Crossovers in two German cystic fibrosis families determine probe order for MET, 7C22 and XV-2c/CS.7.

We have followed the segregation of the probes pJ3.11, 7C22, pB79a, and MET through cystic fibrosis families in the German Democratic Republic with two affected sibs. Two families with a crossover between MET and the CF phenotype were detected. In one of these families recombination was also observed between the DNA probe 7C22 and CF, and between the markers XV-2c and CF, which suggests that XV-2c, MET and 7C22 are all on the same side of CF. The other MET recombinant family is informative with XV-2c and does not recombine, which excludes the genetic order XV-2c--MET--CF if multiple recombinant events are disregarded. These two families together demonstrate that recombinations may occur in a very small genetic interval, which has important implications for prenatal diagnosis based on data from linked markers.