Medical Genetics But it wasn't until 1959 that Jerome Lejeune and Patricia Jacobs, working independently ′rst determined the cause to be trisomy (triplication) of the 21st chromosome Cases of Down syndrome due to translocation and mosaicism were described over the next three yearsMedical Genetics But it wasn't until 1959 that Jerome Lejeune and Patricia Jacobs, working independently, first determined the cause to be trisomy (triplication) of the 21st chromosome. Cases of Down syndrome due to translocation and mosaicism were described over the next three years