Work Step by Step
A karyotype is a picture of the diploid complement of chromosomes of a cell that shows homologous chromosomes paired together in order of decreasing size. Because the karyotype shows the chromosomes and their numbers, only a defect in chromosome number would appear in a karyotype. Options A, C, and D are incorrect because these are conditions that are not caused by defects in the number of chromosomes. Option B is correct because in trisomy 21, we can clearly see that there are three copies of chromosome 21 in the karyotype.