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  1. #91
    Registered User Tsanford's Avatar
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    Re: A Lesson in Basic Genetics

    Think of it as a ladder. Each rung devided in half.

    Sent from my SM-G900T using Tapatalk

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    Alright I made this picture using paint (please excuse my poor artistic skills) of what I'm talking about. Tell me if this is right because this is how I imagine a DNA helix would look. If this is correct then idk how OP's illustrations could be correct also.

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    This isn't a scientifically accurate guide at all, it is information to understand inheritance, the DNA structure plays very little role in what we deal with, within the hobby. However each shoestring in her example should be stated to represent both sides of the DNA, not just one.

  4. #94
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    Re: A Lesson in Basic Genetics

    Quote Originally Posted by Mysnakeislong View Post
    Hi, I just finished reading your thread and I have to say it really is awesome and has taught me A LOT but there is one thing that I'm not sure is correct. Ok so I'm confused by the 2 strands of DNA you use to represent each morph. Wouldn't it be better to represent the DNA for each morph as a helix instead of 2 strands? I thought that DNA was a helix and if it is then i'm not sure if your illustration is correct. The reason I say this is because if you combine the 2 strands the way you visualized them then it won't create a regular DNA helix. Realistically one strand would only have one gene per allele, it is only when you combine the other strand that you get the second parent gene which forms the helix. With your illustration it looks like you are trying to represent one strand as an entire DNA helix because you have the 2 gene dots which would only happen with a fully formed helix. Idk if I'm right about all this or if i'm just totally confused. Anyways, I would appreciate if you got back to me on it just so I can make sure I'm understanding the basics correctly. Thanks again for the awesome guide.

    -Sean
    You are right; that illustration is not correct. IMO, this is less pretty but more correct.

    Pair of homologous chromosomes
    -o-o-A-o-o-o-B-o-o-o- = the double helix of DNA in one chromosome. Each o represents a gene.

    -o-o-a-o-o-o-b-o-o-o- = the double helix of DNA in the other chromosome. Each o represents a gene.

    By the way, alleles have different DNA sequences but the same location in a chromosome. In my pair of chromosomes, A and a are alleles because they have the same location. B and b are also alleles. A and b are not alleles, and neither are a and B. A, a, B, and b are all genes.

    Link: http://ghr.nlm.nih.gov/handbook
    The link is to the Genetics Home Reference, which can be freely downloaded. The first chapter has material about genes and chromosomes and DNA. It may be helpful. By the way, the Home Reference shows a chromosome as X-shaped because that is the stage that is visible. Each X-shaped chromosome is dividing and will eventually become 2 rod-shaped chromosomes before unwinding into an invisible thread.
    Last edited by paulh; 08-12-2015 at 04:58 PM.

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    Re: A Lesson in Basic Genetics

    Images of a DNA double helix.

    http://photobucket.com/images/dna

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    Re: A Lesson in Basic Genetics

    yussss

  9. #97
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    Re: A Lesson in Basic Genetics

    Here's a resource to get your feet wet in genetics. The video helps put things into context.

    http://learn.genetics.utah.edu/

  10. #98
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    I didn't read back 10 pages, but I hope someone corrected the codominant thing, since there are no codominant ball python mutations that we know of.
    We work with incomplete dominant traits, VERY different from codominant traits.

    Incomplete dominance is a blending of traits.
    When you make a pastel mojave the two traits blend to make a yellowed out mojave.
    If these two mutations were codominant parts of the snake would look like a pastel and parts would look like a mojave. They wouldn't blend together like incomplete dominant traits.

    Jerry
    Jerry Robertson

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  12. #99
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    Different texts have different definitions of codominant and incomplete dominant. I know of three that are more or less accepted.

    The oldest definition is that in intermediate dominance, the heterozygote is more or less intermediate in appearance between the two homozygotes. For example, in many flowers the heterozygote produces pink flowers, one homozygote has red flowers, and the other homozygote has white flowers. In codominance, the heterozygote has some areas that look like one homozygote and other areas that look like the other homozygote. For example, the human A blood type, B blood type, and AB blood type.

    If we use the traditional definition, we also need to add a third category--overdominance. The heterozygote's appearance is outside the range of the two homozygotes.

    Here is the most recent definition that I have seen. In incomplete dominance, one allele has a functional product and the other allele does not. This produces a blend--pink flowers instead of white or red flowers. In codominance, both alleles produce functional products. If the test is sensitive enough, both alleles can be detected. For example, in the human AB blood type, both A and B antigens occur on the surface of the same red blood cell and can be detected. In less sensitive tests, a blending can occur as in the Tonkinese cat. Both the Burmese and Siamese alleles produce melanin, but different amounts. The Tonkinese cat has one of each allele and produces an amount of melanin roughly intermediate between that of the Burmese and Siamese cats.

    The third definition basically says that nobody has figured out whether only one or both alleles have a functional product. Until that is figured out, call the two alleles codominant to each other, because "codominant" requires fewer keystrokes to type than "incomplete dominant". And define codominance as each of the three possible genotypes producing its own characteristic appearance.

    IMO, a five part classification scheme (dominant, recessive, incomplete dominant, codominant, overdominant) is too inconvenient for newbies and most breeders.

    Does the pastel situation parallel the Burmese cat or the pink flower? I don't know, and I don't think that anyone knows. So the second definition cannot be used with accuracy. And do breeders care about the number of functional gene products? I've bred a lot of birds, and the main thing I wanted to know was the identity of the bird's genes.

    That leaves the third definition. There was a paper in Bioscience magazine in 1995 (Three neglected advances in classical genetics) where the author stated he had used that definition successfully in introductory genetics courses.

  13. #100
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    Re: A Lesson in Basic Genetics

    There are no codominant traits in ball pythons.

    Pastel + mojave makes pastave where the two colors blend together.
    If they were codominant there would be parts of the snake looking like a pastel and parts looking like a mojave.
    Jerry Robertson

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