Fróðskaparrit - 01.01.1998, Blaðsíða 4

10
HUMAN POPULATION GENETICS IN THE FAROEISLANDS
1 cM often seen in large, panmixed popula-
tions. The search for linkage disequilibri-
um in association studies among unrelated,
or very distantly related cases, with anon-
onymous markers may be impossible for
diseases in which mutations have occurred
several times at several loci, none of which
may need to be frequent (de la Chapelle,
1993; Houwen et al., 1994; Kaplan et al.,
1995) .
For complex diseases with locus hetero-
genecity it is possible that only some
among several susceptibility loci are pre-
sent, and this is advantageous. The smaller
the group of founders the greater the prob-
ability of only some susceptibility genes
may being present. Furthermore, in small
populations gene frequencies may fluctuate
randomly from generation to generation
and alleles may be lost. Bottleneck reduc-
tions in population size due to disasters
may further decrease the number of disease
genes present (Holgate, 1966; Nei et al.,
1975; Neel and Thompson, 1978; Spiess,
1989).
Recently an approach has been applied,
using a search for shared chromosomal
segments among affected individuals in an
isolated population who were related a few
centuries ago, and in a genome wide scan
used to map a gene for beningn recurrent
intrahepatic cholestasis. This approach
makes the localisation of recessive or dom-
inant disease genes possible by means of a
genome wide scan with a few hundred
markers genotyping only a few affected in-
dividuals and their parents (Houwen et al.,
1994; Freimer et al., 1996; Escamillaeía/.,
1996) .
As this method compares only the haplo-
type of affected individuals it can be ap-
plied to diseases with low penetrance.
Material and methods
Population and history of the Faroe Is-
lands
The Faroe Islands is a small group of is-
lands in the North Atlantic Ocean situated
between Norway, Iceland and Scotland. Of
the 18 main islands, 17 are inhabited. The
islands cover an area of 1.399 km2.
The origins of the population is not
known, but thought to be mainly Norwe-
gian with admixture from the British Is-
lands. Thus, disease genes found in the
Faroese population are potentially more
relevant for a larger population than muta-
tions in other genetic isolates in Europe
such as Finland and Sardinia. Irish monks
are believed to have visited the islands from
about A.D. 725 or earlier. The islands have
been raided by the Vikings from about 795
and settled from around 825 probably espe-
cially by emigration from the westem part
of Norway around Bergen. The islands
were thus mainly populated at the same
time as Iceland and it is believed that the
majority of the land was already occupied
around year 900. While the population in
Iceland has been estimated to around 65000
in year 965 probably only a few thousand
people inhabited the Faroe Islands at that
time. Isolation was imposed by distance, by
commercial monopoly and legislation and
the principal contacts have been with Nor-
way, Iceland and Denmark. However, the
islands have had some contact with mer-
chants, smugglers, pirates and naval forces