title: Trisomy 21 and trisomy 18 in half-siblings.
mesh_terms: D000015:Abnormalities, Multiple / Q000235:genetics; D002902:Chromosomes, Human, 16-18; D004314:Down Syndrome / Q000235:genetics*; D005260:Female; D006801:Humans; D008297:Male; D014314:Trisomy