Recessive and dominant "white"

Discussion in 'General breed discussions & FAQ' started by Yard full o' rocks, Dec 15, 2009.

  1. Yard full o' rocks

    Yard full o' rocks Chillin' With My Peeps

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    I guess I didn't pay enough attention in biology class and in college my major didn't require it (Industrial Eng)....so,

    Can someone tell me how to differentiate recessive white and dominant white and how can you tell what you have in a bird? Does it depend upon the breed? Is there some way to tell by looking at the bird and any of their features?

    I.E. - white plymouth rocks....dominant or recessive?

    white leghorns?

    the genetics thing has really caught my attn and I am trying to learn

    THANKS

    Scott
     
  2. tadkerson

    tadkerson Chillin' With My Peeps

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    Last edited: Dec 15, 2009
  3. Sonoran Silkies

    Sonoran Silkies Flock Mistress

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    I'll let Tim fill in hte details, but if you've got a bird that is partially white, chances are that it is dominant white, which is very leaky, especially with red pigment. Different breeds to tend to carry one or the other. I leghorns carry dominant white; silkies and cochins carry recessive white. Not sure about other breeds.

    Dominant and recessive white work differently at the cellular level, and I am still trying to grasp the differences. Recessive white requires a copy from each parent. It acts as an OFF switch, turning off whatever other colours or patterns may be genetically present. If a bird has one copy it does not work--there is no sign from the appearance that it is present; however it can be passed to offspring and make its presence known when it pairs up with a copy from another bird. Dominant white works with either one or two copies--one copy turns off expression of black pigment; two copies turn off the presence or red pigment. However there are circumstances (I don't understand these very well) such as exchequer leghorns where black pigment is allowed into some of the feathers.
     
  4. tadkerson

    tadkerson Chillin' With My Peeps

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    Suze is correct about the two different genes working on the cell level.

    Any biochemists that are reading this please give me some license to overly simplify the process.


    There are certain chemical pathways that are used to make pigments. The chemical pathway to make red pigments and black pigments starts out as only one pathway but then the pathway forks. One chemical pathway leads to black pigment and the other pathway leads to red pigment. Genes can turn on or turn of the chemical pathways at the fork. If the black pigment pathway is switched off the bird makes red pigment. If the red pigment pathway is switched off the bird makes black pigment.

    Once the pigments are made, the pigments must be packaged into special vesicles (plastic bags) and transferred to the follicle in the skin where the pigments can be placed into the developing feather. Inside the follicle there are special cells that make the feathers. These feather making cells place the pigment into the feather.

    An important enzyme that is needed to make black or red pigments is called tyrosinase. Tyrosinase is used in the first part of the pigment making pathway. The tyrosinase enzyme helps break down tyrosine ( an amino acid) which is needed to make black or red pigment.


    Recessive white

    The gene that controls the production of tyrosinase is the dominant C gene. Birds that have a dominant C gene make tyrosinase which then converts the tyrosine to a chemical called doba. The dopa eventually becomes red or black pigment.

    Two recessive c genes prevent the production of the enzyme tyrosinase so the chemical pathway to make pigments is switched off. No tyrosinase means no pigment so you do not get color in a feather. Pigments can not be made if tyrosinase is not produced by the cell.

    Dominant white

    Dominant white is a bit more difficult to explain. Dominant white causes a construction error in the vesicles. The dominant white gene prevents the proper construction of the vesicles (plastic bags) that transport the black pigment to the follicle. The construction error prevents the black pigments from being transported into the cells that make the feathers. The bird can make black pigment, it is not transported into the cells that make the feathers.

    Red pigment is different. The construction error does not effect the transfer of the red pigment. That is why dominant white birds show red pigment and not black pigment.

    You can think of it like this- black pigment comes in big boxes and red pigment comes in small boxes. The small boxes can be transferred but the big boxes can not be traansferred. The vesicle is not constructed so it can transfer the big boxes but it will transfer the small boxes.

    You can get some black in a bird that that only carries one dominant white gene because not all of the vesicles are improperly constructed so some feathers can be black.

    You can get red in a bird that has one or two dominant white genes because the construction of the vesicles does not effect the transfer of the red pigment.


    Mottling gene

    I am not sure how the mottling gene works on the cell level. It may be like the recessive white gene or it may be like the dominant white gene. The mottling gene is only programmed to disrupt the pigmentation of feathers on certain areas of the body and not all over the body like the dominant or recessive white. How it works has to do with embryology and I do not have time to give a lecture on the subject. You will have to trust me on this one.

    Tim
     
    Last edited: Dec 16, 2009
  5. Sonoran Silkies

    Sonoran Silkies Flock Mistress

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    BLESS YOU, Tim! That makes a lot of sense. I understand best at first by use of analogy, and you did an excellent job explaining!
     
  6. onthespot

    onthespot Deluxe Dozens

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    Someone gave me a white ameraucana hen weekend before last. I have been wondering if she is dominant or recessive. I 'm guessing she is recessive, as she is pure, blazing white. Any guesses as to what colors are usually behind recessive white Ameracuanas? Maybe just about anything, but I would think she came from a decent breeder, because her eggs are blue and she looks typey. I don't have pics.
     
  7. Sonoran Silkies

    Sonoran Silkies Flock Mistress

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    There is no "usual colour behind" recessive white, regardless of breed. Probably the closest you can come would be the e-allele most common to the breed. And since some breeds are known to have multiple e-alleles, that wouldn't be of much help in those breeds. If you want to know what is behind a specific recessive white bird, breed it to a non-white.
     

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