Segregation patterns and phenotypes of unbalanced offspring in a large family with (10;18) chromosome translocation.

We describe a large family in whom a balanced 10;18 chromosome translocation is segregating through five generations. Six severely mentally retarded relatives and an abnormal fetus further define the phenotypic expression of dup (18q21----qter). Other segregants detected prenatally included a fetus with deletion 18q21----qter and two fetuses with dup(18pter----q21) owing to tertiary trisomy. One of the latter also had an extra X chromosome; this might be another example of possible nonhomologous pairing in man.