The incidence of cystic fibrosis (CF) in Finland is one tenth that in other 
Caucasian populations. To study the genetics of CF in Finland, we used a 
combined molecular and genealogical approach. Out of the 20 Finnish families 
with a living CF patient, 19 were typed for eight closely linked restriction 
fragment length polymorphisms (RFLP) at the MET, D7S8, and D7S23 loci. The 
birthplaces of the parents and grandparents were traced using population 
registries. Allele and haplotype frequencies in Finland are similar to those of 
other European and North American populations, but are modified by sampling: two 
regional CF gene clusters, evidently the results of a founder effect, were 
identified. Generally, the gene was evenly distributed over the population, 
carrier frequency being estimated at approximately 1.3%. We conclude that CF in 
Finland is caused by the common Caucasian mutation(s), and that the low 
frequency of the gene can be explained by a negative sampling effect and genetic 
drift.