35. If non-disjunction occurs in meiosis I, Which of the following scenario is most likely to occur?
(1) Two gametes will be n+1 and two will be n-1
(2) one gametes will be n+1, two will be ‘n’ and one will be n-1
(3) Two gametes will be normal and two will be n-1
(4) Two gametes will be normal and two will be n+1
Concept:
-
In meiosis I, homologous chromosomes (each still with two sister chromatids) should separate and go to opposite poles.
-
If nondisjunction happens in meiosis I, both homologs go to the same pole.
-
At the end of meiosis I, one daughter cell has n+1 chromosomes, the other has n–1.
-
Meiosis II then separates sister chromatids normally, so each daughter cell produces two gametes:
-
From the n+1 cell → two gametes with n+1 chromosomes.
-
From the n–1 cell → two gametes with n–1 chromosomes.
-
-
Thus, the four gametes are: 2× (n+1) and 2× (n–1).
Option-wise explanation:
-
Two gametes will be n+1 and two will be n–1 – correct
-
Exactly matches the pattern produced by nondisjunction in meiosis I.
-
-
One n+1, two n, one n–1
-
This pattern (2 normal, 1 n+1, 1 n–1) is characteristic of nondisjunction in meiosis II, not meiosis I.
-
-
Two gametes will be normal and two will be n–1
-
Would require one daughter cell to be normal and one to be n–1, which is not what happens when homologous chromosomes fail to separate in meiosis I.
-
-
Two gametes will be normal and two will be n+1
-
Similarly incorrect; nondisjunction in meiosis I doesn’t yield any normal gametes.
-
Therefore, when nondisjunction occurs in meiosis I, the most likely outcome is two gametes with n+1 and two with n–1 chromosomes (option 1).
