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Genetic inheritance 101

  1. Nov 20, 2014 #1


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    I can't believe I'm asking a genetics 101 question, but I guess everything gets rusty after a while.

    'Someone close to me' is convinced that she has no genes from her paternal grandfather. Since he is the only Italian in her ancestry, she believes that, genetically, she has no Italian heritage.

    Her logic is thus:
    - she has two X chromosomes
    - that means she got an X from her father
    - her father has an X and Y, thus he must have gotten the X from his mother, which means he must have gotten his Y from his father.
    - since she has no Y, she did not get her paternal grandfather's genes.

    Which means she has no traits (eye colour, nose shape, risk factors, and racial heritage, etc.) inherited from her grandfather.

    SCTM is no fool in biology, and is actually better educated than I about it, but I think there's a gap in her picture of chromosomes.

    She knows the chromosomes divide and recombine during cell division, but it seems she believes they all do so together, meaning there's no way he genes could contain any from her paternal grandfather.

    I am trying to convince here that she is only describing the single sex chromosome, and that there are 22 other chromosomes, any number of which, from 0 to 22, she might have inherited from her paternal grandfather. I'm not 100% sure I'm right, or exactly where/how the genes/chromosomes get mixed.

    Can someone point me to a concise description or diagram that shows how chromosomes get mixed and recombined from one generation to the next?
    Last edited: Nov 20, 2014
  2. jcsd
  3. Nov 20, 2014 #2


    Staff: Mentor

  4. Nov 20, 2014 #3


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    In concise terms, this as well as the process called "crossing over" during gametogenesis means that nearly all genetic material is thoroughly mixed. Crossing over (it's also shown in jedishrfu's link) happens between all homologous chromosome pairs (i.e., all but the 23rd pair) during meiosis - the cell division process that takes a cell with paired chromosomes and outputs cells with just one set(sperm, ovum).

    Here's a diagram depicting the above (shown after initial duplication of chromosomes):
    http://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/PH709_DNA-Genetics/Crossing%20Over.png [Broken]
    Last edited by a moderator: May 7, 2017
  5. Nov 20, 2014 #4

    jim mcnamara

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    Simply stated: you have one half of one parent's DNA , one half the other parent's DNA. The same is true for the parent. So the DNA contribution from a grandparent to grandchild is 1/4 of his/her DNA. So for chromosome 1 -> 22, the one-fourth assertion is correct. There is crossing over during meiosis (gamete formation, haploid cells with 23 chromosome). This applies to sperm formation egg formation as well. Crossing over swaps DNA sequences across chromosomes, potentially creating new combinations of genes.

    Ain't no way she has zero paternal grandparent DNA unless she knows something you don't know about fidelity.

    Nice graphic Bandersnatch!
    Last edited: Nov 20, 2014
  6. Nov 28, 2014 #5
    As Bandersnatch and Jim said, your friend shares approximately 1/4 of each of her grandparents' genomes including the paternal grandfather. As you suspected, the sex chromosomes behave a bit differently. Your friend is female, so received one X from each parent. Her paternal X however came from his mother, so your friend did not receive (much) DNA from her paternal grandfather. (Actually not physical DNA, just the variant information encoded by his DNA, so calling it "his DNA" is a simplification too). The X chromosome is large but not compared to the entire set of autosomes, so your friend got plenty of DNA (that is, his genetic variation) from that grandfather. There is one correction to this statement however. To allow the X and Y chromosomes to pair during mitoses and meiosis, there is a small amount of genomic information including a handful of protein-coding genes that are shared by the X and Y. There is recombination in those segments, so it is possible for a female to receive a bit of genomic DNA from the paternal grandfather, from one of these two "pseudo-autosomal" regions. That is a small correction however to the main point, that 1/4 of autosomal genomic information is transmitted to your friend from her paternal grandfather.
  7. Nov 28, 2014 #6
    I think human genetic inheritance is more complex than that of peas or beans.
    There are dominant and recessive genes in humans too along with a lot of other unknown biological processes they experience to create their offspring with polymorphic traits.
  8. Nov 28, 2014 #7
    Actually many plants, especially those that have been domesticated, are polyploid, having more than two copies of their entire genome in each cell. Thus their genetics tends to be more complex than humans', not less!
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