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Sex cell?

  1. Sep 4, 2007 #1
    Sex cell???

    Quick question:

    Does every sperm cell in a male testicle contain the same genes, or can some sperm cells contain different genes, even though they are from the same person?
  2. jcsd
  3. Sep 4, 2007 #2


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    Well, clearly the sperm carry either X or Y chromosome, and they come from the same man, so it stands to reason the other chromosomes (sets of genes) can have different representations/combinations of genes.
  4. Sep 4, 2007 #3


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    Every sperm is different. During the process of meiosis (specialized cell division), a process called recombination occurs. This process randomizes the genetic content in each resulting sperm cell.

    - Warren
  5. Sep 5, 2007 #4
    What is the reason, genetically, that a brother or sister is only 1/2 related to you?

    This seems strange if you are both the product of the same parents.
  6. Sep 5, 2007 #5


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    If one is refering to half-brother or half-sister, then that means sharing one parent in common, i.e. either same mother or same father, but not both.
  7. Sep 5, 2007 #6

    If you have the same parents, you are not 1/2 related.

    You have 23 sets of 2 chromosomes (1 set from your father and 1 from your mother). Crossover will produce variation amongst the genes contained within the gamete, however, that doesn't mean you have "different" genes. We all have the same genes. Although they may vary slightly from individual to individual, they still are the same genes with the same basic functions. The difference mainly lies in how these genes are expressed.
  8. Sep 5, 2007 #7
    Strange. I'm reading a book by Richard Dawkins, The Selfish Gene, and in many instances, he states that brothers and sisters are only 1/2 related....???
  9. Sep 5, 2007 #8
    A good way to understand how so many different results can come out of only two parents, is to go back to the basics (historically) and look into Mendel's pea-plant experiment.

    Once you understand the basic process of meiosis, and apply some basic combinatorics to it, it should be pretty clear. I learned this through this textbook I have that has diagrams similar to this one:

    EDIT: this is a better picture

    The big letter are dominant alleles, and the small letters are recessive alleles.
    also note the 3:1 ratio of the dominant (and that even though some pea plants will exhibit the dominant trait, they still carry the recessive, which is how some traits skip a generation).

    try and look for an article on his experiments and the principles he discovered that has similar diagrams; they really help.

    there are also other processes that go on such as genetic recombination, crossovers, and mutations that ensure the diversity of the final products.

    if you look into all these terms you should get a picture of why it's so unlikely that two, four, or even a thousand babies from the same parents would be the same.
    Last edited: Sep 5, 2007
  10. Sep 5, 2007 #9
    See this link for full story:


    Below is the important text showing what Dawkins means when he states that brothers and sisters are 50% (1/2) related:

    .....The 50% refers collectively how much "genetic material" you get from each parent. You get 50% of your genes from your mom and 50% of your genes from your dad.

    Essentially you have two copies of each gene—one copy from your mom and one copy from your dad. Here is the strange part, the copy that you get from your mom may or not be the same copy that your sibling gets from your mom.

    Remember each of your parents has two copies of most of their genes too. When the egg or sperm that made you got made, only one copy of each gene was put in.

    The copy that gets put in is chosen randomly through a process called meiosis. What this means is that you have a 50% chance of getting one of their two copies.

    That probability doesn't seem impressive until you consider that you have around 25,000 genes. Throw in a 50% chance of getting one copy versus your sibling getting another copy and that makes meiosis a serious gene scrambler.

    So, because of this scrambling you and your siblings are 50% genetically identical....
  11. Sep 6, 2007 #10


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    Yes, he's referring to each sibling being half related to each parent, not to each other. Dawkins is discussing population genetics, not individual genetics, and probability of any particular genetic allele being passed from generation to generation.
  12. Sep 7, 2007 #11


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    Siblings are also 50% related to each other. Considering only somatic alleles (sex chromosome alleles bias the result, but the overall contribution is small), each sib is 50% related to a single parent. On average, the alleles that one sib, the other and one parent have in common is 25%. Since there are two parents, the overall similarity between the sibs from the contributions of both parents is 50%.

    Francis Galton came up with simple probabilistic rules to estimate hereditary relationships, and they are applicable in elementary fashion to even the most complex family trees. Of course, all this assumes the parents are unrelated.
  13. Sep 8, 2007 #12


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    Maybe on average in the whole population, but the problem I have with this idea is that the variation around the mean would be pretty large. Afterall, identical twins are siblings, and 100% genetically related, and you could have two siblings who managed to get the completely opposite set of genes from each parent with no genes in common (very improbable, but possible).
  14. Sep 9, 2007 #13


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    Of course. I'm referring to statistical population averages. In the case of a sib-parent relationship, the relatedness is always (almost) 50% (the "almost" is to discount the sex chromosome contribution and meiotic cross-over variation) at an *individual* level. In the case of a sib-sib relationship, the relatedness can vary from 0 to 100%, but it's 50% on *population* average.
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