Segregation of the marker chromosome der(20) in the sperm of a male with karyotype 46,XY[961/ 47,XY+mar[4].

The influence of the marker chromosome on reproductive failure is difficult to assess, especially in the case of low-rate mosaicism. The aim of our work was to examine the meiotic segregation of the marker chromosome der(20) in the sperm of a normal male whose wife had experienced three miscarriages, and therefore to determine whether there occurred a gametogenic tissue-specific mosaicism. The proband was a phenotypically normal 35-year-old man, referred for pre-conceptional counseling after his wife had experienced three miscarriages. Chromosome cytogenetic analysis was performed on peripheral blood lymphocytes using routine protocols. Sperm chromosome complements were obtained after penetration of zona-free hamster oocytes. Dual-color FISH analysis was performed using directly labeled probes identifying X, Y, 9 and 20 chromosomes. A small marker chromosome der(20) was identified in 4% of the proband's lymphocytes and 8.25% of his sperm, which indicated mosaicism among the gametogenic cells, in which the proportion of cells containing the der(20) marker may reach a minimum of approx. 16%. In this case, we addressed a problem whether a revealed marker der(20) was the cause of reproductive failure or just a coincidental finding. In our view, each case of low-rate mosaicism of the marker chromosome should be individually assessed.