Maternal transmission of ring chromosome 21.

A psychomotor-retarded infant with minor dysmorphic signs and a karyotype 46,XY,r(21)mat in lymphocytes is reported. The mother is phenotypically normal but shows the same unstable r(21). This is another case demonstrating that a chromosomal aberration does not necessarily lead to infertility by meiotic failure. Nevertheless, segregation of ring chromosomes is problematic for two reasons: mitotic problems of the ring structure itself and synaptic difficulties during the pachytene stage.