similar types of variation together within the same component (Chase et al. Thus it make sense that these sighthounds would have a craniofacial configuration predicted to enhance horizon scanning (Miller and Murphy 1995; McGreevy et al. With >400 documented breeds worldwide, complex traits including morphologic traits such as body size, bone length and width, and skull shape, as well as disease susceptibility or even behavior could be disentangled by studying dog breeds, and the results would likely be applicable to other mammalian systems, including human (Karlsson and Lindblad-Toh 2008; Shearin and Ostrander 2010; Ostrander 2012). The Genome sequence, comparative analysis and haplotype structure of the domestic dog. Other phenotypes such as rostrum angle are poorly captured by such methods because landmark data are typically rotated to determine best fit prior to shape analysis, which effectively removes palatecranial base angle variation. Angulation between the skull base and hard palate also differs substantially between dog breeds. Thus, the shape diversity of dog skulls is probably best described using morphometric approaches that are contextually appropriate. Molecular clock estimates from mitochondrial DNA suggest domestication started as early as 135,000 years ago (Vil et al. 2004), as is necessary for spotting prey. The https:// ensures that you are connecting to the The site is secure. An unavoidable consequence of this approach is that direct phenotypegenotype relationships are broken. In dogs, a number of craniofacial anomalies can contribute to brachycephaly, including a reduction in the length of bones that form the rostrum, chondrodysplasia of the cranial base, and changes in the palate position relative to the cranial base (Figure 2; Huber 1974; Nussbaumer 1978). This suggests that, among incipient dogs, the skull was at the leading edge of several anatomical changes that would transform wolves. Dorsal, lateral, and ventral perspectives of various breeds of dogs. Zhang Z., Ersoz E., Lai C.-Q., Todhunter R. J., Tiwari H. K., et al., 2010. Lindblad-Toh K., Wade C. M., Mikkelsen T. S., Karlsson E. K., Jaffe D. B., 2005. To compensate, we and others have utilized study designs that include multiple breeds with the same trait, which likely share a common ancestral mutation (Goldstein et al. 2012). Roberts T., McGreevy P., Valenzuela M., 2010. Elsewhere, ancient dogs were smaller than wolves (Napierala and Uerpmann 2010). As proof of principle, during the 40 years it took to domesticate silver foxes, in a well-documented experiment performed in Novosibirsk, Russia, changes in cranial dimensions, among other morphologic features, were also noted as a correlate to tameness (Trut 1999; Zeder 2012). Permutation, population stratification, multiple test correction, and the use of mixed models (Kang et al. Runt-related transcription factor 2 (RUNX2), which is considered to be a master regulator of osteoblast differentiation, was shown to have a modest correlation between its total allele length and alanine/glutamine ratio vs. physical traits such as dorsoventral rostrum bend and midface length (Fondon and Garner 2004). Detection of length-dependent effects of tandem repeat alleles by 3-D geometric decomposition of craniofacial variation. While both remain potential candidates genes, no causal variants have been reported to date. 2010) to isolate the effects of individual QTL. The .gov means its official. The rapid development of both genetic (Mellersh et al. 2010). 2012). Rethinking dog domestication by integrating genetics, archeology, and biogeography. They note that inactivation of RUNX2 causes human cleidocranial cysplasia. Animal models such as mice and zebrafish can provide means of testing variant functionality though transgenesis, exogenous overexpression, and knockdown. 2008; Zhang et al. 2010; Schoenebeck et al. Online Mendelian Inheritance in Man, OMIM McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University, Baltimore, MD, c1966. 2010). This makes it difficult to go from marker to variant of interest. Statistically significant associations that exceed correction for multiple testing are indicated in blue. In practical terms, cross-contamination by allometric variation can be minimized by regressing shape by size during skull quantification and, later, by including a size covariate in the linear regression used for the GWAS. Human induced rotation and reorganization of the brain of domestic dogs, The earliest Ice Age dogs: evidence from Eliseevichi 1, Genetic analysis of craniofacial development in the vertebrate embryo. 2012). 8600 Rockville Pike As one would expect, the BMP3 mutation was absent from dolichocephalic breeds, with one exception. For obvious reasons, testing putatively causal variants identified in dogs requires biological surrogate(s), making the effort that much more challenging. Regardless of how continual morphological changes occur at the molecular level, mapping nascent causal variation will require new approaches. In many cases, these distinctions are pronounced in both the skull and its dentition. Many skulls available for morphometric analysis lack postcranial skeletons, necessitating animal size estimation from the skull itself. Yet the overall size of dogs brains relative to that of wolves has decreased by nearly 30% (Coppinger and Schneider 1995; Zeder 2012). 2011; Levi et al. However, the composition of genetic profiles and craniometric data used between the two studies differed. We thank Maud Rimbault and Jonine Figueroa for helpful comments on this manuscript. Beyond understanding the mechanisms of dog craniofacial diversity, identification of causal genetics is necessary for understanding just what makes dog breeds so morphologically diverse in the first place. J.J.S. Federal government websites often end in .gov or .mil. The x-axis lists marker location by chromosome. In practice, the strong LD within dog breeds is a double-edged sword: GWAS are quite successful at finding genetic associations; however, the resulting critical intervals are often extensive LD blocks that can extend hundreds of kilobases or more. We also used GWAS to map canine traits, favoring treatment of skull shape as a quantitative trait (Boyko et al. GAPIT: Genome Association and Prediction Integrated Tool, Biometric analysis of the skull base of domestic dogs. about navigating our updated article layout. Even in adulthood, a vast number of modern breeds display some sort of growth arrest, likened to wolf neoteny. Through the process of domestication, the modern dog emerged. Craniofacial diversity exists between and within breed dogs. 2005). In addition to targeted transgenics, the similarity in mammalian genomic architecture makes interrogation of canine intergenic variation possible using mice. Certain skull characteristics, such as the angle between the palate and the neurocranium, appear static during wolf development, whereas in dogs the angle differs substantially from that of wolves throughout development, leading to the conclusion that dog skull shape is neomorphic (Drake 2011). Brachycephalic breeds, such as the bulldog, pug, and Boston terrier, are easily recognized by their short pushed-in faces, underbite, and widely placed, shallow orbits. 2006; Parker et al. These examples of brachycephalic breeds display extreme airorhynchy, meaning that their rostra angle dorsally. A simple genetic architecture underlies morphological variation in dogs. 2012). Variation of BMP3 contributes to dog breed skull diversity, Canine morphology: hunting for genes and tracking mutations. The y-axis represents the strength of the association [log10(P-value)]. Such confounders make GWAS in dogs prone to generating false-positive associations. With a few exceptions, modern dog breeds are smaller and have snouts and crania that are proportionally intermediate to wolf neonates and adults. Schoenebeck J. J., Hutchinson S. A., Byers A., Beale H. C., Carrington B., et al., 2012. government site. The pertinence of answers awaiting canine geneticists, we believe, reaches beyond the dog, as we have demonstrated by our discussion of canine craniofacial biology and genetics. Lipka A. E., Tian F., Wang Q., Peiffer J., Li M., et al., 2012. Canine TCOF1: cloning, chromosome assignment and genetic analysis in dogs with different head types, Biometric analysis of brachycephaly in domestic dogs. (2010) were based on linear measurements, while Schoenebeck et al. Principal components analysis (PCA), which is commonly used to categorize shape across fitted datasets, also has shortcomings. An official website of the United States government. PMC legacy view Nearly 70 years would pass until genome-wide association studies (GWAS) would confirm Stockards predictions (Boyko et al. The biological function of BMP3 is not well understood; however, a number of studies suggest that it can inhibit TGF- signaling and that it restricts osteogenesis (Bahamonde and Lyons 2001). Prominent differences across breeds include palate shape (p, indicated by white dots), neurocranium shape (nc, enclosed by blue dots), cranial base length (cb, red line). Klinorhynchy, the hallmark downward-pointing snout of bull terriers, is morphologically opposite to the rostrum angle observed in breeds such as the boxer and bullmastiff. Bethesda, MD 20894, Web Policies Elevated basal slippage mutation rates among the Canidae. Chase K., Carrier D. R., Adler F. R., Jarvik T., Ostrander E. A., et al., 2002. SOMETIME during the Paleolithic, a remarkable transformation occurred. The breed average skull traits used in our studies were obtained predominantly from museum specimens within the United States and Europe. Also, despite genetic isolation from one another, skull shape continues to rapidly change within many breeds (Figure 3B). Available at: Both ends of the leash: the human links to good dogs with bad genes. As a result of artificial selection, dogs radiated to fill niches in our lives, becoming our herders, guardians, hunters, rescuers, and companions (Wilcox and Walkowicz 1995). This debate can be settled only by developing a genetic understanding of the variation underlying canine skull diversity. Haworth K. E., Islam I., Breen M., Putt W., Makrinou E., et al., 2001. This raises the possibility that the effects of the BMP3 mutation extend beyond the skull. 2010). Evolution of working dogs, pp. The authors reported a variant at the locus TCOF1 that appeared to be correlated with head shape; however, interpretation of this finding was quickly disputed when additional breeds were examined (Hunemeier et al. In this Perspectives, we discuss the origins of dog skull shapes in terms of history and biology and highlight recent advances in understanding the genetics of canine skull shapes. Human brachycephaly is associated with morbidity and is diagnostic of many syndromes, including Aperts, Crouzons, and Pfeiffers syndromes (Miraoui and Marie 2010; Johnson and Wilkie 2011; Ursitti et al. Cranial suture biology: from pathways to patient care. In dogs of all breeds, coronal synostosis is normally absent. An integrated linkage-radiation hybrid map of the canine genome, Fibroblast growth factor receptor signaling crosstalk in skeletogenesis, A new palaeolithic dog from central Europe, Relations and interactions between cranial mesoderm and neural crest populations, The problem of the posterior skull base (Hirnstammbasis) in Dachshund skulls, Biometric analysis of the skull base in small and medium sized dogs. Small numbers of gray wolves adopted a new pack masterhumans. 2010; Lipka et al. The range of sizes among dogs extends beyond that of wolves, giving dogs the distinction of being the most morphologically diverse terrestrial mammalian species known (Stockard 1941). Thus, to be recognized as a bulldog, it was insufficient for a dog to be squat in stature and display a shortened snout. 2010; Boyko et al. Adding to breed relatedness are the remnants of genetic bottlenecks and the use of popular sires among breeders, resulting in genetic substructure and allele frequencies that often deviate from HardyWeinberg equilibrium. At the other end of the continuum is the dolichocephalic appearance of breeds such as the Saluki, Borzoi, and collie. 2004; Lindblad-Toh et al. Most prominent were the studies conducted by Charles Stockard, whose detailed dog pedigrees include designed crosses and backcrosses of morphologically disparate breeds (Stockard 1941). Many small dogs from toy and teacup breeds feature brain cases reminiscent of human hydrocephalus. National Library of Medicine Each dot represents a single marker (a SNP). For example, mutation of Treacher Collins-Franschetti syndrome 1 (TCOF1), whose product normally facilitates ribosome production, results in hypoplasia of frontal and zygoma bones (Dixon et al. Figure is adapted with permission (Nussbaumer 1982). Careers, Cancer Genetics Branch, National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland 20892, Supporting information is available online at, dog, craniofacial, genome-wide association studies (GWAS), genetics, morphology. These dogs tend to have narrow, sometimes elongated, snouts and orbitals that are less forward set. and J.J.S. Kettunen P., Nie X., Kvinnsland I. H., Luukko K., 2006. Smith L. B., Bannasch D. L., Young A. E., Grossman D. I., Belanger J. M., et al., 2008. Used in conjunction with selective sweep mapping, it has been possible to detect breeder-selected genetic variation. 2150 in, Teaching an old dog new tricks: SINEs of canine genomic diversity. Disentangling morphologically allometric variation (size-related) from nonallometric variation is a formidable challenge. Guyon R., Lorentzen T. D., Hitte C., Kim L., Cadieu E., et al., 2003. This comparison resulted in detection of a large association on canine chromosome (CFA) 1, as well as a number of other associations that were not described further. Pathways to animal domestication, pp. 2008). As we delve deeper into the mysteries of skull morphology, it becomes increasingly apparent that complex and rare genetics are at play in dogs and defining their contributions will be far from routine. gratefully acknowledge support from the Intramural Program of the National Human Genome Research Institute. Fine mapping is also fraught with challenges. As museums continue to add DNA preservation as a facet of their repositories and as sequencing technologies continue to improve, it is possible that dog morphology association studies using direct phenotypegenotype relationships will be possible. 1997). In a dolichocephalic dog, the width of the rostrum and zygomatic arches is reduced, and the rostrum tilts ventrally relative to the neurocranium. Using household pets in biological research is unorthodox, yet it is this animals symbiosis with humans that makes it uniquely suited to address the genetic basis of domestication, evolution, morphology, and disease. By comparison, zebrafish offer rapid and accessible development and are amenable to gene overexpression, morpholino-mediated knockdown, and certain types of transgenic approaches. However, the idea that pedomorphism is a major driver of canine craniofacial variation is not without controversy. Linkage disequilibrium mapping in domestic dog breeds narrows the progressive rod-cone degeneration interval and identifies ancestral disease-transmitting chromosome. Z. Tierzchtg, On the variability of dorso-basal curvatures in skulls of domestic dogs. Skulls are arranged chronologically from the oldest (top) to the most modern (bottom). Lateral views are articulated so that the skull base (red line, wolf) is approximately parallel between breeds. 2010; Boyko et al. 2010). (A) Surface scans of a wolf skull morphed to illustrate the differences between brachycephalic, ancestral, and dolichocephalic skull states of canids are shown. Histological development and dynamic expression of Bmp26 mRNAs in the embryonic and postnatal mouse cranial base. Like other large domesticates, modern dogs exhibit an increased brain-to-body-size ratio (termed encephalization). 2012; Zhou and Stephens 2012), which take into account genetic potential, fixed effects, and kinship, are essential tools for reducing incidence of false positives that are encountered on a genome-wide scale. Extensive and breed-specific linkage disequilibrium in Canis familiaris. official website and that any information you provide is encrypted Dog fanciers quickly recognized that structured breeding could be harnessed to transmit desirable traits. HHS Vulnerability Disclosure, Help Today most dogs share little resemblance to their lupine ancestors. Examples include a Pekingese (1), French bulldog (2), Chow Chow (3), Bernese Mountain Dog (4), German Shepherd (5), and Borzoi (6). They examined 37 tandem repeats located within coding regions of developmentally relevant transcription factors in 142 dogs from 92 breeds. 2012). As one might expect, each model has its strengths and disadvantages. However, the cranial base of brachycephalic breeds, as well as some dolichocephalic breeds, is disproportionate to overall body size (Stockard 1941; Lps 1974). Whole-genome sequencing may provide the answer, although the analysis of such data is still enormously challenging and arguably less well suited to detect structural variants such as copy number variants and tandem repeats. While the extensive LD, relatedness, and cryptic genetic structure are potential confounders in nearly all dog GWAS studies, mapping skull traits is uniquely complicated phenotypically, largely because of interbreed differences in scale. Brachycephaly, which means short head, is a term borrowed from human medicine. Second, it also became clear that the population structure of domestic breeds would allow geneticists to overcome many of the difficulties faced when doing either linkage or association studies in human populations. The ideas of Fondon and Garner (2004) are particularly interesting. Both premature fusion of the coronal suture (bilateral coronal synostosis) and defective endochondral ossification at the synchondroses of the skull base are contributors of brachycephaly (Cendekiawan et al. and transmitted securely. The most daunting hurdle of any genetic mapping study is proving genetic causality. Szabo-Rogers H. L., Smithers L. E., Yakob W., Liu K. J., 2010. 2012). 2005), put the dog on par with traditional model organisms for performing genetic studies. Sutter N. B., Eberle M. A., Parker H. G., Pullar B. J., Kirkness E. F., et al., 2004. The neurocranium also bears discussion. Reiterating the genetic complexity of brachycephaly, the mutation was absent from medium-to-large brachycephalic breeds including the boxer (medium), bullmastiff (giant), and Dogue de Bordeaux (giant). But how such selection could work on a grand scale was not immediately appreciated and attempts to parse the genetic mechanisms that dictate how canine traits such as skull shape are passed from parents to offspring were not formally studied until decades later. New directions in craniofacial morphogenesis, Early canid domestication: the Farm-Fox Experiment: foxes bred for tamability in a 40-year experiment exhibit remarkable transformations that suggest an interplay between behavioral genetics and development. Postnatal expression in rats indicates that Bmp3 is highly expressed at synchondroses, suggesting a role in chondrogenesis (Kettunen et al. On the basis of live observation and analysis of skeletal preparations of parents and progeny, Stockard concluded that breed-defining skull shape features, such as the bulldogs shortened rostrum, did not follow patterns of Mendelian inheritance.

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