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I don’t know if I’m ready to teach genetics again

I always wonder where they get their unwarranted confidence from. This person seems to have confuse the Y chromosome with the entire genetic complement, or something. I don’t want to have to untangle their thinking right now.

That’s not how DNA works. Your brother would have his father’s DNA and his mother’s DNA. You, as a girl, would have your mother’s DNA and your father’s mother’s DNA. You have only half of your father’s DNA. You do not have your father’s father’s DNA.
And this is why (if one’s father has any brothers) it’s difficult to prove a girl’s paternity. All the brothers would have the same X chromosome so any brother could be the father. It’s been pivotal in the Thomas Jefferson/Sally Hemmings controversies.

You don’t necessarily have the same X chromosome as your brother. It’s not hard to figure out a girl’s paternity because there are all these autosomes. I think someone got a vague hint of how sex is inherited and garbled everything up beyond that. But they still get to tell someone else they don’t understand how DNA works!

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Comments

  1. HidariMak says

    Poorly taught science classes, poorly remembered decades later, being filtered through the collective ignorance of their sub-classes of social media groups. Garbage In, Garbage Out.

  2. Reginald Selkirk says

    from an evolutionary viewpoint, 25 percent of a child’s genes come from each parent, about 6 percent from each grandparent, 1.5 percent from each great-grandparent, and so on.

    Michael Shermer in Why People Believe Weird Things, C 1997, hardcover first edition, second printing

  3. timmyson says

    I’m embarrassed to say despite a generally strong education, I’m weak here. A brother could have gotten either x chromosome from their mother, right?

    @2 don’t 50% of one’s genes come from each parent, and 25% from each grandparent? I don’t understand this snippet at all.

  5. Reginald Selkirk says

    @3,4 timmyson

    Rephrase: a brother could have gotten either of their mother’s x chromosomes.

    More likely, a brother would likely get an X chromosome that was a mixture of genes from his mother’s two X chromosomes, courtesy of recombination.

  6. Reginald Selkirk says

    @3 timmyson

    @2 don’t 50% of one’s genes come from each parent, and 25% from each grandparent? I don’t understand this snippet at all.

    Good for you, because it is horribly wrong.

  7. timmyson says

    I didn’t realize the genes got shuffled between chromosome pairs on the regular. Is that a thing for each pair of chromosomes? I’ve heard described “such and such gene on chromosome N” so I thought they were more or less stuck on a chromosome unless a mutation messed with it.

  8. Reginald Selkirk says

    @7
    Mitosis, Meiosis, and Fertilization

    Meiosis
    When egg and sperm form, they go through a special type of cell division called meiosis. One purpose of meiosis is to reduce the number of chromosomes by half. The other is to create genetic diversity.

    Meiosis begins like mitosis: the cell copies each chromosome. But unlike in mitosis, homologous chromosome pairs line up and exchange pieces-a process called recombination. Remember, homologous chromosomes have the same genes but with slight differences. Recombination increases genetic diversity by putting pieces of slightly different chromosomes together.

    Next, the newly recombined homologous chromosomes are divided into two daughter cells. Then the sister chromatids are pulled apart into a total of four reproductive cells. Each of these cells has one copy each of 23 chromosomes, all with a unique combination of gene variations.

    Recombination between homologous chromosomes during meiosis is normal and commonplace. The mother has two X chromosomes, so yes, they would be expected to recombine. So her children, whether boys or girls, would likely get an X chromosome from her that is a recombination of her 2 X chromosomes.

    Meanwhile, a father only has one X chromosome, so his daughters would probably get his X chromosome from him with no recombination.

  9. silvrhalide says

    @7 It’s called sister chromatid exchange. The sister chromatid exchange can act as a repair template for damage to a damaged or mutagenic chromatid–it provides a “good” copy or template to repair the damaged or “error” copy. It happens during cell division when the chromosomes double prior to the actual division (mitosis).
    https://en.wikipedia.org/wiki/Sister_chromatid_exchange

  10. Reginald Selkirk says

    @7

    Recombination of homologous chromosomes (autosomal chromosomes in either parent, and X chromosomes in the mother) is common. The likelihood that two alleles (gene variants) on the same chromosome will get inherited together is called linkage, and depends on the distance between the two genes. This is related to the possibility of a reconbination site between them.

    Lots of other things can happen too; nonhomologous recombination, gene duplication, point mutations, etc. and one could discuss the frequency of these events. Bu I don’t feel like regurgitating an entire textbook.

  13. petesh says

    Golly gosh, my sister’s face looks more like our dad’s did than mine does, even though I am male, and my face looks more like our mum’s. I suspect that my genitals look more like dad’s, though. Never really checked that, and he’s long gone.

  14. CompulsoryAccount7746, Sky Captain says

    Wikipedia – Sally Hemings

    Hemings was the third generation of women in her family to be impregnated by a free man during her enslavement and the second to be impregnated by the man she was enslaved to.
    […]
    In 1787, at 14, Hemings accompanied Jefferson’s daughter to Paris where they joined Thomas Jefferson. […] At some time during her 26 months in Paris, Jefferson is believed to have begun intimate relations with her.
    […]
    Multiple lines of evidence, including modern DNA analyses, indicate that Jefferson impregnated Hemings several times over the years they lived together on Jefferson’s Monticello estate, and historians now broadly agree that he was the father of her five children. […] her ability to consent is dubious given Jefferson’s near-complete control over Hemings as his property and the fact that she was between 14 and 16 years old when he began having sex with her, while he was in his 40s.

     
    The Wikipedia article describes the DNA testing controversy.

    Following renewed historical analysis in the late 20th century, the Thomas Jefferson Foundation empaneled a commission of scholars and scientists who worked with a 1998–1999 genealogical DNA test that found a match between the Jefferson male line and a descendant of Hemings’ youngest son, Eston Hemings. The Foundation’s panel concluded that Jefferson fathered Eston and likely her other five children as well.

    A rival society was then founded, the Thomas Jefferson Heritage Society, which commissioned another panel of scholars in 2001 that found that it had not been proven that Thomas Jefferson fathered Sally Hemings’ children; the panel, however, was unable to disprove that Thomas Jefferson had fathered her children.

    In 2018, the Thomas Jefferson Foundation […] affirmed that it was treating as a settled issue that Jefferson was the father of her known children.

     
    Here’s what the foundations each said.

    Thomas Jefferson Heritage Society – 2001 Commission Report (pdf, p7)

    The tests merely establish a strong possibility that Sally Hemmings’ youngest son, Eston, was fathered by one of the more than two dozen Jefferson men in Virginia at the time, seven of whom there is documentary evidence to believe may well have been at Montecello when Eston was conceived.

    Dr. Foster [who performed the tests] has readily acknowledged that the DNA tests do not suggest that Thomas Jefferson was Eston’s father as opposed to someone like his younger brother Randolph or one of Randolph’s sons.
    […]
    These tests compared nineteen markers on the Y chromosome of fourteen individuals: five living male-line-descendants of two sons of Thomas Jefferson’s paternal uncle, who was assumed to have the same Y chromosome as Jefferson’s father and thus Jefferson himself; three male-line-descendants of three sons of the paternal grandfather of Peter and Samuel Carr; five male-line-descendants of two sons of Thomas Woodson; and one male-line-descendant of Eston Hemings. The results showed a match between the haplotypes of the Jefferson descendants and the Eston Hemings descendant, but no other matches.

    In plain words, they showed that a descendant of one of Sally Heming’s children carries Jefferson genetic markers, not those of Carr brothers, which effectively rules out possible paternity Sally Hemings’ youngest child by any of the Carr brothers and points to some male Jefferson as his likely father. As we discuss below, the circumstantial case against some of Thomas Jefferson’s relatives appears significantly stronger than the case against him.

    Thomas Jefferson Foundation – 2018 Statement

    The DNA study did prove paternity of a Jefferson family member and corroborated the ample documentary and oral history evidence. […] The summary of the most important evidence proving Jefferson’s paternity is listed below.
    […]
    While there are some who disagree, the Foundation’s scholarly advisors and the larger community of academic historians who specialize in early American history have concurred for many years that the evidence is sufficiently strong to state that Thomas Jefferson fathered at least six children with Sally Hemings. […] While it remains possible, though increasingly unlikely, that a more comprehensive documentary and genetic assemblage of evidence could emerge to support a different conclusion, no plausible alternative with the same array of evidence has surfaced in two decades.

  15. roughcanuk says

    They must of heard this from TikTok. I understand that it is the repository of all true knowledge.

  16. Reginald Selkirk says

    @14 CompulsoryAccount7746, Sky Captain

    These tests compared nineteen markers on the Y chromosome of fourteen individuals:…

    So they only looked at one chromosome, and they only looked at a discrete number of ‘markers’ – they didn’t sequence the entire thing. With today’s technology it is possible to do much better, and not that much genetic material is needed.

  18. nomdeplume says

    “Marjorie Taylor Greene tells Bill Maher she believes extraterrestrials are demons” – another step on the downward spiral.

  20. Tethys says

    Yesh, it’s obvious this dude is conflating the Y chromosome with DNA in general. It’s also obvious that he doesn’t understand that men pass 50% of their DNA to any offspring, but only the males will carry his Y chromosome. There are 22 other chromosomes worth of DNA which can be recombined and passed to offspring, only one carries the DNA that determines sex.

    You do get 50% of your DNA from each parent, but you don’t necessarily get 25% of each grandparent because your parents recombined their genomes before passing them to offspring. Other than the one sex linked chromosome, each child receives an entirely random 50% of their parent’s DNA. IIRC, you can carry variable amounts of each grandparents genome between 22% -28%. It becomes more uneven with each generation.

    Genealogists use a unit called a centimorgan to measure genetic linkage. Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.

    the physical chromosomal distance corresponding to one centimorgan varies from place to place in the genome, and also varies between males and females since recombination during gamete formation in females is significantly more frequent than in males. Kong et al. calculated that the female genome is 4460 cM long, while the male genome is only 2590 cM long.

    X chromosomes are physically larger than Y chromosomes. Everyone inherits their mtDNA exclusively from their mother, and males inherit their Y chromosomes exclusively from their fathers, but those are but a small fraction of the entire genome.

    https://en.wikipedia.org/wiki/Centimorgan

  21. raven says

    “Marjorie Taylor Greene tells Bill Maher she believes extraterrestrials are demons” – another step on the downward spiral.

    You have to deduct points from her because she didn’t even think this delusion up.

    It is a very old and common fundie xian belief.
    UFOs exist and are piloted by demons.

    Ufologist John Keel and some Christian denominations like Jehovah’s Witnesses associate UFOs with demons.

    It is a common belief among Jehovah’s Witnesses and widespread among the loonier fringes of xian fundamentalism.

    It’s polykookery.
    If you believe in a lot of nonfactual and counterfactual ideas, you might as well collect them all.

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