I've been trying to do some research for a story with the working title "The Nancy Boys and the Case of the Confused Chromosome".
In this story, the Nance (aka Nancy) Boys are showing how DNA tests work for a science fair project. The results of their test fall into the hands of the school principal, and the boys wind up having to dress as girls until they can solve the mystery of how they both show two X chromosomes in their DNA's instead of an X and a Y.
The problem I'm running into is I know almost nothing about DNA. Does anyone know of any way a test could show two X's, even though the subject has an X and a Y? I thought about just having the test results switched the the tests from a female, but if there's any other ways the test could have some kind of error, I'd love to hear about it. :)
Maybe a chromosomal
Maybe a chromosomal abnormality that makes the y look like an x, and the analyst in a hurry mistakes it for an x especially since the y determining factor is absent.
DNA tests subject to interpretation
As noted below, DNA tests are subject to interpretation. I suggest that neither the principal nor the students are as adept at interpreting them as the folks in the lab cited below. Once the error was made, it simply becomes impossible for the armatures to correct the result.
As a final the test can be submitted to a lab blindly asking to determine the sex of the subjects.
From The Telegraph UK.
False DNA test led father to reject daughter
By Macer Hall
12:00AM GMT 11 Feb 2001
A DNA testing firm used by the Child Support Agency has admitted incorrectly telling a man that he was not the father of his daughter.
Cellmark Diagnostics, one of seven Government-approved laboratories performing up to 10,000 paternity tests each a year, says staff "misinterpreted" the results in the case. A laboratory error occurred despite the company's claim that its tests were "99.99 per cent" accurate.
Hugs
Patricia
Happiness is being all dressed up and HAVING some place to go.
Semper in femineo gerunt
Ich bin eine Mann
Umm
I can't be the only one who realizs that 99.99% is 1 error in 10,000 tests.
XX men
Nature will find a way! There is this...
http://en.wikipedia.org/wiki/XX_male_syndrome
There is also Klinefelter's Syndrome which is XXY, but that would require a mistake in reading the DNA results or sloppy testing.
http://www.dnatestingfaq.com/dna-testing-issues/dna-testing-...
The most obvious answer is simply a botched test or perhaps one where the results were misread or mixed up with another pair of twins with a similar names and/or ages.
So somewhere out there the Hardy Girls are most unhappy about their own situation? :)
Hugs
Grover
I was think of Klinefelter's
I was think of Klinefelter's Syndrome also.
There are XX "Males"
Google and Wiki are great resources if you want to keep it real.
http://en.wikipedia.org/wiki/XX_male_syndrome
Male and Female are obsolete designations as there are thousands of combinations in between. :)
Gwendolyn
Barr Bodies
An older chromosomal test for sex is the Barr Body test. Basically, since humans only use one X chromosome, people that have more than one (typically, 46,XX women) have one that's inactive. It stays inactive by staying clustered into a tight ball. If a cell sample is stained, and viewed under a microscope, this little lump of DNA can often be seen. This Barr Body test is fairly simple, and crude by modern standards. It's probably something that could be done entirely in a typical high school lab, unlike most DNA analysis. Searching for Barr Body test will give you all sorts of details. The reason it may be useful to you is that in at least one study, it was found to have a 4.9% false positive rate. That is, what appeared to be Barr Bodies were visible in samples taken from 4.9% of people with a 46,XY karyotype.
I Think
What is most important is the fact that genes are what determines what (but not who) we are. Chromosomes, like the XX or YX pair or other variations, contain genes, but just identifying chromosomes does NOT tell what genes are in that chromosome. Geneticists know what genes are usually on a given chromosome, but life always has variation.
The male XX syndrome in the wikipedia link happens when the X and Y of a usual male are pulled apart leading to the making of a sperm. All the chromosomes (which usually come in pairs, except the sex chromosomes) line up as a cell is going to split. Mostly and for all non sex-organ tissue, all chromosomes are doubled before a cell splits; when the split starts half the chromos are pulled into the nucleus of one of the cells and the other half of the chromos go to the other. This is called mitosis; having 2 of each chromo plus the sex chromo set is called a diploid number of chromo's.
When gametes are formed (in humyns these are eggs and sperm) only half the diploid number is needed (the haploid number), one of each pair, go into the new cell. When a sperm and egg fuse, both halves of the diploid number go into the fertilized egg, making the normal (diploid) number of chromos. The cell division where only half the chromos go into each new cell is called meiosis. During this, all chromo pairs line up and one of each pair is pulled into each new cell. As the cells line up, a leg (1 of 3 ) of a Y chromo may cross a leg (1 of 4) of the X chromo. These crossing legs could get tangled and as the chromo pair are pulled apart the tangled area might be stronger than other parts of the chromo. The legs break and the Y leg stays with the X chromo and vice versa. If the Y leg of the X contains the gene for maleness (called SRY) the embryo will develop into a male. Looking at the chromos (at the magnification of a light microscope) will not show that the X is unusual. Gene testing is needed to tell if the X has the SRY gene.
Hugs and Bright Blessings,
Renee
Maybe their test results
are tappered with, or fouled up by labratory incompetence.
May Your Light Forever Shine
Dramatic License
Hmm...
I'm busy drafting the first chapters of a futuristic story where an attempt to genetically modify the human race has gone slightly awry, and resulted in approximately 8% of the male population having their XY pairs of chromosomes turned into XX pairs at puberty or just after. Conveniently, the action takes place so many years afterwards that although everyone knows this is happening they've forgotten how to put it right.
What would we do without dramatic license?
Two kinds of tests
There are two kinds of tests that can distinguish an XX from an XY. The old style karyotype (see Wikipedia) is a very finicky laboratory procedure that requires catching a cell in the process of division, extracting and staining the chromosomes, and then sorting them by size and banding pattern. Since it identifies all 48 chromosomes, and since the X and Y look very different, it's very unlikely that there would be a mistake.
The modern way is to do some form of DNA sequencing that would identify features on each of the chromosomes. A commercial quality DNA sequence [u]probably[/u] won't miss a Y chromosome because the SRY (male sex-determining) gene is something that people usually want to know about, as well as the other male-specific genes on that chromosome.
I seriously doubt that a high school student would have the experience to do a karyotype. However, an upper-class high school might have a gene sequencer; the low end machines don't cost much more than a decent automobile and being able to do your own gene sequences could be a very nice perk for an advanced biology class.
I'd go with an error by a commercial lab, frankly. It makes my head hurt to figure out why a high school principal wouldn't know that anything their students do would probably have errors.
A simple...
There's a simple way for a guy to have XX chromosomes in their blood. (Well simple to tell.)
If you have a blood/bone marrow disease, and they replace your bone marrow... You can end up with one set of chromosomes in your blood and a different set in other parts of the body.
I suspect that a boy who had a blood transfusion could - for a while - have blood cells with XX chromosomes mixed in with his XY.
(The other case... XY and female. Does happen. even Fertile Females with XY. Even Fertile Females who have Kids including girls with XY, who have kids...)
Another possibility is an error. (There are such things as false positives and negatives.)
I'd be surprised to find any headmaster take functional boys and FORCE them to dress as girls for no reason other than the chromosomes... And, if it went as far as bathrooms, etc. and any of the girls parents found out about it... At a minimum said headmaster would go to jail, if the school didn't get shut down. But, that's just talking about the "real world" not what might happen in a story.
Annette
why go to all them
why go to all them lengths?
the simplest accident to show false results is swapped samples. happens all the time by mistake. could also be on purpose or a joke gone wrong.
Not DNA sequencer, probably
Xaltatun: Not DNA sequencer, probably (most top notch universities send their research or educational sequencing to commercial sequencing labs, high schools are much less likely to have one than a microbiology research university), but PCR and electrophoresis equipment, that they might have, though still not very likely.
I think you miss a couple sets of methods though - antibody marking in various forms of assays, as well as immunofluorescence microscopy. The latter can be used to detect chromosome anomalities; the former can be used for battery gene analysis, or even gene expression (mRNA or Protein) analysis, without having to go to the expense of sequencing it all.
PCR with gel electrophoresis can be used to clone and compare DNA fragments, or gel diffusion can be used, or any of a number of different visualisation techniques. Crude (you really want to do bulk comparisons of high number of different fragments rather than a few per gel), but could be used.
Mosaic
I believe a mosaic would work. That's where different cells actually have slightly different DNA, because of mutations. One part of the body could have cells that show up with XY as a mutation with the majority of the cells having XX.
I've done a little research at a library at the University of British Columbia. They have a good section on it.
- Terry
Awesome!
Thank you for all the awesome feedback! This is some excellent gris for my story writing mill. :)
Contamination in the lab. A
Contamination in the lab.
A joke played by lab.
Ummm...
...at the risk of being out of place, I thought of a few reasons not mentioned. Acts of war, God and nature cannot be precluded. Such things as excess radiation from the plant melting down across the street. Tornados (Heaven forbid) or Hurricanes...but not in basketball this year. Misplacement due to the use of samples during a penny poker game. A tech pipettes too much chloroform and falls off his stool exposing the fact that he is wearing panty hose. This creates such a ruckus in the lab that a typo is suddenly showing not only an extra 'Y' but two extra 'C's' and a '3' in the middle of Nance's name. Or...the test is outsourced to a friendly Middle Eastern country that suddenly goes into revolt belatedly joining the Arab Spring movement. During the ensuing blood bath (I prefer a lightly scented oil with maybe rose petals...sometimes milk but it does give me the whimsies)the slides become mixed up with a batch of HepC slides sent in from Hamburg. The DNA machine misreads the HepC as XYY creating a real scare in the billing department. Just a few extra thoughts. :)
Okay...so its seven in the morn and I've had a really poopie shit of a sleepless night AND the pint I had earlier has given me acid reflux from hell. Mom's not around and my dear Kevin is sleeping off his premier solo singing appearance at a new local pub.
May You All Have A Blessed and Blissful Day...
Just another faaaaaaaaaaaaaahhhhhhhhhhhhkn Irish...
Brrrrrrrrrrrrrrrrrat