With sex chromosomes, there are not two copies in the mammalian male. So, for this special case, how does cross-over work?
Since there is only one Y chromosome in males (none in females), it is kind of a moot point. There is no sister chromatid to recombine with. Even if there was recombination between the 2 Ys in a tetraploid cell, after DNA replication, recombination would not change the Y because they are exactly the same. So any recombination at that point would not alter the chromosome at all. This is one reason why the Y can be used to trace the ancestral origin of males. Also, there are actually very few genes on Y.
My question was actually subterfuge. I thought that the reply would be as yours, Gregory, but appreciate having it explicitly stated. The real point concerns the X's, and in particular, those of the somatic line, since one is a Barr body. The more important question is this:
Are X chromosomal sequences as reported in the literature like unto the autosomes, that they are also a collage?
Another way to ask this question is, does the Barr body get sequenced?
The fact that one X is inactive, the Barr body, does not affect the DNA sequencing reactions because this is done on naked DNA. So yes, in females, any sequence for the X is also a mixture of the two alleles on X.
OK. Then, may I suggest that moving forward on the issue of cross-over character is perhaps best facilitated by some means of removal of the Barr body prior to sequencing of a cell. Something like staining that is specific to the proteins that cover the Barr body. Have you critical commentary on such methods?
Dear William
There are many different events found between male and female occurs during meiosis, and the crossover, and according to Sermon & Viville (2014) we can write briefly in the following points:
In male meiosis, telomeres of all homologs are attached to the nuclear membrane next to a morphologically distinct site in the nucleus, the sex body. The sex body contains the transcriptionally inactive X and Y chromosomes that are only partially paired. Pairing occurs in a region of the sex chromosomes which is containing homologous genes, termed the “pseudoautosomal region” (PAR).
Certain areas of the chromosomes are particularly accessible to recombination enzymes and form recombination hot spots. One such recombination hot spot is in the pseudoautosomal region of the X and Y chromosomes where they share homology.
In male meiosis, an obligatory exchange provides for the presence of a chiasma physically linking the sex chromosomes. This is important for the separation of the X from the Y chromosome at first meiosis of spermatogenesis. In oogenesis, the two X chromosomes pair all along their length and may possess chiasmata on long and short chromosome arms.
Pairing: ( male ) Pairing between all homologous; X/Y pairing in pseudoautosomal region of the heterologous gonosomes. Unpaired regions inactivated.
( female) the pairing of all autosomal chromosomes; reactivation of the second X chromosome and full pairing and recombination between both X chromosomes.
Recombination: ( male meiosis) Usually at least one exchange between all autosomal homologs and an obligatory exchange between the gonosomes in the pseudoautosomal region.
(female meiosis) Usually at least one crossover and exchange between all autosomal homologs and between the two X chromosomes; and higher recombination rates in female meiosis compared to male meiosis.
Daughter cells: (male meiosis) Four haploid sperm containing different combinations of chromosomes from each parent and recombinant chromatids with some maternally and paternally derived alleles.
(female meiosis) One diploid and one haploid first and second PB, and a haploid set of chromatids in the egg after fertilization comprising different combinations of chromatids from each parent as well as recombinant chromatids with some maternally and paternally derived alleles.
For more information, please, read in the textbook:
Sermon K., St´ephane Viville.2014. Textbook of Human Reproductive Genetics. Published in the United States of America by Cambridge University Press, New York.
Good Luck
Crossing overs occure only in homologous chromosomes while sex chromosomes in both sexes are not homologous at all. However translocation takes place in sex chromosomes
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