Genetics of X-Chromosome, Mother-to-Son

[Iggy]

@Riemann

But I’m puzzled as to where centromere origin could possibly be relevant in practice. I’ve never done karyotyping, but I’m skeptical that there’s any distinguishable variation in centromere morphology under a microscope, let alone enough to reliably determine which grandparent the centromere came from. And if you’re using PCR or whatever to genotype people, obviously you’ll look directly at the locus you care about, it’s not relevant which grandparent the centromere came from.

Perhaps it was just for the convenience of classroom discussion, but my Human Genetics and Cytogenetics professors made the same distinction.

Yeah, Cytogenetics. Old school stuff. This was before PCR improvements were popularized to something resembling what we do today. Before the Human Genome Project. Southern blots were still a relatively new idea.

Cytogentics classroom and lab sessions were still taught to learn various staining techniques to create banding patterns (q-banding, FISH, etc) to try to elucidate deletions and structural rearrangements were still part of the curriculum. Assembled karotypes too. Some cases were relatively easy to distinguish line of decent for a particular chromosome when you had a pericentric inversion which *might *be related to infertility in some cases, for example.

I suppose the young whippersnappers no longer have to learn that stuff.

[TheUser]

@Iggy

Right.

By convention the centromere, the compressed point of the X that you see in many karyotypeimages of diplotene chromosomes, is the point that determines the parentage of a chromosome. Even though each chromosome you get is a mixture of DNA from your grandparents’ X chromosomes, we say you received your maternal grandmother’s X if you get an X chromosome with a centromere derived from your maternal grandmother. This is so even though, due to crossovers, much of the chromosome you get may be copies of your maternal grandfather’s DNA.

The AR gene sits very close to the centromere of the X chromosome at Xq11-12. A crossover between the centromere and the site of the AR gene is highly unlikely simply due to their close proximity. So if you receive an X chromosome derived from your maternal grandfather (same centromeres) then it is highly likely you also got his allele at the AR locus too. These factors are said to be linked.

There is no particular favoritism, so a man is just as likely to have received an X chromosome derived from his maternal grandfather as on derived from his maternal grandmother. But whichever he gets he is likely to also get that grandparent’s AR allele as well.

Very useful info, thanks for that.

Iggy, what do you make of recent discoveries pertaining to non-AR-related genes influencing male pattern baldness? One in particular concerns Chromosome 20. A few recent articles suggest that this would fall outside the sphere of the X-chromosome.

Male-pattern baldness susceptibility locus at 20p11

http://www.wsj.com/articles/SB122393553747430381

*Humans get one copy of chromosome 20 from their mother and one from the father. The gene variation for balding is neither dominant nor recessive, but additive… Men with one affected copy were 3.7 times as likely to show early hair loss, and those with two copies were 6.1 times as likely.

The McGill study calculated that about one in seven Caucasian men have both the chromosome 20 variation and the AR gene, which increases their risk of early baldness sevenfold.*

Do you know anything about how common this Chromosome 20 gene is? 1 in 7 men have both the AR risk and this one, but how many men just possess this chromosome in general? And I would assume it’s non-androgen-related?

[Nava]

@Riemann

The OP did not originally say this, but it has since become apparent that OP is considering an X-linked male phenotype.

In the OP he says

How is that possible – that would mean that the son is totally unrelated to 50% of his maternal tree?

My response referred to that line. There is more than one chromosome, and more than one type of DNA, and the DNA that gets inherited from your ancestors is the whole pile of DNA, not just the sex pair. Not having inherited your maternal grandmother’s X-chromosome doesn’t mean you’re “genetically unrelated” to her.