What causes XYY Syndrome?

Question

It's obvious how a misdisjunction can result in klinefelter's syndrome (XXY) but I don't see how this can result in XYY syndrome. Your parents have a collective total of only one Y chromosome so how can any misdisjunction result in a child with two Y chromosomes?

Answer 1

Hi,

During the process of non-disjunction the XX (mom) or XY (dad) fail to separate during meiosis. As you can see in the diagram posted, the failure to separate occurs during either Meiosis I or II, the end result of the gamete determines at which stage it occurs.  During the process of fertilization (the egg and sperm from each parent coming together), if a sperm, having both an X and a Y chromosome fertilizes an egg having a single X chromosome, or a normal Y- bearing sperm fertilizes an egg having two X chromosomes, an XXY male is conceived. (Diagram: a-3 and b-1,2). Hope this helps. Feel free to contact me if you have any questions or need further clarification.

Answer 2

Klinefelter Syndrome, XXY is a misdisjunction during oogenesis. This is actually a misdisjunction during anaphase I or II during spermatogenesis. Thus the sperm that fertilized the egg had 2Y (YY), rather than just Y.

Thus and XYY Syndrome, or Jacob's Syndrome would have a genetic composition of:

---X from the mother

---YY from the father