The figure below shows a karyotype from a normal boy. A karyotype is a photograph of a magnified view of the chromosomes from a human cell, with pairs of homologous chromosomes arranged next to each other and numbered. An extra copy of any of the chromosomes in the top row results in such severe abnormalities that the embryo always dies. In contrast, an extra copy of chromosome 21 results in less severe abnormalities so the embryo can often survive to be born as a baby with Down syndrome. What do you think is the reason for this difference?