#finally a use for my 4/5 of an anthro minor 🙏

[wrizard]

wrizard's super basic guide to y-chromosome-based identification!!

for those interested, on this fitzcovery day:

a dear friend asked me to explain why i felt completely insane about the phrase "genetic distance of one" and, as usual, i got overexcited and wrote an entire thing about it complete with goofy images! it's on twt HERE, but i figured it would also be nice to pop it up here also. SO. with the caveat that it has been many years since my last bio class and this is VERY OVERSIMPLIFIED. here's

Human DNA is grouped into chromosomes. We generally have TWO of each chromosome: 22 pairs (numbered 1-22), plus one pair of sex chromosome (typically either two X-chromosomes (XX), or one X-chromosome and one Y-chromosome (XY)). That's 23 pairs, or 46 chromosomes, in total.

When producing sex cells, matching chromosome pairs will RECOMBINE (swap bits of information) - eg. one Chromosome 4 will remix itself with the other Chromosome 4, making TWO UNIQUE C4s. When the cell splits into two sex cells, each sex cell will carry ONE unique C4.

That's sexual reproduction! Every new offspring is genetically unique - new combinations of traits pop up quickly, and if they improve reproductive fitness, can be passed on to future offspring. This allows for rapid adaptation and changes in a species over time.

But what about Y-chromosomes, which don’t have pairs? They can't recombine in the way paired chromosomes can - which means Y-chromosomes pretty much only change via mutation (errors in copying DNA). Mutation is VERY slow, especially compared to recombination.

This means that when an XY parent passes down their Y-chromosome to a child, chances are high that chromosome will have few, if any, changes – as opposed to X-chromosomes, which recombine in both XX parents and children, shuffling genetic information all over the place.

Due to this slow rate of change, Y-chromosomes can be more easily tracked through the generations than other human chromosomes. A Y-chromosome might be passed down nearly unchanged for hundreds of years from genetic father to genetic son.

GENETIC DISTANCE refers to the measurement of difference between two sets of DNA. The lower the genetic distance, the more closely related the two samples are likely to be. A genetic distance of 1 means the samples are close to identical.

Because we know how slowly Y-chromosomes change over time, we know that if the Y-chromosomes of two people have a low genetic distance, this implies that those people are paternally related – even if the two people live/lived hundreds of years apart.

In the case of Captain James Fitzjames, genetic data was extracted from a set of unidentified remains (a molar from a disarticulated mandible). 17 genetic markers from the molar’s Y-chromosome were compared to the Y-chromosome of a confirmed paternal relative of the Captain.

Those 17 markers were the same in both samples, giving the two Y-chromosomes a genetic distance of one – meaning, with the genetic information available, the living relative and the unidentified decedent are more than 2000 TIMES more likely to be paternally related than not.

EDIT: DOIP I MISREAD THE CHART 16 of 17 match, not all 17!!

Along with all the information we have from the historical record, the context of the remains, and this new comparative genetic analysis, we can safely conclude that this particular set of remains belong to Captain Fitzjames.

160 years isn't long in the grand scheme. Every identified set of remains is another reminder that these were people, not just a distant curiosity. It's humbling to remember not just that we have identified Cpt. Fitzjames, but that still, today, we have a genetic distance of one.

Photos and Y-chromosome comparison chart taken from Stephen, Fratpietro, and Park's paper "Identification of a senior officer from Sir John Franklin’s Northwest Passage expedition" from the Journal of Archaeological Science: https://www.sciencedirect.com/science/article/pii/S2352409X24003766?via%3Dihub

hope my nonsense is helpful and/or informative and/or at least made you smile!! if you like this sort of thing :) cheers doves