Non disjunction

[groovyangel2008]

Hi,
I wonder if anyone can help me...I was reading about men who have XYY chromosomes instead of having the XY chromosome and apparently this occurs as a result of non disjunction in meiosis 2! however why is this the case, why can it not be because of non disjunction in meiosis 1?
Help will be much appreciated
Thanks

[darkido]

Hi there!

I think that this is because Meiosis I is causes the separation of homologous chromosomes (eg. XX in females and XY in males[though i am not too sure if we can call XY homologous]) So after meiosis I you get 2 haploid cells, one with the X chromosome and one with the Y chromosome.

In Meiosis II, separation of the sister chromatids of each chromosome happens. This is where non-disjunction of the Y chromosome can lead to a formation of a gamete with a double copy of the Y chromosome(YY gamete)

If non-disjunction occurs in Meiosis I, you will have both X and Y chromosomes in one cell. Which will lead to formation of 2 gamete with XY chromosomes if Meiosis II occurs normally. This will lead to XXY syndrome if fertilization is successful.

It might be possible to form a YY gamete from non-disjunction in both Meiosis I and Meiosis II, but that is highly unlikely. After all genetics is a game of probability.

I am just theorising up there, I wouldn't be surprised if I am wrong. Hope it helps you getting to the right answer though.