Question

Nondisjunction of chromosome #21 in which of the following (1-7) could result in a fetus/child with trisomy 21 (47,XX+21) or (47,XY+21)? To indicate your choices, type an “R” at the end of each RIGHT choice, and a “W” at the end of each WRONG choice.

I. Anaphase 1 in spermatogenesis

II. Anaphase 2 in spermatogenesis

III. Anaphase 1 in oogenesis

IV. Anaphase 2 in oogenesis

V. The first anaphase of a normal zygote.

VI. The anaphase in one of the 2 cells produced from the first normal division of a normal zygote.

VII. The anaphase in one of the 4 cells produced from the first normal division of a normal zygote.

Answer 1

ANSWER :-

1. According to me, non-disjunction is the process during which the homologous chromosomes are not able to separate and thus lead to abnormality in terms of number of chromosomes. This process can occur during meiosis as well as mitosis.
2. In case of Trisomy 21 also called as Down's syndrome, there is an additional chromosomes which occurs in the 21st pair due to non-disjunction event occuring during Meiosis I as well as Meiosis II in both the spermatogenesis and oogenesis process. This is associated with autosomal chromosomes and thus is not associated with sex chromosomes.

Option 1 :- Anaphase I in spermatogenesis (R). (Non-disjunction usually occurs during Anaphase Stage at the time when the chromosomes separate themselves and migrates to opposite poles).

Option 2 :- Anaphase II in spermatogenesis (R).

Option 3 :- Anaphase I in oogenesis (R)

Option 4 :- Anaphase II in oogenesis (R)

Option 5 :- The first Anaphase of a normal zygote (W). (Mosaicism has been seen in individuals but in case of mitotic non-disjunction past fertilization event, the event does not happen or has less probability of occurence accept for affected the sex chromosomes and hence this condition can only be assumed to be passed on through non-disjunction events occuring during Meiosis).

Option 6 :- The Anaphase in one of the two cells produced from the first normal division of a normal zygote (W).

Option 7 :- The Anaphase in one of the four cells produced from the first normal division of the zygote (W).

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