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Re: crossover

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  • Royan Webb
    ... I spoke with a Dr. of microbiology over the weekend and she stated that yes, chasma CAN form anywhere. She s also looking to see if she has time to
    Message 1 of 11 , Nov 1, 1999
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      A budgie told me lovebirds said:
      > From: "lovebirds" <chris@...>

      > Terry, Clive, Inte, Ralph et al.

      > From a purely chemical point of view, I cannot see why chiasma cannot form
      > at any random point.

      > (and as a long time Lovebird breeder, I don't place a lot of faith in
      > D'Angieri in several areas).

      > What is being discussed here is so valuable Terry, I think sometime soon we
      > need to recruit a professional Microbiologist/Molecular Geneticist for us.
      > Someone with no knowledge of avian genetics might in someways be more useful
      > than one with preconceived ideas. Can any of our members dragoon someone to
      > guide us?

      I spoke with a Dr. of microbiology over the weekend and she stated that yes,
      chasma CAN form anywhere. She's also looking to see if she has time to
      actually join the list.

      _ _
      (') (`} Email mailto:bandr@...
      //) / ) ICQ:14256691 nick:MLO
      " / / Theropod Aviary
      //" http://www.globaldialog.com/AdventureCentral/aviary/

      And the budgie says:
      'He is not an honest man who has burned his tongue and does not tell the
      company that the soup is hot.'' - Yugoslav proverb
    • Jaynee Salan
      From: Jaynee Salan Pieds1usa@hotmail.com Terry and All - I would like to jump into this discussion, if you don t mind. The research team I work with has
      Message 2 of 11 , Nov 1, 1999
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        From: Jaynee Salan Pieds1usa@...

        Terry and All -

        I would like to jump into this discussion, if you don't mind. The research
        team I work with has found that Chiasmata formation can
        occur anywhere along the DNA chain and can lead to crossovers at any
        point. Mutations occur because of additions, deletions and/or
        substitutions of base pairs. Let me explain further for the benefit of
        everyone that would like to read this post.

        Homologous chromosomes come together as pairs during prophase of Meiosis 1.
        This pairing, or synapsis, is very precise, with homologues aligning with
        each other on a gene-by-gene basis. During synapsis, genetic information is
        often exchanged between the paired homologous chromosomes. A segment of a
        chromatid changes places with
        the equivalent segment of a homologue. Crossing over is usually a
        reciprocal process in which identical lengths of chromatids are exchanged
        between the two homologues. Therefore, there is no loss of
        genes, but rather an exchange of genes between the two chromosomes.

        Although no genes are lost, new genetic combinations nevertheless
        result from the exchange of maternal and paternal chromosome segments.

        Crossing over occurs with the formation of a chiasma. Let me explain
        a little further. The four closely associated chromatids are called tetrads
        or bivalents. Exchange of genetic material, or crossing over, occurs
        between nonsister (homologous) chromatids; this event is visible in the
        microscope by the appearance of X-shaped regions called chiasmata (singular,
        chiasma). The long arms, we call them, of the X-shaped figure that we see
        in the microscope are referred to as chromatids with the union in the middle
        of the chiasma referred to as the centromere.

        The recombinant frequency that you have mentioned we call Chromosome
        Mapping, the ordering and spacing of genes on a chromosome on the
        basis of crossover frequencies. In the construction of such maps, crossover
        frequency becomes the map distance. Frequency of recombination between two
        genes is a measure of how far the markers are from each other along the
        chromosome. Genes located very close to each other would be less likely to
        become separated by a crossover event between them than would genes that are
        far apart. For example, if alleles A and B appear in the off spring in
        their parental combination 85% of the time and as recombinants 15% of the
        time, we would conclude that the two genes are linked (are on the same
        chromosome) and are 15 map units apart.

        Although this approach is widely used today to map the chromosomes of
        many animal species, the information about gene loci determined from
        crossover data is relative, not absolute. Recombination frequencies tell us
        the sequence of linked genes and provide clues about
        comparative distances between them, but this approach discloses neither the
        actual locations of the genes on the chromosome nor the absolute distances
        between the genes (in nanometers, for instance).

        Clive - In your post on Oct. 28th you mentioned long stretches of DNA
        with no apparent function. We refer to this phenomena as INTRONS and

        Much of the DNA of most organisms does not code for protein. The noncoding
        segments of DNA have given us one of the most surprising and
        important dicoveries in recent biological research. Such unexpected
        interruptions are puzzling. It is as if unintelligible sequences of letters
        were randomly interspersed in an otherwise intelligibly written document.
        Most of the more complex genes are interrupted by
        long segments of such noncoding regions, which are called intervening
        sequences or INTRONS. The coding regions, which introns interrupt, have
        come to be called EXONS because they are expressed (translated).

        After an RNA transcript is made, the introns are removed and the exons that
        flanked them are joined to produce an mRNA molecule with a continuous coding
        sequence. This process is called RNA Splicing or
        RNA Processing and is also required for the production of tRNA
        (transfer RNA) and rRNA (ribosomal RNA). In fact, we can tell if a
        gene is active when we look at a sequence of DNA in a microscope because it
        puffs itself up, due to the strong chemical bonds present. We refer to these
        active genes as "puff" genes.
      • Brett Doran & Sharon House
        Inte Wrote:
        Message 3 of 11 , Nov 2, 1999
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          Inte Wrote:

          <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Can a
          spangle be modified so it looks like a "normal"? Have you already heard of
          the "cleartail" Budgerigar coming out of spangles? These are normal birds
          with a white or yellow tail. Is it a modified spangle or what? And what
          about the inconsistend results crossbreeding sex-linked clearbodies and
          lacewing Budgerigars.>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.

          As I have photos of those Cleartails they look like Spangles which are
          extremely heavily marked. I would like to get some of them to test pair them
          myself. Being in Australia it is impossible at this time to get birds in
          unless you are a zoo owner or such like.

          As far as inconsistent result go with Clearbodies bred with Lacewings all I
          can say is that I don't consider the results we get to be inconsistent. I
          treat the Lacewing as though it were an Ino but keep in mind that a
          crossover can occur and so I expect to bread a Cinnamon or Cinnamon
          Clearbody now and then. This happened last year. I paired a Clearbody Dark
          Green/Opaline/Lacewing/Blue cock with an Opaline Cinnamon Light Green/Blue
          hen. Results were. Lacewing hens as expected in Opaline and Normal. Opaline
          cocks. Clearbody Opaline hen. The only bird produced which some would say
          was unexpected was a Cinnamon Cock. I just consider this a crossover event.

          Down Under Budgies
          ICQ 17926314
        • Ralph T. Hopkins
          At 10:17 AM 10/30/99 +1000, Terry Martin
          Message 4 of 11 , Nov 3, 1999
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            At 10:17 AM 10/30/99 +1000,"Terry Martin" <sbankvet@... wrote:
            Clive, Inte, Ralph and Chris
            Thanks for your replies.
            I suppose what I was really getting at is can a chiasma form
            >anywhere along the DNA chain, leading to crossover at any point, or do
            >chiasma only form when particular base pair series occur?
            If it is purely random at any point along the DNA chain, then the
            >potential for crossover has significance for the blue locus where we have
            >heteroalleles. If the altered segments for two different heteroalleles are
            >separated by a significant distance, then crossover has a real chance of
            >happening. And if it did, then a 'new' wildtype allele would be formed on
            >one chromosome and a new 'double damaged' allele on the other.

            I had the chance to pose this question to a geneticist this morning using
            the blue locus as an example. His answer was simply: "Yes".

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