Re: In first meiosis non disjunction what will be origin of disomic gamete

Hi Neal!

From your question it is clear that you need to clear your basic concept 
regarding meiosis first. Meiosis I is a reductional division or a division 
where chromosome number is reduced to half. Meiosis II is an equational 
division where chromosome number remains the same. Meiosis usually occurs 
in the gonads (testes in males and ovary in females).

Consider this example, there is an organism which has say 46 chromosomes. 
Not during meiosis I its chromosme number will reduce to 23 chromosmes and 
during meiosis II its chromosme number will remain 23 although the number 
of daughter cells increases.

The somatic cells in our body have 46 chromosomes. Half of this has been 
donated by our mothers and fathers. Now, chromosomes are tranferred to the 
offspring from both the parents, mother and father, each of whom on the 
other hand had received their chromosomes from their respective mother and 
father. For example, you have received your chromosomes from both your 
parents, not just your mother or not just your father.

Disjunction or separation of homologous chromosomes (chromosomes coming 
from both parents) is a common phenomenon and is the ususal way of 
maintaining chromosome number in the offspring. But NON-disjunction means 
that homologous chromosomes don't get to separate during meiosis I and I 
hope you can easily guess the consequenses, the daughter cell will be 
having the same number of chromosmes as the mother cell, i.e. reduction in 
chromosome number ( a key characteristics of meiosis I does NOT occur). 

Usually it is found that if non disjunction has taken place in meiosis I, 
it cant be restored to normalcy in meiosis II, which is purely an 
equational division similar to mitosis where chromosome number of both the 
mother and the daughter cell remain the same.

Hope this has cleared your concept a bit.