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Background vs Foreground melanin

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  • Lists@Theropod.org
    Could someone explain in detail just what the terms background and foreground melanin distribution mean? With examples if possible :D Royan _ _ ( } ( }
    Message 1 of 3 , Jan 2, 2001
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      Could someone explain in detail just what the terms background and
      foreground melanin distribution mean? With examples if possible :D

      Royan
      _ _
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      " / / Theropod Aviary
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      And the budgie says:
      When did my wild oats turn to prunes and All Bran?
    • Clive Hesford
      Message text written by INTERNET:Genetics-Psittacine@egroups.com Royan said, ... foreground melanin distribution mean? With examples if possible :D
      Message 2 of 3 , Jan 3, 2001
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        Message text written by INTERNET:Genetics-Psittacine@egroups.com

        Royan said,

        >>Could someone explain in detail just what the terms background and
        foreground melanin distribution mean? With examples if possible :D<<

        Since I'm probably responsible for first introducing these terms I'll try
        to clarify them for you.

        The difference is seen to best advantage in the budgerigar where, simply
        speaking, the black 'markings' are produced by heavy deposits of melanin
        which can be described as foreground melanin for fairly obvious reasons.

        Background melanin is deposited more lightly and it is less obvious that it
        is present underlying all green (or blue,etc.) areas of a bird.
        Constructional blue colouration (green when psittacin is present) would be
        all but invisible were it not for this background melanin absorbing all
        other wavelengths derived from the scattering of incident light.

        Background melanin becomes visible in its own right in the Grey varieties
        where feather structure is modified and there is little differential
        scattering.

        More technically background melanin is confined to the central regions of
        the medullae, which form the inner core of the barbs, where it fulfills its
        role of absorbency.

        The much heavier foreground melanin deposits (I believe, though Inte could
        go into more detail) occur more or less throughout the solid keratin of
        which the feather is formed, but certainly so at or near the surface of the
        outer sheath or cortex.

        More technical names (such as medullary melanin and cortical melanin) could
        be used but I believe those used here are very descriptive and more likely
        to be understood by ordinary breeders like ourselves.

        Usually foreground and background are reduced in tandem in diluted vareties
        but, in the budgie anyway, there are varieties in which they vary
        independently so there must be a degree of separate genetic control. This
        would not be so obvious in other parrot species where there is not usually
        so much contrast between the two types of melanin deposits. However, many
        do have significantly darker flights and/or tail feathers so it's worth
        looking out for in these. I'm not very familiar with PF Lovebirds but I
        believe the American Dilute (or is it Suffused?) shows a darker pencilling
        around the perimeter of many feathers which indicates the presence of
        foreground melanin and maybe a differential reduction in comparison with
        the Japanese Dilute. Dirk has something along these lines on his webpages.

        Sorry to go on at such length but you did say 'in detail'. ;-)))

        Clive.


        Clive Hesford,
        Cheltenham, UK.

        Mail: CliveHesford@...
        Internet: http://ourworld.compuserve.com/homepages/clivehesford/
      • Terry Martin
        Royan, Clive I think the two terms are excellent to use when describing changes we see in various colour morphs. The background melanin is contributing to body
        Message 3 of 3 , Jan 4, 2001
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          Royan, Clive

          I think the two terms are excellent to use when describing changes
          we see in various colour morphs. The background melanin is contributing to
          body colour, whilst foreground melanin contributes to markings. When we
          describe changes in a colour morph, these are the things we are observing,
          we cannot actually see the melanin distribution within the feather, although
          we can theorise about it. So unless we are describing feather sections
          microscopically, using the terms foreground and background melanin and
          describing the changes in them is the better way to go, using medullary and
          cortical only when the microscope is used to confirm it.

          As Clive mentioned, some dilute alleles specifically target melanin
          within one region or the other. We had a discussion on this a long way back
          with greywing, clearwing and dilute budgies. The terminology is also useful
          when investigating the changes caused by the opaline mutation. The action of
          this allele in all species seems to target foreground melanin more strongly
          than background melanin. And in at least two species (budgies and Bourkes)
          it even increase background melanin, whilst reducing foreground melanin.

          Terry
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