Two different structural abnormalities of chromosome 13 in offspring of chromosomally normal parents with two fragile sites.

Two siblings were found with different structural abnormalities involving their maternally inherited chromosome 13. The proband exhibited a ring 13 and a small fragment: 46,XX,r(13) (p11q34), +f, while her clinically normal brother carried a dicentric Robertsonian translocation: 45,XY,dic(13;15) (p11;p11). Both parents had normal karyotypes in peripheral blood and skin fibroblasts. The structural abnormalities of chromosome 13 may be due to an unstable gonadal 13;15 translocation in the mother. In addition, two autosomal fragile sites were segregating in this family. The mother had a fragile (16) (q22) which was inherited by the proband. The father and paternal grandmother possessed a fragile (12)(q13) which was not inherited by either child. The expression of both fragile sites was dependent on culture conditions.