Marker chromosomes in parents to children with Down's syndrome.

The incidence of an extra marker chromosome in the normal population is low, about 0.26 per 1,000, and that of trisomy 21 is about 1.25 per 1,000. The incidence of both these chromosomal abnormalities in the same family should be very low, if this occurs by chance. Thirteen cases of Down's syndrome in 10 families in which one of the parents had an extra marker chromosome have been reported earlier. In three of these families there were two siblings with Down's syndrome. The present report describes two families in which an extra marker chromosome was found in one of the parents and in which three of the offspring had a trisomy 21 karyotype. From the findings at Q- and AgNOR banding of the chromosomes it seemed probable that in one case this extra marker chromosome was an isochromosome for the short arm of an acrocentric chromosome. It seems very likely that marker chromosomes in healthy mothers are of pathogenetic importance for non-disjunction, resulting in a trisomy 21 offspring. A finding of an extra marker chromosome in one of the parents should therefore be taken into consideration in genetic counselling.