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Thread: Breeding/selecting

  1. #11
    zappafan's Avatar
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    Feb 2003
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    Bobz, I've read that even if you self a plant it still allows for "genetic recombination",and therefore it varies somewhat from the original plant. Any comments about this are very appreciated.

  2. #12

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    Apr 2003
    Cambridge, MA, USA
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    Zappafan, what you are describing is indeed recombination. *It is due to the fact that organisms often (usually) have two copies of the "same" gene - one from each parent. *If these two copies are identical, the organism is said to be homozygous for that gene. *However, if the two copies are "different", then the organism is heterozygous for that gene, and when it produces gametes, there is a (theoretically) 50:50 chance that each offspring will get one or the other copy. *

    If you've heard of "dominant" or "recessive" traits, it is referring to the behaviour of the two copies of the same gene. *A dominant trait is one that is expressed in the organism even if there are two different versions of the gene, a recessive trait won't be expressed unless BOTH copies of the gene are the same.

    A good example of this is albanism. *Typically, this is a recessive trait/mutation, and a plant must be homozygous in the anthocyanin-free gene to be albino (a simplified case, of course).

    So, to get back to your original question. *Say the plant in question is heterozygous in three genes. *That means that there is a 0.5 x 0.5 x 0.5 (x 2) = 50% probability that the offspring will have the same genetic makeup as the parent. *Ok, i'm pretty sure i just did that probability wrong (always was a weakness), but you get the idea (i hope). *

    This is why it's important to have multiple clones of a plant in existence - to make sure that all the possible genes are represented. *Consider that once a gene becomes homozygous in an organism, there is no way its offspring can have a variation in that gene except through a random mutation.

    Is that clear?

    All of that is covered, in a rather more gradual manner, in most Jr. High Bio courses, but most of us (me included) forget until we hear it again in high school, college, etc. It's the Mendel and the peas experiments, if that rings a bell (no, that's pavlov [img]http://www.**********.com/iB_html/non-cgi/emoticons/wink.gif[/img] j/k).

    Here's a more graphic example:

    R = dominant gene for flower coloration (red flowers)
    r = recessive gene for flower coloration (colorless (white) flowers)

    Parent plant:
    Rr (one copy of each, heterozygous).

    If there are four offspring:

    RR Rr Rr rr ==> 50 % chance that offspring will have same gene makeup as parent.

    The above is a probability, and is of course subject to variation in real life and small samples.

    So, in this example, the parent had red flowers (R is dominant), one of the offspring had white flowers (homozygous for the recessive trait), and three had red flowers. Of those three, one would never give white-flowered offspring if selfed. In that way, genetic variation can be "lost" in small populations.
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