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I am reading one answer about meiosis:

During meiosis I, the sex chromosomes separate and enter different sperm or egg cells (gametes).

I assume that sex chromosomes refer here to homologous chromosomes that experience crossing-over after meiosis that is exchange of genetical information.

Meiosis I refers here to female meiosis I and to male meiosis I. In both cases, homologous chromosomes separate resulting in haploid cells after meiosis I. These homologous chromosomes enter sperm or egg cells according to the writer, which I cannot understand, since the given chromosomes, now sister chromatids after meiosis I, do not enter physically anything, they just divide into haploid cells, gametes, after meiosis II.

What is the meaning of the word "enter" in the given sentence?

I am confused by the word "enter". I am assuming that the word refers to enter something physically which is not the case in my opinion here.

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After meiosis I, the resulting cells are haploid, not diploid, but they have 2n chromosomes. –  Gergana Vandova Feb 8 '12 at 19:48
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X-inactivation occurs post-fertilization, only if fertilized by a X-carrying sperm. In meiosis I, HOMOLOGOUS chromosomes (meaning your mom and dad's copy of your genes) separate. In meiosis II, SISTER CHROMATIDS (i.e. copies of your mom's or your dad's chromosomes) separate. –  jp89 Feb 9 '12 at 22:26
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@Masi You have some flaws in your logic here. The first polar body does not divide into more polar bodies. Once a polar body, always a polar body. At the end of female meiosis II, your total products is 2 polar bodies and 1 mature ovum. –  jp89 Feb 11 '12 at 20:54
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@Masi And to address the question of X-inactivation, x-inactivation only occurs in diploid XX cells under regulation of the XIST gene from the non-inactivated X chromosome. When homologous chromosomes separate in meiosis I, x-inactivation cannot occur since the XIST gene product from either the paternal or maternal chromosome cannot act on the other X chromosome because they're simply not in the same cell anymore. –  jp89 Feb 11 '12 at 20:57
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@Masi See here (uic.edu/classes/bms/bms655/lesson9.html), here (ncbi.nlm.nih.gov/books/NBK10008/#A4696) and here (faculty.stcc.edu/AandP/AP/AP2pages/reprod/oogenesi.htm). There's also a great diagram in my medical genetics book but it's not on the internet. –  jp89 Feb 11 '12 at 22:24
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1 Answer

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I seem to now understand the sentence. I read the clause in other words:

During meiosis I, the sex chromosomes of the gametogonia separate in anaphase I and II and become eventually different sperm or egg cells after cell differentiation and maturation: male spermiogenesis and female maturation into ovum.

Female thing does not seem to have a special name for the maturation thing. Female does not seem to have differentiation part at the last part similarly as male, since only one ovum is formed after the sequence of events. Sex chromosomes refer to your parents genotypes. [jg89]

Problem seems to be solved now.

I would not personally use the word "enter" in a such sequence. It is a shortcut. Better would be "enter eventually" or "enter after a sequence of events". I prefer explicity instead of implicity.

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Ok, I am not sure whether the last female step has anything to do with differentiation. It seems that it just a maturation step, since there will only be one ovum, while 4 male spermatozoa where the differentiation makes sense. I will fix it. Please, reply if I am wrong. –  Masi Feb 11 '12 at 17:39
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