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Genes - Why are Brothers different?

  1. Nov 6, 2012 #1
    Genes - Why are Brothers different?

    This question might seem childish but i still don't understand it.

    Why are two Brothers different. Children get 23 chromosomes from their father, and 23 chromosomes from their mother. So each child has 46 chromosomes, 23 pairs, half from the mother and half from the father.

    So why aren't brothers identical? After all, each male child should have 23 chromosomes from his father and 23 from his mother? So what makes brothers different from each other? If i DNA test two brothers, would they not have exactly the same 46 chromosomes?

  2. jcsd
  3. Nov 6, 2012 #2


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    In your case you have brothers that have inherited chromosomes from the mother and from the father, forming 23 pairs. Now what happens when one of the brothers has offspring? He must pass on 1 chromosome of each pair: either the chromosome of his mother, or the chromosome of his father. There is the difference. For each pair either the maternal or the paternal chromosome is passed on, allowing many different combinations to form.

    In addition there is meiotic recombination, ensuring that the genetic material between the pairs of chromosomes is shuffled creating unique genetic material in each egg/sperm.
  4. Nov 6, 2012 #3
    Thanks Monique,
    I googled some of the words in your post and i have been reading this Wikipedia page;

    So two brothers will always end up with different DNA, no matter how many children a couple have.

    Ok, i'm getting a clearer picture now. This topic is in fact very complex, its not simple.
    Thanks Monique,
  5. Nov 7, 2012 #4


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    Even without homologous recombination the number of possible combinations is very large.

    There are 23 chromosomes, of which there is a pair each: that would mean 2^23 = 8388608 possible unique egg/sperm. Now if the egg and sperm combine into an embryo there are 2^23 x 2^23 possible unique embryos = 70368744177664 (70 trillion).
  6. Nov 7, 2012 #5
    I have to admit that i still don't fully understand the biological mechanism. But the proof is in the pudding. Brothers never turn out as identical clones unless the mother actually has identical twins. Brothers are always genetically different.

    I'm just going to have to do more reading on this topic,
    Thank you Monique, thanks for the help,
  7. Nov 7, 2012 #6


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    What biological mechanism do you not understand? Chromosomes come in pairs (homologous, but not identical) and a parent can only pass down one copy of each.

    Maybe you should read up on mitosis and meiosis, the basic principles of cell division.
  8. Nov 7, 2012 #7


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    Something you might be missing John, is assuming that chromosomes separate together. Maybe you think that Dad's 23 chromosomes he passes on are either all the 23 from his mom or 23 from his dad. This isn't the case. For the most part, chromosomes separate independently.

    Law of independent assortment

    In fact, that whole page (Mendelian inheritance) would be useful reading for you.

    *Caveat, not all things are inherited in a Mendelian fashion, but for the novice of biology and genetics this is a good basic place to start.
  9. Nov 7, 2012 #8
    Monique and Bobze,
    Thanks for your help.

    I was very unprepared when i posted this message. Its as simple as this; I need to spend some time reading up on this topic if i want to understand it. So you have given me some very helpful tips to get started.

    Thanks guys,
  10. Nov 10, 2012 #9
    Many of the differences between “identical” twins originate in the placental environment. Although the twins are genetically the same, the zygotes become implanted in different places in the uterus. Even if the twins share a placenta, they have to be attached to the placenta in different positions. Therefore, the environment of the twin embryos has to start out slightly different.

    The environmental differences in different parts of the uterine wall may be small. However, they get amplified by the competition between the embryo for the resources supplied by the mother. One twin gets slightly “short changed” by the mother. The twin that starts out getting slightly more resources can draw out even more resources from the mother. Therefore, one twin may be slightly undernourished or underoxygenated than the other early in development. Therefore, one develops differently from the other. The developmental differences at birth can persist throughout their lives.

    There have been many studies of the effects of embryonic competition on the development of twins. Here are a few links on the subject.

    “The researchers found that epigenetic profiles vary significantly among both types of twins. They speculate that the differences arise partly randomly and partly from environmental factors, such as differences in blood supply, exposure to infection and the weight of each twin’s placenta.
    In an unexpected twist, identical twins who shared a placenta show greater differences between their epigenetic profiles than identical twins who did not, suggesting that competition for resources may play a role.”

    Identical triplets and identical quadruplets vary even more than identical twins because of this competition.
    “Of course, pregnancies with more than two babies have occurred throughout history. However, once the number of babies reaches three, overexpansion of a woman's uterus begins to cause difficulties. The implantation of several embryos and placentas in the endometrium of the uterus results in a competition for space and inevitably some implant in an area without good circulation. During a pregnancy, it is essential that the uterus be well perfused to sustain the fetus with nutrients and oxygen. A lack of oxygen can cause central nervous system damage in the fetuses that implanted in a less than desirable area. Since the human female was not made to carry an indefinite number of fetuses, multiple gestations can have many of the following complications:”

    The competition for placental resources can result in the ultimate difference between identical twins in humans and in animals. One twin can survive and the other die before birth. This study showed that this is quite common among horses.
    “Multiple ovulations tend to be more common in barren and maiden mares then in lactating mares… Most twin pregnancies will terminate, either by early foetal reabsorption or the birth of small retarded foals.
    Placental competition leads to insufficiency and usually results in abortion.”

    I don’t think this is as common in humans as in horses, but it does happen in humans. Elvis and Jesse come to mind.

    The amplification of differences due to competition is a special example of what physicists call spontaneous symmetry breaking. Spontaneous symmetry breaking describes any physical process where an initial difference between different physical states is amplified by a nonlinear interaction. The result is that the final states are far different even though the initial states had no apparent difference.
    “Spontaneous symmetry breaking is a mode of realization of symmetry breaking in a physical system, where the underlying laws are invariant under a symmetry transformation, but the system as a whole changes under such transformations, in contrast to explicit symmetry breaking. It is a spontaneous process by which a system in a symmetrical state ends up in an asymmetrical state. It thus describes systems where the equations of motion or the Lagrangian obey certain symmetries, but the lowest energy solutions do not exhibit that symmetry.
    Consider the bottom of an empty wine bottle, a symmetrical upward dome with a gutter for sediment. If a ball is placed at the peak of the dome, the situation is symmetrical with respect to rotating the wine bottle. But the ball may spontaneously break this symmetry and roll into the gutter, a point of lowest energy. The bottle and the ball retain their symmetry, but the system does not.”

    In the case of identical twins, the “underlying laws” are the genome of the twins. The genome of the twins are invariant to exchange of the individuals. The system consisting of the two twins is almost invariant to individual exchange, but initially there is a slight asymmetry due to uterine location. The competition between the twins is an interaction that amplifies the initial difference so that the “identical” twins can be noticeably different even from birth.

    Please note that competition between twins can continue after birth. Therefore, the twins can become more different as they grow up. This starts to be a free choice. The competition amplifies the difference whether or not the initial difference is prenatal.

    I once saw a Jeffrey Springer TV program where three sets of identical twins were interviewed. Although I generally avoid such sensationalist programs, the topic interested me enough to make an exception.

    One set of twins made a decision early in life to try to be as different from each other as they can possibly be. The other two sets of twins made a decision to be as similar to each other as they can possibly be. The twins that made the decision to be different dressed differently, went to different vacation spots, and tried to make their experiences as different as possible. The two sets of twins that decided to be as similar as possible married each other!
  11. Nov 10, 2012 #10
    Thanks for that information. Some good stuff there, and a few interesting links.

    Are Jeffrey Springer's TV shows the same kind of thing as Jerry Springer? Sounds like it.....LOL

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