Meiosis / AAMC 9 Question

[HinduHammer]

This has been addressed in several other threads over the years, but I haven't seen a satisfactory answer. Please let me know if you can explain it - otherwise I really think the test makers may have made a mistake...

If two cycles of DNA replication in a spermatocyte occurred before tetrad formation what would be the product after normal meiotic division?

A. One tetraploid sperm
B. Four diploid sperm*
C. Four haploid sperm
D. Eight haploid sperm

I was under the impression that 2n comes from having one copy from both mother and father (diploid), as such having a spermatocyte with 2X the genetic material is just 2X the same material from the father (twice as many "n" materials). This is the same reason that the tetrads are still considered n (I think), and that after meiosis 1 even though the daughter cells still have 2 chromosomes they are considered "n" because the genetic material is the same and thus not homologous.

I'm going to quote other people's issues with this question too, so you all can see where the confusion lies (in case my above explanation didn't convey it clearly)"

@thestrokes14 said
I basically assumed that identical chromosomes were not considered homologous. Also, I define (as Examkrackers does) that diploid implies homologous pairs. I am guessing my first assumption is why I got the wrong answer. If an organism has two identical copies of a chromosome, although not necessarily homologous, it would be considered diploid then?

I'm confused on about this question too. No one has really addressed the OP's issue with it. Wouldn't the cell still be haploid since it has copies of identical chromosomes, not homologous ones?

I know where the confusion stems from here.

Just because you multiply genetic material doesn't mean you increase in ploidy number.

Normal somatic cells multiply their DNA by 2 during S phase, yet they still stay diploid, since they are not gaining any new genetic material, just replicating that which was already there.

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Can anyone explain the test makers logic here? Further is my understanding of meiosis incorrect? Thanks.

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[jordansnfootbal]

I'm assuming the answer is B.?
If it is ONE replication gives you 4 copies of each gene TWO replications give you 8 copies of each gene. You know that one replication is normally what occurs before mitosis resulting in haploid spermatids so you can assume two replications gives you diploid spermatids....its easier to explain with a diagram

 

[MolonLabe16]

Hmm, this is how I interpreted the question, hope this helps. I'll use a normal human chromosome complement.

Normal Cell (or spermatocyte), pre-synthesis
- 23 chromosomes, 23 unique chromatids, from father ( n)
- 23 chromosomes, 23 unique chromatids, from mother ( n)
- 46 chromosomes total (2n)
- one full set of genetic material

Normal Cell (or spermatocyte), post-synthesis 1
- 23 chromosomes, 23 pairs of unique chromatids, from father ( n)
- 23 chromosomes, 23 pairs of unique chromatids, from mother ( n)
- 46 chromosomes total (2n)
- two full sets of genetic material

--> this is normally where Meiosis I/Mitosis would normally start but there is another round of synthesis

Spermatocyte, post synthesis 2 (assuming everything gets duplicated and chromosomes can't have 4 chromatids)
- 23 pairs of unique chromosomes, (23 pairs of unique chromatids)x2, from father (2n)
- 23 pairs of unique chromosomes, (23 pairs of unique chromatids)x2, from mother (2n)
- 92 chromosomes total, 46 pairs of chromosomes (4n)
- four full sets of genetic material

Post Meiosis I (x2 daughter cells)
- 46 chromosomes, 23 pairs of unique chromatids, mix from father and mother due to independent assortment
- 46 chromosomes total (2n)
- two full sets of genetic material

Post Meiosis II (x4 daughter cells)
- 46 chromosomes, 23 unique chromatids, mixed again due to segregation, chromatids separate
- 46 chromosomes total (2n)
- one full set of genetic material

So the result should be 4 diploid cells