Somatic cells from an individual with Down syndrome (trisomy 21) have copies of the genes on chromosome 20 and copies of the genes on chromosome 21. See Section 13.3 (Page) . A karyotype showing chromosomes 1-22 plus X and Y chromosomes. For chromosomes 1-22, size and color banding patterns match for each of the two chromosomes in the pair. There are three copies of chromosome 21, which are circled. Chromosome X is significantly larger than chromosome Y.