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In a gene test for a son and a mother by FamilyTreeDNA, the son is shown as having a match with the mother on the X chromosome from about 2 700 000 to about 154 900 000, indicating that parts of the PAR1 and PAR2 are not matching.

What is the reason for this?

a20
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1 Answers1

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In the Abstract of the article The Human Pseudoautosomal Region (PAR): Origin, Function and Future, authors A Helena Mangs and Brian J Morris say:

The pseudoautosomal regions (PAR1 and PAR2) of the human X and Y chromosomes pair and recombine during meiosis. Thus genes in this region are not inherited in a strictly sex-linked fashion. PAR1 is located at the terminal region of the short arms and PAR2 at the tips of the long arms of these chromosomes.

Figure 1 in the article The human pseudoautosomal regions: a review for genetic epidemiologists by Antonia Flaquer, Gudrun A Rappold, Thomas F Wienker & Christine Fischer published in the European Journal of Human Genetics volume 16, pages771–779 (2008) shows the PAR1 and PAR2 regions on the human X and Y chromosomes.

The X and Y chromosomes can recombine during meiosis, but only at their tips. The non-matching regions are usually very small, and are usually not useful for genetic genealogy.

Resources:

Jan Murphy
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  • Thank you for your reply, but I still do not understand. Meiosis occurs before fertilization, so the Y chromosome is irrelevant. All the genetic material in the X chromosome should come from the mother, so there should be, barring spontaneous mutations or read errors, a 100 % match of the X chromosome for a son to his mother. No? – a20 Jul 18 '23 at 13:14
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    @a20 I'm not satisfied with this answer, either. I wanted to give you something to start with while we're waiting for a better answer. – Jan Murphy Jul 20 '23 at 00:37