Introduction

While the focus of our studies involving Cohorts 1 and 2 was on dental development and morphology, we decided to concentrate more on oral health and disease, particularly dental decay, when studying Cohort 3. The major developments in molecular biology have also enabled research involving this cohort to make use of new technologies and approaches to study genetic effects more directly.

Dental decay (caries) is the most common chronic disease affecting Australian children, despite the implementation of public health initiatives, such as fluoridated drinking water and toothpastes. The disease can cause pain and systemic infection, lead to speech and learning problems, and is a predictor for poor general health. Treatment for dental caries inflicts a huge economic burden on society, accounting for 6.5 per cent ($AUS5.3 billion) of total health care expenditure in Australia per year (Armfield et al., 2009; Ha et al., 2011).

In 2004, we submitted an application for funding to the NHMRC for a new initiative involving Australian twins and their families. The title of this grant application was ‘Tooth emergence and oral streptococci colonisation: a longitudinal study of Australian twins’. Although the application was supported, funding was only provided for three years rather than the five years that had been requested. The study commenced in 2005, with the twins and their families comprising what is now referred to as Cohort 3. The chief investigators on this project were Grant Townsend, Kim Seow, Theo Gotjamanos, Toby Hughes, Neville Gully and Lindsay Richards.

The first aim of the project was to clarify the influences of genetic and environmental determinants on variation in the emergence (often referred to as tooth eruption) of the human primary teeth by applying modern methods of genetic model- fitting to longitudinal data obtained from young monozygotic and dizygotic twin pairs. The intention was to compare variability in both the timing and sequencing of tooth emergence within and between the twins, and to relate these findings to measures of physical development and other pre- and postnatal factors. It was also planned to apply linkage analyses to detect quantitative trait loci (QTLs) for dental development using genome scans of DNA derived from cheek cells.

Introduction

In 1994 the Craniofacial Biology Group decided that it would be valuable to carry out a comprehensive study of dental and facial development and morphology in young twins in the 4-6-year-old age interval to supplement previous studies of teenage twins. At that time, there had been no detailed studies of the teeth or faces of young twins with primary (deciduous) teeth. It was also not known whether special features of the twinning process affected dental and facial features of young twins. It was planned, if funding allowed, to re-examine these twins again around the ages of 9 to 11 years and then at around 12 to 14 years of age. This would then become one of the few longitudinal studies of twins focusing on teeth and faces to be carried out worldwide.

The aim in setting up this study of young twins, who are now referred to as Cohort 2, was to extend the use of newly developed methods of genetic model-fitting to data obtained from teeth and faces of both monozygotic and dizygotic twins, so that we could clarify in more detail than previously the roles of genetic and environmental determinants on observed variation. We also aimed to compare the expression of bilateral dental and facial features on the right and left sides, in both twins and singletons, to see the extent to which the twinning process affects the determination of body symmetry. This included our desire to explore more fully the fascinating phenomenon of mirror imaging, where one twin of a pair mirrors the other for one or more features.

As has been emphasised earlier, the primary teeth provide an excellent model system to study genetic aspects of dental and facial development. They also have advantages for investigating some of the questions relating to observed developmental differences between dental and facial features on the right and left sides (disturbed laterality) thought to be associated with the twinning process.

Most of the development of the primary tooth crowns occurs between approximately four weeks in utero and birth, with all the crowns being completely formed by around twelve months postnatally. This developmental span enables an assessment of how various pre- and perinatal factors, such as maternal health, smoking and alcohol consumption, placenta type and birth weight, might influence dental structures.

Publications

1980s

Brown T, Townsend GC, Richards LC, Travan GR (1987). A study of dentofacial morphology in South Australian twins. Aust Dent J 32:81‑90.

Sekikawa M, Namura T, Kanazawa E, Ozaki T, Richards LC, Townsend GC, Brown T (1989). Three‑dimensional measurement of the maxillary first molar in Australian Whites. Nihon Univ J Oral Sci 15:457‑464.

Townsend G (1988). Research in dentistry: growth and development. Aust Dent J 33:375‑378.

Townsend GC, Corruccini RS, Richards LC, Brown T (1988). Genetic and environmental determinants of dental occlusion variation in South Australian twins. Aust Orthod J 10:231‑235.

Townsend GC, Richards LC, Brown T, Burgess VB (1988). Twin zygosity determination on the basis of dental morphology. J Forensic Odontostomatol 6:1‑15.

Townsend GC, Brown T, Richards LC, Rogers JR, Pinkerton SK, Travan GR, Burgess VB (1986). Metric analyses of the teeth and faces of South Australian twins. Acta Genet Med Gemellol 35:179‑192.

Travan GR, Townsend GC, Brown T, Richards LC, Burgess VB (1987). Application of the SIR system in a study of South Australian twins. Proceedings of the Annual Conference USIR Australasia.

1990s

Brown T, Townsend GC, Richards LC, Travan GR, Pinkerton SK (1992). Facial symmetry and mirror imaging in South Australian twins. In: Craniofacial Variation in Pacific Populations. Brown T, Molnar S, editors. Adelaide: Anthropology and Genetics Laboratory, The University of Adelaide, pp. 79‑98.

Corruccini RS, Townsend GC, Richards LC, Brown T (1990). Genetic and environmental determinants of dental occlusal variation in twins of different nationalities. Hum Biol 62:353‑367.

Dempsey PJ, Townsend GC, Martin NG (1999). Insights into the genetic basis of human dental variation from statistical modelling analyses. Perspec Hum Biol 4(3):9‑17.

Dempsey PJ, Townsend GC, Richards LC (1999). Increased tooth crown size in females with twin brothers: evidence for hormonal diffusion between human twins in utero. Am J Hum Biol 11:577‑586.

Dempsey P, Schwerdt W, Townsend G, Richards L (1999). Handedness in twins: the search for genetic and environmental causes. Perspec Hum Biol 4(3):37‑44.

Dempsey PJ, Townsend GC, Martin NG, Neale MC (1995). Genetic covariance structure of incisor crown size in twins. J Dent Res 74:1389‑1398.

Introduction

There is an interesting lineage of researchers who influenced Grant Townsend and contributed directly or indirectly to the establishment of the three major studies of Australian twins and their families at the Adelaide Dental School. These studies commenced in 1983 and still continue today.

Townsend studied the subject ‘Genetics and Human Variation IH’ presented by Professor John Henry Bennett from the Department of Genetics at the University of Adelaide in 1973. This was part of his Honours year, referred to as a BScDent (Hons) degree. This subject included a practical component which involved Nick Martin as a tutor in the laboratory. Another member of the staff in the Department of Genetics at that time was Dr George Mayo, who was more than happy to answer questions about genetic aspects of dentistry and to help subsequently with any issues that arose in writing up papers from Townsend's PhD (Figure 4.1). Nick Martin has continued to be a magnificent supporter of the Adelaide Dental School twin studies over the past thirty-five years, always prepared to provide advice and to encourage and challenge us to ‘push back the frontiers of knowledge’.

JH Bennett subsequently put together the combined works of Sir Ronald Fisher, who spent time in Adelaide during the 1960s. Fisher was a mentor to Kenneth Mather and John Jinks, who supervised Lindon Eaves's PhD; and Eaves, in turn, was a supervisor of Nick Martin's PhD.

After being appointed to the academic staff of the Adelaide Dental School in 1977, Townsend and his PhD supervisor, Professor Tasman Brown, published some papers presenting the main findings of the thesis entitled Tooth size variability in Australian Aboriginals: a descriptive and genetic study (Townsend and Brown, 1978a,b; Townsend, 1978; Brown and Townsend, 1979; Townsend and Brown, 1979a,b). Townsend was well aware of the ongoing importance of the records collected in the 1950s and 1960s during the longitudinal growth study of Aboriginal Australians living at Yuendumu in the Northern Territory (Brown et al., 2011). He was keen to establish another ongoing study that would enable questions about the genetic basis of dental development and morphology to be further addressed, and he realised the potential of studying twins.

Three broad phases of research can be identified where twins have been studied to understand more about the contribution of genetic and environmental factors to human dental and facial variation. The number and types of studies have increased dramatically since the early 1900s.

Studies of twin resemblance: hereditary and environmental influences

1920s-1940s

An initial phase of studies can be identified in the period from the 1920s to the early 1940s (see Table 3.1 at the end of this section for a summary of these studies). Several researchers around the world began studies of twins which highlighted resemblances of dental structures. Through being able to identify similarities in dental tissues they began to separate what could be thought of as genetically influenced (or hereditary) factors from environmentally influenced ones (Bachrach and Young, 1927; Goldberg, 1930; Korkhaus,1930; Newman 1923, 1928a,b, 1930; Newman et al.,1937). Indeed, publications involving Horatio H Newman commenced as early as 1913.

Henriette Bachrach and Matthew Young (1927) studied the resemblances of dental features in twin pairs who had been grouped into so-called identical pairs (development of more than one embryo from a fertilised egg) and non-identical pairs (development of embryos from separate fertilised eggs). The reason for forming these groupings conformed to the general belief that identical twin pairs shared the same inherited characteristics and that non-identical twin pairs shared only part of their inherited characteristics.

Using these two twin categories they set out to compare the degrees of influence that heredity and environment had upon chosen dental characteristics. These included tooth eruption times, dental caries in deciduous and permanent teeth, enamel hypoplasia, the state of dental occlusion and gingivitis. With the exception of total caries prevalence, data collected for all of the other features (particularly in subjects displaying different degrees of malocclusion) showed that correlations were higher in identical twin pairs than in non-identical twin pairs. This study and its choice of multiple dental features provided an important foundation for future studies involving dental morphology and disease in twins.

A paper written by Samuel Goldberg (1930) reported on dental arch size and shape in identical twins. His research considered resemblances of arch symmetry and asymmetry, as well as mirror imaging, in monozygotic twin pairs. Goldberg attempted to determine both the degree of arch variation and the severity of dental malocclusion.

The original research team

Grant Townsend

Lindsay Richards

Tasman Brown

Sandy Pinkerton

George Travan

Craniofacial Biologyand Dental Education Group 2015

Abdulaziz Almajed

Poppy Anastassiadis

Peter Anderson

Nor Atika Md Ashar

Manpreet Bariana

Corinna Bennett

John Berketa

Vishesh Bhojwani

Michelle Bockmann

Alan Brook

Tasman Brown

Ketki Chandewar

Anh Diep

Mohamed El-Kishawi

Frances Greenwood

Linda Hassanali

Toby Hughes

Helen James

John Kaidonis

Sophie Karanicolas

Kenneth Koh

Felicity Lam

Dimitra Lekkas

Chelsea Mann

Mustafa Mian

Suzanna Mihailidis

Ruba Odeh

Premal Patel

Caroline Petroff

Renée Phillips

Sandy Pinkerton

Rabiah Rahmat

Sarbin Ranjitkar

Chris Redwood

Daniela Ribeiro

James Rogers

Ruth Rogers

Graham Scriven

Komal Shah

Catherine Sims

Vicki Skinner

Catherine Snelling

Karen Squires

Richard Jonathan Taduran

Vivian Toh

Ikuko Tomo

Soichiro Tomo

Grant Townsend

Ryuji Ueno

Daniel Waller

Abbe White

Tom Wilkinson

Scott Williams

Tracey Winning

Robin Yong

Visiting researchers, collaborators and other key contributors

Wadian Abdul-Wahed

Hiro Aboshi

Christina Adler

Michael Aldred

Lassi Alvesalo

Mary Apps

Peter Arrow

Mark Bartold

Hannah Berman

Jasmine Blight

Lucas Bockmann

Dorret Boomsma

Louise Brearley Messer

Walter Bretz

Gary Briscoe

Bridget Brown

Gina Browne

Lotte Bullens

Viv Burgess

Hanny Calache

Angus Cameron

Emmanuel Chan

Ke Chen

Rebecca Chen

Kitty Cheshire

David Chubb

John Clement

Deborah Cole

Robert Corruccini

Tom Coxon

Jeffrey Craig

Paula Dempsey

John Diamanti

Kim Dooland

Craig Dreyer

Shosei Eguchi

Tennent Emerson

Alistair Evans

Luca Fiorenza

Judy Ford

Antonio Gagliardi

Jinlong Gao

Rakesh Garayia

Evangeline Gotjamanos

Theo Gotjamanos

Neville Gully

Edward Harris

Yuh Hasegawa

Shirley Hastings

David Hay

Tuomo Heikkinen

Denice Higgins

Jenny Hong

John Hopper

Neil Hunter

Tadashi Ideguchi

Elka Johansson

Ikuo Kageyama

Voula Kaidonis

Eisaku Kanazawa

Sunita Kapali

Kazutaka Kasai

Samvit Kaul

Mohd Fadhli Bin Khamis

Nima Kianoush

John Kibble

Jules Kieser

Nicky Kilpatrick

Inger Kjaer

Shintaro Kondo

Raija Lähdesmäki

Herve Lesot

Judith Littleton

Helen Liversidge

Pamela Long

Michelle Luciano

Lucy Ludlow

David Madsen

Zuliani Mahmood

Nick Martin

Judith May

John Mayhall

Elise McConnell

Linda Mealey

Sarah Medland

Pascale Mehanna

Marion Morgenstern

John Mulley

Sen Nakahara

Koh Nakajima

Mike Neale

Leanne Ng

Ruba Odeh

Greg Ooi

Richard Osborne

Tadashi Ozaki

Tracey Parish

Kathleen Paul

Brian Penhall

Liu Ping

Pertti Pirttiniemi

Ron Presswood

Kris Anne Pulanco

Josephine Quinn

Jonathan Race

Quentin Rahaus

David Richmond

Jackie Rovensky

Fiona Rowett

Loreta Rupinskas

Richard Saffery

Wayne Sampson

Bhim Savara

Wendy Schwerdt

Richard Scott

Mitsuo Sekikawa

Tsuneo Sekimoto

Kim Seow

Rob Sergi

Tarciso Sindeaux

Patricia Smith

[…]

Introduction

The birth of twins has been of interest throughout human history. From the earliest recorded times a multiple birth generated considerable attention both within and outside the family. The fact that these offspring often shared the same physical characteristics led to many explanations about the possible underlying causes. These explanations often related more to the depth of the imagination than to any factual evidence. Without knowing the complexities of the twinning process, early societies had to construct explanations that ordinary people could understand and accept, and it is not surprising that many of these explanations were centred on the mythical aspects of their cultures and religions.

Attitudes toward twins and twinning varied considerably in different parts of the world and could change quite dramatically over time. In pre-industrial societies there appeared to be two distinct attitudes towards them. One linked more to the unexplained (mythological) supernatural aspect and the other to the practicality of living with twins. The lives of twins depended on their cultural acceptance by society. If they were seen as freaks of nature, then some societies would have no compunction in killing them, and this occurred widely throughout history. Reasons for killing twins varied, but most concerned the idea that twin creation went against the laws of nature. One example of this may be seen in the belief that it was ‘animal-like’ for a mother to produce two children at the same time, or that to have given birth to two babies must mean two fathers. Other beliefs likened twinning to the practice of adultery or even the involvement of an evil spirit, which could well have decided the fate of twins (Bryan, 1983).

The practicality of living with twins was also an important factor in determining how they were treated, particularly in nomadic societies where it became almost impossible to breastfeed two babies when food was scarce and the population was continually on the move to find food and water. In contrast, other societies accepted and even welcomed twins if, for example, the chief of a tribe had twins and the tribe then prospered. If the birth of twins coincided with ‘good omens’, then the likelihood of good fortune would normally be associated with them.