[Approaching the gene of mucoviscidosis. New data].

We used 5 polymorphic probes strongly linked to the gene of cystic fibrosis (CF) to perform the genotypical study of 48 families with at least one child presenting with the disease. The last Km19 and XV2c probes showed a very important linkage imbalance with the CF gene (allele 2 = 6.6 kb of Km19/Pstl, chi 2 = 56; allele 1 = 2.1 kb of XV2c/Taql, chi 2 = 21). These two markers define a B haplotype which confers a relative risk of 55 to be gene carrier. From these data, the predictive value for an individual presenting with this haplotype to be heterozygous was computed to be 1/5. Presently, the risk of 1/20 for a randomized subject to be gene-carrier should be reexamined after study of this genotype. These results are very important practically, as they modify the classical data of genetic counselling concerning cystic fibrosis for the couples with a risk higher than 1/4.