Expert: Dana Krempels, Ph.D. - 10/6/2008

Question
Hi again i was just wondering if u could help me better understand the processes of non-disjunction. Really i don't need to understand the term so much as what happens to the male XY chromosomes during meiosis 1 and 2. I've drawn a image on the way i think the processes works the link is here: http://img222.imageshack.us/my.php?image=meosao9.jpg is this correct, if not why? thanks a lot!

Answer
Dear Chris,

The diagram you've drawn isn't quite correct.

Remember that before meiosis (or mitosis), each chromosome duplicates itself, but stays attached to its "twin", forming a pair of sister chromatids.

The X and the Y chromosomes are considered homologous partners, though they are hetermorphic (of different shape), unlike the homomorphic autosomes.  But they still line up together at meiosis I.  Hence, the drawing you have for Meiosis I should have a cell with *two* X chromosomes and *two* Y chromosomes on the top line.

The first division, the reduction division, separates the homologous pair, but *not* the sister chromatids.  So one of the cells on the second line should have two X chromosomes (still attached to one another) and the other should have two Y chromosomes (also still attached to each other).  That's the end of Meiosis I.

The third line (representing the result of Meiosis II) should have four cells.  During the last, equational division of meiosis, the sister chromatids separate and travel to different sister cells.  So of those last four product cells (after Meiosis II), two should have a single X chromosome, and two should have a single Y chromosome.

(You don't need the second drawing at all.  Both divisions of meiosis are represented in either one, though you don't have the chromosomes distributed correctly.)

Hope that helps.  Good luck in your lab and classes!

Dana