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Chromosomes and genes

  1. Jul 30, 2010 #1
    I'm not a biology student. So, I humbly request you to limit yourself to the basics, please do not indulge in infighting. If you ignore the request, you would be investing your efforts at the wrong place and perhaps it would be considered a pretentious show of knowledge.

    There are 23 pairs of chromosomes in a human body cell, 46 chromosomes in total. All cells have identical genes but themselves are different excluding the 23rd pair sex chromosome which has different set of genes.

    Female sex chromosome is called 'Y' and male 'X'. If the combination is:
    XX = female
    YY = male
    XY = YX = ?

    1: What would be the result of 'YX' combination?

    2: How many genes are there per human cell? I read somewhere it's close to 35,000. Is this number for 23 pairs or just 23 chromosomes?

    3: Are the genes of all humans the same? I think they aren't - if they were, the humans would look alike. It's a silly question, probably. Perhaps, my question is an inquiry on something else. If humans started from the same, then why is the pool full of so many different genes?
  2. jcsd
  3. Jul 30, 2010 #2


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    1. XX is female and XY is male. Therefore, the YY combination is impossible.

    2. Yes, the number of human genes is somewhere around 35,000. This counts only genes on the 23 different chromosomes.

    3. For the most part, almost all of our genes are nearly identical. When you think of the basics of what our genomes must do, we are all very similar. We all have basically the same metabolic pathways, the same types of cellular components, and the same body plan. Of course, some genes are different between individuals, which leads to different traits such as hair color, height, eye color, etc. But, in the greater scheme of things, these are differences only in the minor details of our biology.

    As for how all of the diversity arose, the answer is mutations. Our mechanisms for copying and repairing DNA are not perfect, so sometimes mutations can arise that change traits. If these changes are harmful, they are not likely to be passed on, but if the changes are neutral or beneficial, they will stick around in the population and contribute to our genetic diversity.

    In addition, geneticists are beginning to appreciate another source of genetic diversity called copy number variation. Sometimes, our replication machinery will duplicate whole segments of a chromosome, giving some individuals extra copies of certain genes.
  4. Jul 30, 2010 #3
    Hi jackson6612, I don’t know if you are really interested in book recommendations - I know that you did say that you are not a native English speaker, but your command of English does seem to well within that needed foir the books I would like to mention. Like you I am not a biology student or a life sciences professional, but I wanted some grasp of these kind of issues. There are, of course many excellent books about this subject. But the ones in particular I wanted to mention to you are those by a guy called Sean B Carroll. He is a microbiologist and research scientist working at an American university. He is also quite a charismatic figure with a real talent for putting his subject across in a deeply compelling way. His books are very accessible and highly readable, and if you read them, I guarantee you will emerge with a much better grasp of the issues you raise in your questions on this thread. Anyway, the two books to look for are:

    Endless Forms Most Beautiful
    (a title that is a wonderful reference from Darwin himself)


    The Making of the Fittest
  5. Jul 30, 2010 #4
    Hi Drasil

    That means the genes responsible for male characteristics possess more influence. Why is so?

    Genome: Life is specified by genomes. Every organism, including humans, has a genome that contains all of the biological information needed to build and maintain a living example of that organism. The biological information contained in a genome is encoded in its deoxyribonucleic acid (DNA) and is divided into discrete units called genes.

    Does this mean the genes as whole are called a genome?

    Are you trying to say that the genes of all of us (human) are nearly identical?

    Ken, thank you for the recommendations. Someday I would have a look on those books.
  6. Jul 30, 2010 #5


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    By default, we develop as females. The presence of a Y chromosome changes our developmental plan to that of a male.

    Note that this is nothing special to maleness. Birds have Z and W sex chromosomes. Males are ZZ and females are ZW.

    Yes, sorry for not defining the term genome. I speak to other biologists so much that sometimes I forget which terms are English and which terms are biospeak. The term genome refers to our entire collection of DNA (not just the genes, but the other regions of DNA that regulate the genes).

  7. Jul 30, 2010 #6
    Hi again

    Thank you. I appreciate the fact that you explained the stuff in layman terms so it can be understood easily. I would post any further questions once I've absorbed all of what you have said.

  8. Aug 10, 2010 #7


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    Also to add a little more for you, on top of what Ygg has written. The reason why "YY" combinations are not possible, is because the "X" chromosome carries more than just "female" genes. Which means having at least one X chromosome is necessary for a human to live. While the Y chromosome contains information on masculinizing (making more "man like") the embryo.

    To your number 2;

    All cells have (essentially) the same DNA. Different types of cells (like liver cells, or brain cells) express different parts of DNA. That simply means, some parts of the DNA are "turned on or turned off". Which parts are "on or off" depends on the type of cell and the stage of growth/life it is in.

    Having only 35,000 genes (which are the directions for specific proteins) may not seem like enough, and our bodies (in fact, all animal bodies) get around this problem (less genes than proteins) by doing something called post-translational modifications. Which sounds like one of those big scary biology words, but isn't. It simply means our cells often alter proteins after their made, which means one protein can take on many different jobs.

    On your last question;

    We all have the same "genes" or spots on our DNA which code for a certain type of protein. What differs is the "flavor" of genes. We call these different gene "flavors" alleles.

    Think about it like ice cream (yum). You could have different freezers filled with ice cream. And in each freezer the ice cream cartons are in the same spots, shelf 1, spot 1, etc. This is like our chromosomes (the freezers) and our genes (the cartons). In each carton though, can be different flavors of ice cream. Maybe in one freezer shelf 1, spot 1 is chocolate, while in freezer two its strawberry.

    We all share the same alleles for many genes, because the jobs of those genes is so important that any variation would be a bad thing, and result in the termination of an embryo.

    As Ygg pointed out though, some traits we have, aren't so critical so variations can persist in our populations.
    Last edited: Aug 10, 2010
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