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  1. #1
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    Default Basic Reproductive biology question

    I've been wondering and researching without much success this question:

    Since a stallion continually creates millions and millions of sperm while a mare makes far fewer eggs, is there more genetic variation possible from the stallion in breeding overall?

    Now I know both the male and female get their potential gametocytes (?) as embryos, and that males have a process where the spermatogonia can reproduce by mitosis as well as meiosis to keep from running out over the male's lifetime, but does that mean that both male and female embryos have the same number of possible variations for lifetime breeding? Is each sperm in an ejaculation genetically different, or are many of them basically identical?

    Because when breeding a mare has a single (or maybe only one or two) eggs available at one time from her limited lifetime supply, while the stallion provides millions of possible hits from his limited lifetime supply. Breeding is already a gamble, but the stallion could be like winning a lottery with millions of entries, while the mare might be more like blackjack or roulette. Or does the mare also have millions of possible hits over her lifetime of breeding--depending on which of her oocytes mature to become eggs and make it to the breeding apparatus?

    Does anyone know the answer to this?
    Last edited by vineyridge; Jul. 27, 2010 at 12:29 PM.
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  2. #2
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    I don't have an answer, but it's a great question!
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  3. #3
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    Well siblings are only considered "half-siblings" if they are out of the same mare because there is less variation than if they were by the same stallion out of different mares. So, I would say you are correct.

    Some stallions are very "prepotent" and stamp their offspring strongly, but this is more rare than if you were looking at the produce of a single mare.
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  4. #4
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    It would seem that the sperm should have a numbers advantage in recombination variation because each time they are produced it is through meiosis, so I would think that because this happens continuously and the mare does it only once at the beginning of her life with limited numbers, that the sperm, by numbers, would have a higher degree of variation.
    But I think there is some mechanical realities to recombination that keeps some things together, more of the time as opposed to others. So I am sure there is some sort of bell curve of recombination with some combinations being more common than others.
    But as far as blackjack and the lottery, any one breeding only needs one of each, so statistically, the "other" sperm are as useful as the eggs the mare didn't produce. But of course if the stallion get thousands of mares in his lifetime, one will tend to see more range of the stallions genetics than a mare that can only produce a few offspring in a lifetime, and it is very possible that there was some of her genetics that was never passed on to an offspring.

    I would like to hear some other opinions too.



  5. #5
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    Quote Originally Posted by SmartAlex View Post
    Well siblings are only considered "half-siblings" if they are out of the same mare because there is less variation than if they were by the same stallion out of different mares.
    The association of sibling with the mare's side of the family greatly pre-dates the knowlege of genetics that you mention.

    One of the historic reasons siblings were only considered siblings - half or full - if they were out of the same mare was because pre-DNA, you KNEW what mare that foal came out of, but you only guessed at what stallion covered her (even if you saw a cover, you really didn't know that some other stallion hadn't also covered her).
    Hidden Echo Farm, Carlisle, PA -- home of JC palomino sire Canadian Kid (1990 - 2013) & AQHA sire Lark's Favorite, son of Rugged Lark.



  6. #6
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    Quote Originally Posted by KBEquine View Post
    The association of sibling with the mare's side of the family greatly pre-dates the knowlege of genetics that you mention.

    One of the historic reasons siblings were only considered siblings - half or full - if they were out of the same mare was because pre-DNA, you KNEW what mare that foal came out of, but you only guessed at what stallion covered her (even if you saw a cover, you really didn't know that some other stallion hadn't also covered her).
    This is so cool! It makes perfect sense!
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  7. #7
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    Ahhh yes, good explanation... Probably applies to humans as well
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  8. #8
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    PArtially a lot of this stems from the homozygousity or heterozygousity of the stallions genetics. Since literally hundreds of thousands of traits are contraolled genetically, only some of which are "visible" to us, we tend to focus on the visible traits that are being passed. The more homozygous genes the stallion carries for particular visible traits (as well as invisible ones like heridity of genetically caused illness) the more he will "stamp" his get and the less "variability" in his sperm he produces



  9. #9
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    Quote Originally Posted by SmartAlex View Post
    Ahhh yes, good explanation... Probably applies to humans as well
    Indeed it does.

    In the Jewish faith, the "Jewishness" (sorry, don't know how else to express it), is carried down from the motherline, not the fatherline.

    In other words, if the mother is a Jew, but the father is not, the child is still considered Jewish.

    I asked a good friend of mine why this is so (she's Jewish), and she gave the same answer: you KNOW who gives birth to this child and is therefore it's mother. Father is always speculative



  10. #10
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    Quote Originally Posted by Kyzteke View Post
    you KNOW who gives birth to this child and is therefore it's mother. Father is always speculative
    Except for my DD toddler who is sweet and cuddly. My family is convinced she was switched in the hospital....
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  11. #11
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    Is each sperm in an ejaculation genetically different, or are many of them basically identical?
    Each of them is slightly different, and this goes for eggs as well--the process of meiosis allows for reshuffling of certain parts of certain chromosomes, and theoretically every gene could be different, although of course the gigantic majority of them are not--just "dad" or "mom" replicated.

    Which is why although full siblings often have strong resemblances, they're not, in fact, the same. This goes for mom's genes as well as dad's.
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  12. #12
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    On a related note. Biologists have finally figured out why it takes millons of sperm to fertilize just one egg.

    Because none of them will stop and ask for directions.



  13. #13
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    Quote Originally Posted by Ravencrest_Camp View Post
    On a related note. Biologists have finally figured out why it takes millons of sperm to fertilize just one egg.

    Because none of them will stop and ask for directions.
    Too funny!
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  14. #14
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    Quote Originally Posted by vineyridge View Post
    Now I know both the male and female get their potential gametocytes (?) as embryos, and that males have a process where the spermatogonia can reproduce by mitosis as well as meiosis to keep from running out over the male's lifetime, but does that mean that both male and female embryos have the same number of possible variations for lifetime breeding? Is each sperm in an ejaculation genetically different, or are many of them basically identical?
    The above quote isn't scientifically accurate; males produce sperm throughout their lifetimes, but the sperm cells themselves don't reproduce. Females don't produce eggs; they are present in a certain number at birth and there will never be any more than that. Both are the result of meiosis, and crossing over (reshuffling of genes) occurs during the first phase of meiosis. Given that, it doesn't seem as though there would be any more genetic variation in sperm cells than in eggs, there are just lots more of them.



  15. #15
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    Quote Originally Posted by Kyzteke View Post
    Indeed it does.

    In the Jewish faith, the "Jewishness" (sorry, don't know how else to express it), is carried down from the motherline, not the fatherline.

    In other words, if the mother is a Jew, but the father is not, the child is still considered Jewish.

    I asked a good friend of mine why this is so (she's Jewish), and she gave the same answer: you KNOW who gives birth to this child and is therefore it's mother. Father is always speculative
    Interestingly, in the early Christian faith it is the father that decides the religion of the child. So, if the father was Christian the child was and it did not matter what the mother was.

    Which begs the question...what is the product of a Jewish mother and Christian father? More importantly...what is the product of a Christian mother and Jewish father? Crazy....



  16. #16
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    yes there are more sperm so likely more variation simply due to numbers. I don't know how many genes a horse has, but as its a random assortment of one of each of them the variety is going to be huge.

    However the randomness of any given sperm or egg is the same. A mares eggs are not going to be less variable in relation to each other. Random is random. But a stallion will have more.

    Now if he is homozygous at many loci and a mare has high heterozygosity then the stallion's sperm will have less variation than the mare's eggs.

    Realistically the number of haploid cells produced isn't a good indication of inheritable variability, heterozygosity is...



  17. #17
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    sper·ma·to·go·nium (-gō′nē əm)

    noun pl. spermatogonia -·nia (-nē ə)
    Zool. a primitive male germ cell which divides into spermatocytes or more spermatogonia
    Apparently this all happens in a seminiferous tubule during the male's lifetime.

    Quote Originally Posted by ynl063w View Post
    The above quote isn't scientifically accurate; males produce sperm throughout their lifetimes, but the sperm cells themselves don't reproduce. Females don't produce eggs; they are present in a certain number at birth and there will never be any more than that. Both are the result of meiosis, and crossing over (reshuffling of genes) occurs during the first phase of meiosis. Given that, it doesn't seem as though there would be any more genetic variation in sperm cells than in eggs, there are just lots more of them.
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  18. #18
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    Quote Originally Posted by vineyridge View Post
    Apparently this all happens in a seminiferous tubule during the male's lifetime.
    Yes. Spermatagonia undergo mitosis to produce either more spermatagonia or spermatocytes, both of which are diploid (2N). Spermatocytes undergo meiosis, ultimately giving rise to mature sperm cells, which are haploid (1N). And you are right – this process occurs throughout the male’s lifetime. However, mature sperm cells do not divide further, by mitosis or meiosis; that was the point I was trying to clear up. The process is similar for oocytes, but it all happens prior to birth – none of it happens after birth. In your original post, you indicated that the mature sperm cells reproduce by mitosis and meiosis, and that was the part that was not accurate. I was under the impression that you understood the process, but the terminology was a little misleading, that’s all!



  19. #19
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    Not worth arguing over, but I did specifically use the term "spermatogonia" in my original post.
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  20. #20
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    Quote Originally Posted by vineyridge View Post
    Not worth arguing over, but I did specifically use the term "spermatogonia" in my original post.
    I understand, but you also implied that sperm cells reproduce through mitosis, and that's not correct. Spermatogonia are not mature sperm cells - they are primordial germ cells and the two are very different. I apologize; I wasn't trying to start an argument. As a scientist, I was just trying to clear up some foggy terminology I noticed in your original post. Carry on!

    I looked back at your original post and wanted to change my response, but rather than edit my words above, I'll just add here:

    You did indeed use the term spermatogonia - I was wrong in saying that you didn't - however, spermaogonia only undergo mitosis, not meiosis. And ultimately, the point I was trying to make is that all primordial germ cells (whether they give rise to spermatozoa or oocytes) undergo similar rounds of mitosis/meiosis in their development towards mature germ cells that it doesn't seem likely that the amount of genetic variation in one compared to the other would be terribly different. And I think that is what you were asking to begin with. Sorry for the confusion!
    Last edited by ynl063w; Jul. 29, 2010 at 10:17 PM.



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