doi: 10.1111/j.1469-1809.2007.00398.x
Admixture and Population Stratification in African
Caribbean Populations
J. Benn-Torres1 , C. Bonilla2 , C. M. Robbins4 , L. Waterman3 , T. Y. Moses4 , W. Hernandez1 , E. R. Santos1 ,
F. Bennett5 , W. Aiken5 , T. Tullock5 , K. Coard5 , A. Hennis6,7 , S. Wu7 , B. Nemesure7 , M. C. Leske7 ,
V. Freeman8,9 , J. Carpten4 and R. A. Kittles1, ∗
1
Section of Genetic Medicine, Department of Medicine, The University of Chicago Pritzker School of Medicine, Chicago, IL 60637
2
Department of Clinical Pharmacology, University of Oxford, Oxford, OX2 6HA, UK
3
Department of Biological and Chemical Sciences, University of the West Indies, Bridgetown, Barbados
4
Genetic Basis of Human Disease Research Division, Translational Genomics Research Institute, Phoenix, AZ 85004
5
Tropical Medicine Research Institute, University of the West Indies, Mona, Kingston, Jamaica
6
Tropical Medicine Research Institute, University of the West Indies, Bridgetown, Barbados
7
Department of Preventive Medicine, School of Medicine, Stony Brook University, Stony Brook, NY
8
Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, Chicago, IL. 60612
9
Department of Urology, Loyola University Stritch School of Medicine, Maywood, IL
Summary
Throughout biomedical research, there is growing interest in the use of ancestry informative markers (AIMs) to deconstruct racial categories into useful variables. Studies on recently admixed populations have shown significant population
substructure due to differences in individual ancestry; however, few studies have examined Caribbean populations. Here
we used a panel of 28 AIMs to examine the genetic ancestry of 298 individuals of African descent from the Caribbean
islands of Jamaica, St. Thomas and Barbados. Differences in global admixture were observed, with Barbados having the
highest level of West African ancestry (89.6% ± 2.0) and the lowest levels of European (10.2% ± 2.2) and Native American ancestry (0.2% ± 2.0), while Jamaica possessed the highest levels of European (12.4% ± 3.5) and Native American
ancestry (3.2% ± 3.1). St. Thomas, USVI had ancestry levels quite similar to African Americans in continental U.S.
(86.8% ± 2.2 West African, 10.6% ± 2.3 European, and 2.6% ± 2.1 Native American). Significant substructure was
observed in the islands of Jamaica and St. Thomas but not Barbados (K=1), indicating that differences in population
substructure exist across these three Caribbean islands. These differences likely stem from diverse colonial and historical
experiences, and subsequent evolutionary processes. Most importantly, these differences may have significant ramifications
for case-control studies of complex disease in Caribbean populations.
Keywords: Caribbean, Barbados, Jamaica, St. Thomas USVI, genetic admixture, ancestry informative markers
Introduction
Understanding genetic ancestry and admixture and their
role in contributing to population stratification is important for biomedical research. Recent gene flow among diverse populations (also called admixture) and non-random
∗
Corresponding author: Rick Kittles, Ph.D., Section of Genetic Medicine, Department of Medicine, The University of
Chicago, 5841 South Maryland Avenue MC6091, Chicago, IL
60637 USA. Tel: (773) 834-2271; Fax: (773) 702-2567. E-mail:
[email protected]
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Annals of Human Genetics (2008) 72,90–98
mating can create subpopulations of individuals with varying ancestral proportions (or population stratification). The
dynamics of population stratification are of special concern due to the growing interest in genome-wide association studies to uncover genetic effects on complex diseases
(Cardon & Palmer, 2003; Hirschhorn et al. 2002). Large
efforts have been employed to utilize ancestry informative
markers (AIMs) consisting of single nucleotide polymorphisms (SNPs) which vary significantly in frequency among
putative parental populations. These AIMs have aided in
understanding the dynamics of admixture within recently
admixed populations, such as some Central and South
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American populations (Bortolini et al. 1999; Bravi et al.
1997; Da Silva et al. 1999; Rodas, 2003; Sans et al. 2002;
Saul et al. 2004; Vallinoto et al. 2003; Seldin et al. 2006),
Hispanic North Americans (Bertoni et al. 2003; Bonilla
et al. 2004a, 2004b; Choudhry et al. 2006; MartinezMarignac et al. 2006; Salari et al. 2005), and African
Americans (Collins-Schramm et al. 2003; Parra et al. 1998,
2001; Seldin et al. 2004; Tian et al. 2006).
Admixture studies in the Caribbean have been scarce
with respect to the overwhelming amount of data readily
available for other admixed populations. Studies of the islands of Tobago, the Dominican Republic, Puerto Rico
and Cuba have shown a gene pool with varying contributions of Native American, West African and European
ancestors (Bonilla et al. 2004b; Martinez-Cruzado et al.
2001, 2005; Miljkovic-Gacic et al. 2005; Torroni et al.
2001). On the other hand, an early study by Parra et al.
(1998) using 10 ancestry informative markers reported that
the Jamaican island population was mostly of West African
descent, with very little European and no Native American
admixture.
The Caribbean region consists of four main geographical regions: the Bahamas, the Greater Antilles (Jamaica,
Cuba, Hispaniola, and Puerto Rico), the Lesser Antilles
(eastern chain of islands and group of islands north of the
Venezuelan coast), and the islands of Trinidad, Tobago and
Barbados. According to historical records three major native groups were encountered by the Spaniards when they
reached the Caribbean in 1492 the Ciboney, the Arawak
and the Carib (Rouse, 1992). Early during the Spanish
conquest (1492–1620) these populations were generally
decimated by disease, warfare and forced labour (SuedBadillo, 2003). In response to the declining native populations, King Charles V of Spain granted permission to export the first large number (4,000) of indigenous Africans
to serve as the primary labourers in the Antilles in 1518
(Emmer & Carrera Damas, 1999). Later in the 17th century other European colonists also imported millions of
enslaved Africans from the sub-Saharan states of West and
Central Africa (Rogoziânski, 1999) to work on plantations
over the course of four centuries.
The three islands considered in this study were St.
Thomas, Barbados and Jamaica. The smallest island, St.
Thomas, has a current population of approximately 53,000
and is located in the northern region of the Lesser Antilles.
In the latter part of the 17th century the Danish began colonization and it became an official Danish possession in
1754. While never as profitable as the English colonies, St.
Thomas also had sugarcane plantations powered by enslaved
Africans and, for a small period of time, indentured Europeans (Hall & Higman, 1992). The enslaved population
on St. Thomas reached its peak in 1802 with an estimated
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Journal compilation 5,737 Africans (Donoghue, 2002). The Danes were the first
imperial power to abolish the slave trade in their colonies
in 1804 (Knight, 1990). St. Thomas was later purchased by
the United States in 1917, becoming a U.S. dependency
(Knight, 1990) as part of the U.S. Virgin Islands.
Southeast of the Lesser Antilles is Barbados, a densely
populated independent island nation with a 2006 census
estimate of 279,912 inhabitants (Dickerson & Johnson,
2006). When the Spanish arrived in 1518 they quickly
depopulated it and eventually abandoned their claim to
the island. By 1627 the English arrived and made Barbados
primarily into a sugarcane colony (Higman, 1984). Consequently, a large number of enslaved Africans were brought
to work the sugarcane plantations and by 1748 there were
four Africans for every European (Knight, 1997). This history is very similar to that of the final island included in this
study, Jamaica. With a census estimate of nearly 3 million
individuals Jamaica is actually the least densely populated
island in the Greater Antilles (Dickerson & Johnson, 2006).
From 1509 to 1655 the Spanish maintained control of the
island and used it mainly as a transit station for slave trade
(Saunders, 2005). In 1655 the British forcibly took the island and it was officially ceded by the Spanish to the British
in 1670 (Knight, 1997). Between 1655 and 1807 Britain
turned to large scale importation of enslaved Africans
and Jamaica became the chief slave market of America
(Higman, 1984). Like many Caribbean islands, St. Thomas,
Barbados, and Jamaica’s contemporary population primarily consist of the descendants of African labourers, white
and mixed people, or more recent migrants from the Middle East, India and China (United States Dept. of State).
Although these islands have distinct histories they share
certain characteristics in regards to the dynamics of the admixture process. Within these former British and Danish
colonies historical records indicate that there were generally three parental populations, West Africans, Europeans
and Native Americans. However, the extent to which each
of these groups has genetically contributed to the current populations is unclear. What is also unclear is the
relative contribution of genetic ancestry to ethnic differences in risk factors, prevalence, and severity of diseases like
prostate and breast cancer, asthma and cardiovascular disease
(Amundadottir et al. 2006; Freedman et al. 2006; Homa
et al. 2000; Kittles et al. 2006; Reiner et al. 2005; Salari et al.
2005; Tang et al. 2006; Ziv et al. 2006). Given the amount
of ethnic variation in disease risk, unraveling the genetic
ancestry of Caribbean populations will aid biomedical research efforts to understand disease etiology and provide
useful information for the design of genetic disease mapping studies. A popular approach to mapping susceptibility
genes in recently admixed populations is called admixture
mapping. This novel methodology attempts to find genes
Annals of Human Genetics (2008) 72,90–98
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J. Benn-Torres et al.
that underlie ethnic differences in disease risk (McKeigue,
2005). In this approach genome-spanning AIMs are
genotyped in the clinical population, to infer ancestry at
each locus and then test locus ancestry for association with
the trait of interest in the population. Admixture mapping
has been successful for mapping disease and trait loci for
multiple sclerosis (Reich et al. 2005), hypertension (Zhu
et al. 2005) and prostate cancer (Freedman et al. 2006).
Admixture proportions and dynamics, and AIM characteristics, all influence the power of admixture mapping
(McKeigue, 2005).
Here, we estimated genetic ancestral proportions and
tested for the presence of population substructure due to
admixture in three understudied Caribbean populations
from Jamaica, St. Thomas and Barbados. A set of autosomal ancestry informative markers (AIMs) was used to
estimate individual and population admixture proportions.
Our results revealed differences in the presence and amount
of population substructure among these three island populations.
Materials and Methods
Population Samples
The study subjects consisted of unrelated individuals who participated in genetic studies on cancer risk among African Caribbeans.
All self-reported as being of African descent and resided on the
Caribbean islands of Jamaica, St. Thomas or Barbados. Fiftyfour (54) Jamaican men were recruited during the year 2000 in a
hospital-based study at the University Hospital of the West Indies
in Kingston, Jamaica. In addition, one hundred and twenty-five
(125) men born in St. Thomas, USVI, were recruited in a clinicbased study on genetic variation and risk factors for prostate cancer during the summer of 2001. Ninety-five (95) Barbadians were
randomly selected from the controls recruited in a populationbased case-control cancer study that began in 2002. The hospital
in Jamaica and clinic in St. Thomas are considered major institutions for the treatment and screening of prostate cancer on the
islands and see patients who are representative of the island inhabitants. For the Barbadians, subjects were randomly selected from
a national database of Barbados’ citizens >21 years of age. Blood
samples were collected from all study participants. The Institutional Review Boards at Loyola University, Howard University,
Stony Brook University and the University of the West Indies
approved the study and written consent was obtained from all
subjects.
Laboratory Methods and Genotyping
Genomic DNA was isolated from lymphocytes using standard
proteinase K digestion, cell lysis, protein precipitation and DNA
precipitation. We used a panel of 28 bi-allelic ancestry informative markers (AIMs) distributed across the genome to estimate group and individual ancestry (IA) proportions in Barbados,
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Annals of Human Genetics (2008) 72,90–98
Jamaica and St. Thomas. These markers show large allele frequency differences (delta, δ) between the ancestral populations
(West Africans, Native Americans and Europeans). The description of this marker set has been provided in previous papers
(Bonilla et al. 2004a; Shriver et al. 2003). Marker frequencies for Native American (Maya, Pima, Cheyenne, and Pueblo,
N = 184), West African (Central African Republic, Nigeria, and
Sierra Leone, N = 279) and European (Spain, Germany, England
and Ireland, N = 243) populations were included in the analysis
as representative of parental groups. Average delta for markers
informative for population pairs was 54% for European/Native
American, 54% for European/West African, and 55% for Native
American/West African.
Genotyping was performed using the Sequenom MassARRAY(tm) genotyping platform with iPLEXTM chemistry according to manufacturer’s recommendations. Briefly, iPLEXTM assays
were designed utilizing the Sequenom Assay Design software,
allowing for single base extension (SBE) designs used for multiplexing. PCR and SBE primer sequences are available upon
request. Multiplex assays were performed to amplify 5–10 ng
of genomic DNA by polymerase chain reaction (PCR). PCR
reactions were treated with shrimp alkaline phosphatase (SAP)
to neutralize unincorporated dNTPs. Subsequently, a post-PCR
single base extension reaction was performed for each multiplex
reaction, using concentrations of 0.625 μM for low mass primers
and 1.25 μM for high mass primers. Reactions were diluted with
16 μl of H 2 O and fragments were purified with resin, spotted
onto Sequenom SpectroCHIPTM microarrays and scanned by
MALDI-TOF mass spectrometry. Individual SNP genotype calls
were generated using Sequenom TYPERTM software, which automatically calls allele-specific peaks according to their expected
masses.
Data Analysis
Population admixture proportions were calculated using the
program ADMIX, which implements a weighted least squares
method (Long, 1991). Individual admixture was estimated with
a maximum likelihood approach (Chakraborty et al. 1986;
Hanis et al. 1986). Differences in individual admixture estimates
across the three islands were assessed using the Kruskal-Wallis H
test. This non-parametric method, appropriate for three or more
groups, uses rank sums to test for differences in group medians. It
requires a dependent variable, in this case the individual ancestry
estimates, as well as an independent variable, the island of sample
origin.
Population structure was assessed for each Caribbean population using three different approaches. Our first approach was to
estimate the number of sub-populations (K) within each island
population using the STRUCTURE 2.1 program (Falush et al.
2003; Pritchard et al. 2000). Running parameters were as follows:
admixture model, independent allele frequencies, and separate alphas, for 50,000 burn-ins and 70,000 iterations. The predefined
number of populations ranged from K = 1 to K = 4. In addition
we compared the percentage of associated unlinked marker pairs.
The expectation, if there is no population stratification, is that
less than 5% of the unlinked marker pairs would be significantly
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Population
N
W. African
European
Native American
Barbados
St. Thomas, VI
Jamaica
95
125
54
89.6 ± 2.0
86.8 ± 2.2
84.4 ± 3.1
10.2 ± 2.2
10.6 ± 2.3
12.4 ± 3.5
0.2 ± 2.0
2.6 ± 2.1
3.2 ± 3.1
Table 1 Global population admixture
estimates (mean% and SE) for Caribbean
populations.
WLS method (Long, 1991)
Population
N
West African
European
Native American
Barbados
St. Thomas
Jamaica
95
125
54
84.1 (56.0–96.4)
81.4 (50.7–94.9)
80.8 (46.8–97.0)
11.8 (2.6–39.2)
13.1 (2.6–42.6)
13.3 (0.7–47.1)
4.0 (0.0–28.3)
5.4 (0.1–32.8)
5.9 (0.9–33.9)
Table 2 Mean individual admixture estimates (mean lower and upper bound)
for Caribbean populations.
ML method (Chakraborty, 1985)
associated. The exact probability for all associations was determined using SAS/Genetics 9.1 (SAS Institute Inc., Cary, NC).
After eliminating those pair-wise associations that were due to
physical linkage of markers, the proportion of significantly associated unlinked markers was calculated. Finally, we evaluated
the relationship between initial linkage disequilibrium (D 0 ) due
to admixture and current pairwise linkage disequilibrium (D t ),
considering a dihybrid model of admixture as described by Pfaff
et al. (2001). Initial disequilibrium levels were calculated as m(1m)δ A δ B , where m and 1-m correspond to the ancestral contribution of West Africans and Europeans, and δ A and δ B are the
frequency differentials between these populations at loci A and B,
respectively. A positive significant correlation between D t and D 0
indicates that the admixture process has taken place more likely as
continuous gene flow from one parental population to the other
and that the admixed population exhibits some level of genetic
structure.
American individual ancestry estimates for each island and
depicts how ancestry is distributed throughout the island
samples.
No statistically significant differences in genetic ancestry
among the three islands were detected using the KruskalWallis H statistic (Table 3). Further examination of the
mean ranks of the individual ancestry estimates showed
that individuals from Jamaica had the highest mean ranks
of European ancestry, individuals from Barbados had the
highest mean ranks of African ancestry, and those from St.
Results
Genetic structure among the three Caribbean populations
was examined using 28 AIMs that distinguish between European, West African and Native American genetic ancestry. Global and individual admixture estimates for each
island are listed in Tables 1 and 2. West African admixture
was highest in the Barbadian sample and lowest in the Jamaican sample. European and Native American estimates
were also highest in the Jamaican sample. The distribution of individual ancestry estimates for each population
are shown in Figure 1. Except for a few individuals who
appeared to have significant European ancestry, subjects
in all islands generally clustered towards the West African
vertex. Two individuals from Jamaica, however, displayed
extremely high levels of Native American ancestry. If these
two Jamaican samples were excluded then the samples from
St. Thomas appeared to show the most non-African admixture. Bar plot in Figure 2 shows the range in Native
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Journal compilation Figure 1 Triangular plot depicting genetic ancestral
components from West Africans, Europeans American and
Native Americans. Samples from Barbados are depicted as green
circles, St. Thomas as pink triangles, and Jamaica as blue
diamonds.
Annals of Human Genetics (2008) 72,90–98
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J. Benn-Torres et al.
Figure 2 Bar plot of % Native American genetic ancestry for each island.
Parental
Component
Barbados St. Thomas Jamaica Kruskal-Wallis P-value
H statistic
European
142.77
West African
157.02
Native American 146.33
150.29
146.41
153.56
159.77
137.28
144.42
1.640
2.186
0.753
0.440
0.335
0.686
Table 3 Kruskal-Wallis mean ranks and
H statistics.
Highest rank is indicated in bold
Table 4 STRUCTURE estimates of K (subpopulations).
K
Barbados
St. Thomas
Jamaica
1
2
3
− 2879.1
− 2886.2
− 2895.8
− 2770.7
− 2696.9
− 2727.8
− 1085.5
− 1073.9
− 1084.9
Ln probability values
Thomas had the highest mean ranks of Native American
ancestry (Table 3).
Results from the three analyses consistently indicated
the presence of population structure in two of the island
populations. The STRUCTURE analyses revealed that
there were two sub-populations (K = 2) for St. Thomas
(Ln P(D) = − 2696.9) and Jamaica (Ln P(D) = − 1073.9),
but only one (K = 1) for Barbados (Ln P(D) = − 2879.1)
(Table 4). The value of K = 2 for the two Caribbean
populations was consistent with what has been observed
in African-American populations (Bonilla et al. 2004b;
Reiner et al. 2005; Tian et al. 2006; Yang et al. 2005). In addition we observed that 13% of unlinked markers (exceeding the 5% expected by chance) were significantly associated in the St. Thomas’ and Jamaican populations, while
only 2.4% were associated among the Barbadian sample.
Finally, Figure 3 revealed a significant positive correlation
between initial linkage disequilibrium (D 0 ) and current
linkage disequilibrium (D t ) in Jamaica (P = 3.8 × 10− 5 )
and St. Thomas (P = 2× 10− 12 ) but not Barbados (P =
0.089). An examination of the slopes (alphas) of the cor94
Annals of Human Genetics (2008) 72,90–98
relation lines for the three islands revealed that Barbados
(α = 0.09) was markedly different from the slopes for
Jamaica (α = 0.48) and St.Thomas (α = 0.40).
Discussion
Case-control association studies continue to be a widely
used approach for characterizing genetic effects on disease.
However, false positive results and failures to detect real
association due to undetected population stratification are
problems which demand serious concern (Choudhry et al.
2006; Kittles et al. 2002). The unique histories of migration and admixture of populations in the African diaspora
warrant studies on genetic ancestral background in order
to provide useful data for the design of genetic association
studies in these populations.
In this study we estimated individual ancestral proportions for three Caribbean populations using 28 AIMs, and
determined the presence of population stratification in two
of our three Caribbean populations. Our findings corroborate historical records that indicate admixture within the
Caribbean occurred among Africans, Europeans, and to
a lesser degree Native Americans. These findings further
demonstrate that the extent of exchange among these populations, while varied, did not significantly differ across the
three islands studied. Additionally, there is evidence of population stratification in both Jamaica and St. Thomas. The
genetic variation of island populations is largely attributable
to founder effect, migration trends, and genetic drift. Gene
flow into an island from individuals with diverse ancestries
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A
y = 0.0923x + 0.0001
2
R = 0.0086
0.08
0.06
p=0.089
0.04
Dt
0.02
-0.06
0
-0.02 -0.020.00
-0.04
0.02
0.04
0.06
-0.04
-0.06
D0
0.15
B
0.10
p=2.1E-12
0.05
Dt
y = 0.4025x - 0.0006
2
R = 0.171
-0.10
-0.05
0.00
0.00
-0.05
0.05
0.10
-0.10
-0.15
D0
C
y = 0.4808x + 0.0037
2
R = 0.0578
p=3.84E-5
0.2
0.15
0.1
Dt
0.05
-0.06
-0.04
-0.02
0
0.00
-0.05
0.02
0.04
0.06
-0.1
-0.15
-0.2
D0
Figure 3 Observed LD (D t ) versus initial LD due to admixture
(D 0 ) between pairs of unlinked markers in the populations of A)
Barbados; B) St. Thomas; and C) Jamaica.
and non-random mating contribute to significant differences in the distribution of ancestral background, or population stratification. Furthermore, the levels of admixture
detected across these islands are likely the result of simultaneously interacting factors that include the depopulation
of indigenous peoples, geographic location, and the hegemonic colonial power. We note that in our comparisons
there likely could be biases due to the low sample size for
the island of Jamaica (N = 54) and due to ascertainment
given the differences in recruitment strategies across island
populations. However, considering the current genetic data
and historical information available neither sample sizes nor
ascertainment biases appear to have negatively influenced
our conclusions. The global admixture estimates for each
island observed in this study are generally congruent with
those observed in other African-derived communities of
the Caribbean (Miljkovic-Gacic et al. 2005; Parra et al.
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Journal compilation 1998; Shriver et al. 2003). The small differences that exist
between the present study and past research are primarily
a function of the number of markers used. More specifically, the results of this study are most consistent with the
Shiver et al. (2003) study that considered only six more
AIMS than were examined in the present study. The admixture estimate for their African-Caribbean sample was
essentially the same as that observed in the current study.
Comparable studies with individual admixture estimates
for similar African-Caribbean communities do not exist
to date. However, we do note that the recent study of a
genome-wide AIM panel for African Americans revealed
that estimates of both population and individual admixture
proportions are most robust when at least 160 AIMs are
used (Tian et al. 2006).
Molecular evidence supports the view of some historians that although the conquest had a profound impact in
these islands, it did not manage to completely wipe out
indigenous peoples, many of whom survived and were
assimilated into the new colonial system (Guitar, 2002).
Nonetheless, the consequences of this conquest are still
very prominent, as evidenced in the small degree of Native American admixture estimates in these three islands.
These values contrast with what has been observed in the
Hispanic Caribbean, where the indigenous component has
been estimated to be as high as 60% in some contemporary
populations (Martinez-Cruzado et al. 2005). In the island
of St. Thomas for example, a closer look at the individual
admixture estimates revealed that Native American ancestry seemed to be concentrated in a few individuals, as less
than a third of the sample had indigenous ancestry levels
over 10%. However, as indicated by the mean ranks, these
few individuals appeared to have higher levels of Native
ancestry when compared to individuals from the other islands. This contrasts with Jamaica which, while it did not
have the highest mean rank for Native American ancestry,
did have the highest average for Native American ancestry.
This apparent discrepancy is the result of a large variance
in individual ancestry estimates, as well as two outlying Jamaicans that have greater than 50% indigenous ancestry.
These differences in the Native American component may
be attributed to survival of residual indigenous communities in the more inaccessible regions of the islands, and
admixture with Maroon populations.
Location also likely played a role in shaping the admixture characteristics of the islands. Barbados’ geographical
location, for example, appears to have been influential in
its historical, cultural, and economic development. Because it is southeast of the island chain that comprises the
Lesser Antilles and northeast winds complicated the access
to Bridgetown, the only seaport, during the sailing ship
era, Barbados could be considered relatively isolated. This
Annals of Human Genetics (2008) 72,90–98
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J. Benn-Torres et al.
isolation may have acted to deter European colonists and
other migrants, thus resulting in lower levels of non-African
admixture. It follows that the mean ranks of West African
ancestry are the highest in Barbados, as are the global and
individual West African admixture estimates. Jamaica, on
the other hand, may represent a ‘crossroads’ within the
Caribbean. It is located just 145 km south of Cuba and is
much more centrally located as part of the Greater Antilles.
Historically, the Greater Antilles were used as transit points
by colonists between Central and South America and Europe (Rogoziânski, 1999). These factors may have served to
make Jamaica more cosmopolitan and thus provided more
opportunities for admixture to occur. The large variance
in both the global and individual admixture estimates in
Jamaica attests to the cosmopolitan nature of the island.
Finally, the dominant colonial power may also have
served in influencing the admixture characteristics of these
islands. The relative impact and volume of the trade of enslaved Africans was far greater in the islands colonized by
the British, which were wholly utilized in the production
of sugar and therefore required more labour. For instance,
while the British islands received almost 2 million enslaved
Africans altogether during the period 1700 to 1810, the
Danish islands imported approximately 57,000 Africans in
the same time frame (Knight, 1997). Consequently, in islands that received higher numbers of Africans, it follows
that there are higher levels of African genetic contributions
to the current population, as is the case with Barbados. Jamaica appears to deviate somewhat from this pattern. This
may be attributable to the factors discussed above as well
as to the size of the island. We should also note that the IA
estimates are statistical estimates based on only 28 markers and thus may have errors. A larger number of markers, especially those informative for the indigenous Native
American groups in the Caribbean, would likely provide
better estimates.
In summary, the unique histories found on each island
helped to shape much of the genetic variation observed
today. Understanding the dynamics of admixture in the
Caribbean within the historical context of European conquest and colonization remains a challenging but nevertheless exciting task. Consequently, further studies are needed
to obtain a clear picture of the events that moulded the
contemporary Caribbean populations. Given the strong interplay of genetics and the environment (diet and lifestyle)
in complex disease etiology, the focus on genetic ancestry
will be of paramount importance in order to tease out genetic and environmental risk factors for disease (Halder &
Shriver, 2003). It is clear that the use of ancestry informative markers (AIMs) and the potential power of admixture
mapping (Hoggart et al. 2004; Montana & Pritchard 2004;
Patterson et al. 2004; Reich & Patterson, 2005; McKeigue,
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Annals of Human Genetics (2008) 72,90–98
2005) will help to uncover genes underlying ethnic differences in disease risk.
Acknowledgements
We thank the participants from each of the studies. Support for
this project was provided by the NIH (1U54CA91431-01) to
RAK and (HG25487) to BN and the Department of Defense
(DADM1717-00-1-0029) to VF. This research was also funded
by the National Center on Minority Health and Health Disparities/NIH, and the National Human Genome Research Institute/NIH.
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Received: 30 April 2007
Accepted: 9 August 2007
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Journal compilation