Calix having X chromosomes from both his mother and his father shows that each X came from a normal gamete, so the issue cannot be explained by a chromosome being broken and reassembled (a chromosomal rearrangement) on just one parent’s X.

Normal X inheritance

In mammals, each X chromosome in the offspring can be “tagged” as maternal or paternal by looking at its specific DNA markers or alleles.

In Calix’s case, one X clearly matches the mother’s X and the other X clearly matches the father’s X, meaning he received a whole, intact X from each parent rather than a structurally altered X created by rearrangement.

What chromosomal rearrangement would look like

Chromosomal rearrangement involves structural changes within a chromosome (like deletions, duplications, inversions, or translocations) and does not normally create an extra full chromosome.

If rearrangement were the cause, you would expect abnormal structure on one X (pieces missing, duplicated, or swapped), not a situation where both X chromosomes can be cleanly traced back—one to the mother and one to the father.

Why this points away from rearrangement

Because both X chromosomes in Calix are full, traceable copies from each parent, there is no evidence that either X was pieced together from fragments in a way that would change total DNA amount.

Instead, the data (including the increased total DNA mass per cell) fit better with an event that adds an entire extra chromosome—such as nondisjunction—rather than with a rearrangement that just reshuffles parts of existing chromosomes.