Medical Genetics Trisomy is having three chromosomes of a particular type. A common autosomal trisomy in humans in Down syndrome or trisomy 21, in which a person has three instead of the normal two chromosome 21s. Trisomy is a specific instance of polysomy a more general term that indicates having more than two of any given chromosome.Medical Genetics Trisomy is having three chromosomes of a particular type. A common autosomal trisomy in humans in Down syndrome, or trisomy 21, in which a person has three instead of the normal two chromosome 21s. Trisomy is a specific instance of polysomy, a more general term that indicates having more than two of any given chromosome