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Consanguinity, Inbreeding, and Genetic Drift in Italy (MPB-39) available in Paperback
- Pub. Date:
- Princeton University Press
In 1951, the geneticist Luigi Luca Cavalli-Sforza was teaching in Parma when a studenta priest named Antonio Moronitold him about rich church records of demography and marriages between relatives. After convincing the Church to open its records, Cavalli-Sforza, Moroni, and Gianna Zei embarked on a landmark study that would last fifty years and cover all of Italy. This book assembles and analyzes the team's research for the first time.
Using blood testing as well as church records, the team investigated the frequency of consanguineous marriages and its use for estimating inbreeding and studying the relations between inbreeding and drift. They tested the importance of random genetic drift by studying population structure through demography of the last three centuries, using it to predict the spatial variation of frequencies of genetic markers. The authors find that drift-related genetic variation, including its stabilization by migration, is best predicted by computer simulation. They also analyze the usefulness and limits of the concept of deme for defining Mendelian populations. The genetic effect of consanguineous marriage on recessive genetic diseases and for the detection of dominance in metric characters are also studied.
Ultimately bringing together the many strands of their massive project, Cavalli-Sforza, Moroni, and Zei are able to map genetic drift in all of Italy's approximately 8,000 communes and to demonstrate the relationship between each locality's drift and various ecological and demographic factors. In terms of both methods and findings, their accomplishment is tremendously important for understanding human social structure and the genetic effects of drift and inbreeding.
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Consanguinity, Inbreeding, and Genetic Drift in Italy (MPB-39)
By Luigi Luca Cavalli-Sforza Antonio Moroni Gianna Zei
Princeton University PressCopyright © 2004 Princeton University Press
All right reserved.
Chapter OneHistory of This Investigation and Structure of This Book
The study of inbreeding, the consequence of the mating of relatives, has an important place in genetics. The similarity of the paternal and maternal contributions caused by the mating of relatives leads to increased genetic homogeneity of inbred individuals. A table of the expected effects of inbreeding in successive generations of selfing (crossing with self), the closest mating possible, which often occurs spontaneously in many plants, appears in Mendel's article, the founding paper of genetics. Thus, Mendel was also the first population geneticist.
Human societies are unique in keeping records of their own ancestry, sometimes, though very rarely, for thousand of years or more. Some breeders of domestic animals, however, have kept records of their animals' ancestry for even longer periods, measured in terms of numbers of generations.
The genetic effect of inbreeding can be estimated by the increase in average homozygosity over that expected by random mating. Homozygosity is the average percentfrequency of homozygotes-individuals receiving the same form of a gene from both parents. The complement to 100% of homozygosity is heterozygosity.
Matings of close relatives (often called consanguineous matings), if repeated for many generations, increase homozygosity to a point of seriously decreasing fertility and individual survival, making prolonged inbreeding incompatible with continuation of life. To obtain stocks of high genetic homogeneity, animal breeders often make recourse to systematic parent-child or brother-sister matings for a number of generations (twenty or more). In the process they lose many inbred lines because of loss of fertility or survival, and at best end with stocks of weak or delicate constitution (inbreeding depression). There are examples of repeated brother-sister marriages in ancient Egyptian and Persian dynasties; but, for reasons probably not of genetic nature, no dynasty lasted for periods long enough to expect serious inbreeding depression. In any case we have no records from which to observe it. But apart from these examples of encouragement of brother-sister marriages, which was still popular in the Egypt of Cleopatra's time, unions between brother and sister and parent and child are condemned and avoided in practically all human societies. The term "incest" refers to these tabooed unions. They occur, though rarely. Children of incest are very rare, of the order of 1/10,000 in an estimate in Michigan (Adams and Neel 1967).
The dangers of close inbreeding must have become known to early humans fairly soon, since practically every society has rules that tend to avoid close consanguineous marriage, and sometimes extend prohibition to remote consanguinities. This is especially true of very small communities, like those of Eskimos living in extreme northern latitudes (Sutter and Tabah 1956), which are under greater risk of reaching high levels of inbreeding. In one remote, very small, and highly isolated Greenland Eskimo community, no marriage closer than third cousins was found in genealogies from living individuals.
Animals also tend to avoid close inbreeding, by social customs that seem to have been favored by natural selection in response to inbreeding depression. In most Primates, social groups are fairly permanent, but young males reaching puberty tend to leave the group into which they were born and join other groups. This custom is clearly effective in limiting close consanguineous matings. Chimpanzees are the only exception, as here it is the females that leave the group. Among humans one likewise observes a greater tendency of females to leave their birthplace at marriage. Wives tend to move to their husbands' residence in 70% of traditional societies (Murdock 1967). This custom has important genetic consequences: mitochondrial DNA (mtDNA), which is transmitted by the maternal line, should show less geographic clustering than Y chromosomes, which are transmitted by the male line. This expectation was confirmed by observation (Seielstad et al. 1998). In fact, the Y chromosome shows greater genetic variation between populations than nonsexual chromosomes and mitochondrial DNA.
In addition, at least in chimpanzees and other mammals, some tendency to avoid brother-sister or parent-child mating is observed. Among humans, the social custom of avoiding marriage of close relatives is paralleled by a similar constitutional safeguard against brother-sister mating, in the form of the so-called Westergaard effect: a tendency to avoid sexual contacts between brother and sister, or, in general, children who have been brought up together. Puberty seems to be the dividing line between social contacts that are unfavorable (before puberty) or favorable (after it) to interest in establishing sexual relations between individuals of opposite sex. Research has shown that children brought up in the same kibbutz, where they were usually raised together, marry very rarely, if ever. An old Chinese custom, which survived in Taiwan until recently, is the adoption of a young girl by a family in which a son was born, so that this girl becomes his future wife. These so-called "minor" marriages have been shown to be, on average, less fertile and less long-lasting than ordinary marriages with girls not brought up in the family (Wolf 1980).
There are, however, social exceptions to the rule of avoidance of close consanguineous marriages, less close than brother-sister. In certain social groups such marriages may be much more popular than would be expected by chance, undoubtedly because of a social preference. In West and South Asia two types of consanguineous marriages are especially common: uncle-niece marriages comprise up to 20% of all marriages in several north Indian tribes, and first-cousin marriages reach 50% or more in many Middle Eastern ethnic groups (Arabs, North Africans, and some Jewish groups). First-cousin marriages are or were high in Japan, especially at a time when marriages were mostly arranged by parents. High consanguinity customs spread around with the people who developed them. Perhaps as a remote consequence of Arab domination in Sicily and southern Italy in the eighth to the eleventh centuries, the frequency of uncle-niece and first-cousin matings became high in these regions and is currently especially high in Sicily. These relatively moderate degrees of inbreeding do not seem to have had a truly damaging effect. They may have contributed to lowering the current frequency of lethal and semilethal genetic diseases, at least in Japanese populations (Cavalli-Sforza and Bodmer 1971, 1999).
By definition, recessive genes are those that are manifested only in homozygotes. In inbred families increased homozygosity is expected, leading to a higher probability of observing recessive inheritance. The study of consanguineous marriages, therefore, has merit for the detection of recessive genes and for the study of their frequency.
In this chapter we summarize the salient points of the history of this investigation, which started in 1951 and is now coming to an end. We then briefly summarize the main properties of consanguinity, inbreeding, and drift, as well as inbreeding effects in humans and some special projects that were part of the investigation. Finally, we summarize the structure of this book.
1.2 HISTORY OF THIS RESEARCH
In 1951 Luca Cavalli-Sforza started teaching a course in genetics at the Faculty of Sciences of the University of Parma, Italy. Among his students was Antonio Moroni, a priest who is now professor of ecology at the University of Parma. At that time Moroni taught natural sciences at the Seminary of the Parma Archbishopric and made Cavalli-Sforza aware of records in the Roman Catholic archives that could be of interest for human genetics: essentially dispensations for consanguineous marriages and parish books of deaths, marriages, and baptisms. In an almost 100% Catholic country, baptisms are a close equivalent of birth records. Newborns are usually baptized very soon after birth, and a very small fraction, probably less than 1%, dies without a chance to be baptized. Moroni was also instrumental in obtaining permission from the higher religious authorities to use these records for genetic investigations. Our investigations began at the bishopric of Parma. They were soon extended to other bishoprics, and eventually to the whole country of Italy. To help with our investigations, a letter from the highest Catholic authorities was sent by the Vatican to all parish priests, asking them to make parish records available for scientific purposes. Genetic research using Roman Catholic records was also started in France by Jean Sutter and his collaborators, at about the same time as ours.
Consanguinity records are to be found in various Catholic archives. Consanguinity itself is very carefully defined and Roman Catholic legislation prescribes with great precision which marriages are completely forbidden, which ones are permitted under dispensation from a higher religious authority, and which do not require dispensation. Priests receive formal teaching about these rules in seminaries in which they also learn to evaluate accurately the degree of consanguinity of candidates for marriage. The need for dispensation has changed through time and is now reduced to a minimum. In earlier times even remote degrees of consanguinity were forbidden and it was essential to ask for dispensations before marriage could be celebrated. A consanguineous marriage celebrated without the requested dispensation would be null and void, generating a very serious social problem for the families.
Chapter 2, on the history of consanguinity regulations in the Roman Catholic Church, examines the historical knowledge available. There are also geographic differences in rules for obtaining dispensation. In peninsular Italy the parish priest must check every pair of prospective spouses for the existence of recent relationship, and if one requiring dispensation is discovered, he must request it from the bishopric. A copy of the request is then sent from the bishopric to the Vatican, and is returned to the bishopric with the Vatican response and then to the parish before a marriage requiring dispensation can be celebrated. In a few regions other than peninsular Italy, dispensations for at least some less close consanguinities could be given by a local Catholic authority other than the Vatican.
At least in peninsular Italy, therefore, there are three sources of records: the priest is supposed to indicate on the parish marriage book that a specific marriage required a dispensation, but we found that in some parishes this was not always carefully done. Folders keeping full records of dispensations are kept in each bishopric, and a slightly less complete set of records is available in the Vatican archives. It is rare that a dispensation is not approved by the Vatican, if it is dispensable. Genealogical trees of the candidate spouses reconstructed by the parish priest are extremely common in the bishopric archives that we investigated, and are available for practically every dispensation requested, but much less frequently in the Vatican archives. These genealogical trees are essential for checking the consanguinity degree calculated by the priest and for testing hypotheses on age and migration effects, to be described in a later chapter. The presence of genealogies in the dispensation folders made it possible to check for errors in the calculation of consanguinity degrees. Errors were nonexistent or exceptional, not surprisingly, since the method of computation is regularly taught to priests at seminaries.
Our work was based initially on bishopric records. After a full study of dispensations deposited in the archives of the Archbishopric of Parma-the diocese of the city where the university in which Cavalli-Sforza taught from 1951 to 1962 is located-those of the two adjacent dioceses of Piacenza and Reggio Emilia were also investigated. The territory of these three dioceses and cities has almost identical ecological structure, extending from the Appennine mountains to the lowest part of the plain of the Po River. The three dioceses form the northern moiety of the administrative Italian region called Emilia, practically at the center of the Po valley, in the northern part of Italy. The Po River flows just north of the city of Piacenza, and continues eastward toward the Adriatic Sea. Parma and Reggio Emilia are on a major Roman road, the Via Aemilia, an almost straight line in the Po valley leading east-southeast from Milan to Bologna. The region around the Po and the Via Aemilia is a very fertile plain. Proceeding from each of the three cities toward the south, one enters first a hilly region and then a mountainous one. Population density is maximum near the cities, which are all located not far from the center of a very prosperous agricultural region. The mountainous region at the southern end of each diocese has the lowest population density. It is a part of the Appennine chain, the crest of which separates Emilia from Tuscany. The hilly region, intermediate between the plain and the mountains, has intermediate population density. The size of villages is on average highest near the cities and decreases regularly toward the mountains. This variety of environments of each diocese has helped in the investigation of the effects of the relevant ecological and demographic variables.
Bishopric records of the islands of Sardinia and Sicily, as well as of some other islands and inland regions of special interest, were also investigated, showing similar effects of demographic variables, along with other characteristics of each region. Records of individual dioceses of the islands were published earlier; their analysis has been repeated by partially new approaches for the purpose of preparing this book.
In later years it was possible to establish a team of young students who copied the consanguinity records of the Vatican archives under direction of Father M. Bracco. These records included all of peninsular Italy from 1911 to 1964, except for Sicily, which had independent rules. It was necessary to visit directly the bishoprics of Sardinia and Sicily, but our survey of Sicilian dioceses was not complete.
Full names of consanguineous couples were available. They were kept confidential, but we had permission to use them for linking them with other records to study the effect of consanguinity on certain phenotypes. The records were eventually transferred to computer tapes, analyzed statistically, and ordered alphabetically.
Results on some of the bishoprics were in part published earlier. But the major analysis, that of the Vatican records, is published for the first time in this volume. A number of other new calculations were carried out on the available records and are included in this book. Other socioeconomic investigations done in Italy were studied and correlated with the consanguinity data. Socioeconomic information came from the Istituto Centrale di Statistica.
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Table of Contents
1. History of This Investigation and Structure of This Book 1
1.1 Introduction 1
1.2 History of This Research 4
1.3 Consanguinity 9
1.4 Inbreeding Measurement 11
1.5 Inbreeding Effects 13
1.6 Random Genetic Drift 18
1.7 Research on Drift in the Parma Valley 20
1.8 Genetic Uses of Surnames 21
1.9 A Summary of Published Studies on Consanguinity and Inbreeding, with Special Reference to Italy 24
1.10 Structure of This Book 26
2. Customs and Legislation Affecting Consanguineous Marriages, with Special Attention to the Catholic Church 29
2.1 Early and Medieval Christian Tradition 29
2.2 Traditional Methods of Consanguinity Evaluation: The Roman and the German Methods 34
2.3 Justifications of the Dispensation Request 35
3. Demographic Factors Affecting the Frequencies of Consanguineous Marriage-A Study in Northern Emilia 39
3.1 Nature and Interest of the Problem 39
3.2 Relations to Population Structure 40
3.3 Number of Sibs, Distribution of Family Sizes, and Observed Abundance of Relatives 43
3.4 Consanguinity Degrees and Observed Numbers of Consanguinity Dispensations in Northern Emilia 47
3.5 Pedigree Types, Pedigree Codes, and Proofs of the Influence of Age at Marriage and of the Sex of Intermediate Ancestors 52
4. Probability of Consanguineous Marriages 69
4.1 Theory of Age Effects on the Frequency of Consanguineous Marriages 69
4.2 Migration as a Factor Affecting the Frequency of Consanguineous Marriages 76
4.3 The Role of Women in Maintaining Family Ties among Relatives 78
4.4 Observed and Expected Frequencies of Major Consanguinity Degrees 83
5. Consanguinity, Inbreeding, and Observed Genetic Drift in the Parma Valley 90
5.1 The Parma Valley and the Origin of This Investigation 90
5.2 Geography of the Parma Valley 91
5.3 Consanguinity and Inbreeding in the Parma Valley 95
5.4 Blood Groups and Genetic Drift 101
5.5 Surnames and Genetic Drift 115
5.6 Correlations of Inbreeding and Drift 118
6. A Computer Simulation of the Upper Parma Valley Population 122
6.1 The Need for a Population Simulation 122
6.2 Structure of the Simulation 123
6.3 The Migration Matrix 129
6.4 Is Drift the Only Cause of Genetic Variation in the Parma Valley? 134
6.5 Expected and Observed Consanguinity 142
7. Islands 149
7.1 Italian Islands 149
7.2 Sardinia 149
7.3 Sicily 175
7.4 Aeolian Islands 183
8. Effects of Inbreeding on Normal and Pathological Phenotypes 192
8.1 Introduction 192
8.2 Normal Quantitative Phenotypes: Stature and Chest Girth 193
8.3 Mortality, Fertility, and Sterility 196
8.4 Incidence of Disease Groups from Surveys of Hospital Populations 199
8.5 Study of Specific Recessive Diseases 204
9. Consanguineous Marriages in Italy: Data from the Vatican Archives 211
9.1 Introduction 211
9.2 Variations of Consanguinity over Time 212
9.3 Geographical Variations: Provinces and Regions 214
9.4 Space-Time Analysis: Four Models of Declining Consanguinity in Italian Regions 215
9.5 Factors Responsible for Space and Time Differences in Consanguinity: Choice of Variables and Their Meaning 222
9.6 Demographic Variables: Birthrate, Death Rate, and Demographic Transition 223
9.7 Effect on Consanguinity of Environmental Variables of Socioeconomic and Ecological Meaning 229
9.8 An Attempt at a General Synthesis 237
10. Geography of Demes in Italy 242
10.1 Population Sampling 242
10.2 Random Mating, Mendelian Populations, and Demes 246
10.3 Comparing Genetic and Demographic Approaches to the Study of Demes 252
10.4 Are comuni (Communes) Demes? 260
10.5 The Negative Correlation Between N and m 263
10.6 Using Surnames for Evaluating Drift 267
10.7 A Drift Map of Italy by Communes 274
10.8 Statistical Observations on the Italian Drift Map 280
11. Conclusions 284
11.1 Human Consanguinity 284
11.2 Inbreeding 291
11.3 Genetic Drift 293
11.4 Demes, Isolates, and Migration 298
What People are Saying About This
This work is a uniquely significant contribution to the fields of historical demography and genetics of human populations. Both the results themselves and the approaches the researchers devised are of tremendous interest.
Mary-Claire King, University of Washington