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Post by lfmpyisi » Wed Nov 13, 2019 7:15 pm

For a human karyotype, are females diploid given that they have 2 copies of the X chromosomes? Are men haploid because they have one copy of the X and one copy of the Y chromosome?
Not: I know that the germ cells are haploid because the cells have undergone meiosis. I am curious to know the ploidy of males and females before meiosis occurs, and chromosome 23 (X and Y) is separated into either X or Y.
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Re: Ploidy

Post by NS_Tutor_Mathias » Thu Nov 14, 2019 1:08 pm

All human somatic cells are diploid while all gametes are haploid. The confusion you're having is between sister chromatids, which are pairs of identical chromosomes, versus homologous chromosomes, which contain all the same genes but may contain different alleles of those genes and therefore not be identical.

So for any given chromosome, your somatic cells can have 4 (!) copies after S-phase:
or for the resulting gametes after meiosis:
A and A and a and a

Where capital-As are one pair of sisters and lowercase-as are another pair of sisters, while capital-A is homologous to lowercase-a. This is important to realize, because meiosis starts only after S-phase (and after G2 and all the relevant cell cycle checkpoints). That means on chromosome 23 for example, I am: XXYY and my gametes are all either singular X or singular Y (no sister chromatids!)

While my sister would be XXXX (or XXX'X' more accurately, since she has two homologous Xs).

To clarify on meiosis:
At the end of meiosis I, sister chromatids stay together, so if we ignore recombination the way it happens on autosomes, my products of meisosis one would be two cells:
XX and YY
Then after meiosis II, those sisters chromatids will separate and I will produce 4 cells:
X and X and Y and Y

If an X of mine fuses with an ovum to form a zygote, then the result will be XX', a daughter. If a Y of mine fuses with an ovum, we get XY, or a son. Either case will of course immediately replicate DNA to become XXX'X' or XXYY respectively.

Small note:
Ploidy (1n/2n etc) is determined by the number of homologs you have, not the number of total copies. So going from G1 to S-phase, a human somatic cell stays diploid. However, just completing meiosis I makes a cell functionally haploid, since now they have only sister chromatids, not homologous chromosomes.

Only autosomes (non-sex chromosomes) are usually used to determine an organism's ploidy. But if you want, functionally, the Y chromosome can be considered a deteriorated homolog to the X chromosome, so we would still have two homologous sex chromosomes.
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