​If a gamete produced following non-disjunction of a chromosome at second meiotic division was fertilized by a normal gamete, what is the expected frequency of trisomic progeny?​

Correct option is A

Explanation-

Non-disjunction in Meiosis II means sister chromatids fail to separate.
This results in two normal gametes, one disomic gamete (extra chromosome), and one nullisomic gamete (missing chromosome).
So from a meiosis II non-disjunction event:
                              4 gametes are produced:
                                                       2 normal (n)
                                                       1 with (n+1) → disomic
                                                       1 with (n–1) → nullisomic

​​When these are fertilized with a normal gamete (n), the possible outcomes are:

Abnormal Gamete                       Fertilization with Normal Gamete (n)                                   Zygote Type
n (normal)                                                      n + n = 2n                                                                     Normal
n (normal)                                                      n + n = 2n                                                                     Normal
n+1                                                              n + (n+1) = 2n+1                                                            Trisomy
n−1                                                              n + (n−1) = 2n−1                                                          Monosomy

Option A -  1/4 → Correct
Only 1 gamete out of the 4 (the n+1 one) gives rise to trisomic progeny.
So, frequency = 1 out of 4 = 1/4
This is the correct and scientifically accurate answer.