A geneticist observes that four gametes donated by an individual contain the fol

Question

A geneticist observes that four gametes donated by an individual contain the following number of chromosomes: cell one has 23 chromosomes. cell two has 23 chromosomes, cell three has 22 chromosomes, and cell four contains 24 chromosomes. Consider the following hypotheses and clearly state why each don or don not support this nondisjunction event. (There is one correct hypothesis.) The sister chromatids did not separate in meiosis 1. The homologous chromosomes did not separate in meiosis 11. The homologous chromosomes did not separate properly in meiosis I the sister chromatids did not separate properly in meiosis II. The sister chromatids did separate properly in meiosis II.

Explanation / Answer

Ans. Ans. Meiotic disjunction occurs in two phases-

Let’s label three pair of chromosomes as – A1 /A2, B1/B2 and C1/ C2 where, 1 = maternal chromosome and 2 = fraternal chromosome.

For simplicity of expression we proceed with A1A2, not with 46 chromosomes as in a diploid cell.

1. Meiosis I disjunction: Homologous chromates separate.

A1 and A2 are called homologues pair. During S-phase, DNA is duplicated.

The resultant-

I. A1A1 remain joined to each other. Are called sister chromatids.

The same happens to A2A2, and rest other chromosomes.

            II. Homologous chromosome A1 and A2 pair together prophase I. Since each of A1 and A2 are duplicated, they are (A1A1) and (A2A2) respectively.

          III. Homologous pairing forma a tetrad – A1A1/A2A2 i.e. four chromosomes remaining together.

            IV. During meiosis I, homologous chromosome separate. That it, (A1A1/A2A2) dissociates to form wo sister chromatids (A1A1) and (A2A2)- one goes to first daughter cells, and the other goes to second daughter cell produced after meiosis I.  

            V. In case of non-disjunction, sister chromatids do not separate from each other, that is, one daughter cell gets no chromosome while the other gets 4 chromosomes at the end of meiosis I.

So, Result of non-disjunction, meiosis I: Cell 1 = 0 chromosome      ; Cell 2 = 4 chromosome

2. Meiosis II disjunction: Sister chromates separates into two independent chromosomes. That is, “A1 A1” separates into two independent A1 chromosomes.

If chromosomes have separated normally in meiosis I, but exhibits non-disjunction during meiosis II- the resultant two daughter cells will have –

            Cell 3 = 0 chromosome       ; Cell 4 = 2 chromosomes

#2. A. Non-disjunction in Meiosis I followed by Non-disjunction in Meiosis II :

2A. 1. Cell 1 (Meiosis I) = 0 chromosome            -- Nondisjunction= 0 chromosomes

            Normal disjunction = 0 chromosomes

2A. 2. Cell 2 (Meiosis I) = 4 chromosome

            Normal disjunction----> 1 chromosome (cell 5) + 3 chromosomes (cell 6)

            Non-disjunction -------> 4 chromosomes (cell 7)

Note: Cell 1, cell 2, cell 7 etc. denote the cell number- all possibilities of respective non-disjunction or normal disjunction.

Analysis of given condition:

A. Incorrect. Sister chromatids do not separate in meiosis I, they separate in meiosis II.

B. Incorrect. Homologous chromosomes separate in meiosis I

C. Incorrect. No- disjunction in Meiosis I but normal disjunction in Meiosis II:

If it were the case the resultant cells would have following chromosome number (See 2A. 2.) -

            Cell 5- 1 chromosome

            Cell 6- 3 chromosome

            Cell 7- 4 chromosomes

But no cell has 2 chromosomes (i.e. 2 chromosomes held together).

So, the non-disjunction is NOT in meiosis I.

D. Normal separation (Disjunction) in meiosis I, no-disjunction in Meiosis II:

Under this condition one cell will have 0 chromosomes (No copy of that chromosomes) and the other cell will have 2 chromosomes (2 copies of a chromosome attached together). See 2.

So, it’s the correct option.

E. Incorrect. If chromosomes separate properly in Meiosis I (question does not says it did not; so it’s assumed to be normal separation) as well as in meiosis II, all daughter cell will have only 1 copy of each chromosome.

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