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Recessive vs Dominant White & Crossing to Non-White--posts compilation

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This is a compilation of multiple posts and threads about recessive and dominant white, and while I copied them to Word to clean them up for reposting as a whole a while back, I still have not done so. th  Hopefully I will get in and edit it into more coherence eventually, but at least this will provide some oft requested information in the meanwhile.  Tim Atkerson wrote a number of these posts, so please give him credit for all the cellular level explanations of how the genes work, as well an any other posts with his name at the head.



11/24/2009 1:21 pm
From: tadkerson
Subject: Re: Ressiseve? Dominat?
Recessive white
A bird needs two copies of the recessive white gene to make the bird white. Sometimes when you cross two black birds they can have recessive white chicks. When a chick inherits two recessive white genes, the genes switch off the chemical pathways that make black or red pigments so you get a white bird. The bird can have the genes that would make it black or red but the recessive white genes prevent the birds cells from making red or black pigments.

Dominant white.
Dominant white is different because it tries to switch off the chemical pathways but does not do a very good job. It is good at switching off the pathway that makes black but has a hard time turning off the chemical pathway that makes red. This causes some of the red pigment to leak through the white.
Sometimes the dominant white gene only switches off the black for a short time then allows the black to switch back on so you do get some black pigment in the feathers. In order to get rid of the black completely, breeders will add additional genes to a bird that also turn off the chemical pathway that makes black pigment.
In order to keep dominant white birds white, breeders want the genes in the bird that  make black pigment and not red pigment. You could say that the dominant white bird is black under the white.  The bird has the genes to make black feathers but the dominant white gene or genes like barring and silver turn off the chemical pathways that make the black pigments.
You can say recessive white genes really do not need help from other genes to make a bird white but the dominant white gene needs help from genes like silver and barring.

Tim
Last edited by tadkerson (12/22/2009 5:49 am)

11/24/2009 3:25 pm
Sonoran Silkies
Subject: Re: Ressiseve? Dominat?
I will try to start with very basics.  My apologies if I am too simple.

Every creature with two parents has two sets of chromosomes: one inherited from each parent.  Except for the sex chromosomes, the chromosomes are matched pairs, each containing locations for specific genes.  These non-sex chromosomes are referred to as being autosomal. 

Within each location on a chromosome, a set of specific variations can occur.  Sometimes there are only two alternatics, sometimes many.  These variations are referred to as alleles.  For the blue gene (since that seems to be the most commonly explained) there are two alleles: blue and not-blue.  Blue is abbreviated Bl, using a capital letter because it is dominant over the alternative allele, not-blue, which is abbreviated bl+.  The + indicated that this allele is wild-type.  Because chromosomes are paired, each bird carries two copies of a blue allele.  They can be two copies of Bl, two copies of bl+ or one of each. When both alleles are the same, it is referred to as being homozygous; when they are different it is heterozygous.

So, each location holds a different gene, of which there can be several variations. 

Kind of like a row of houses, each with a different family.  Only the Adams live at the Adams' house, only the Browns live at the Brown's house, only the Carlisles live at the Carlisle's house.  But, at the Adams there can be Mr Adams, or Mrs Adams, or Baby Alex Adams; at the Browns you might find Aunt Beatrice Brown, Uncle Bob Brown or Cousin Betty Brown.  etc. 

Transfering this analogy to chicken genes, Dominant White would be one house, and living there are Smokey (I^S) Dominant White (I), Dun )I^D), and not-White (i+).  Another house is home to the Recessive Whites: Colour (C+), not-colour (C), red-eye (C^re) and albino (c^a).  And yet a third house is home to the Extended Blacks: Extended (E), Birchen (E^R), Wheaten (e^Wh), not-extended (e+) and Brown (e^b).  Now the rules of this little universe are somewhat complex, but everyone has an identical twin who lives at their home.  Also, in most cases, exactly two of the family is always home.  In a few cases for a few homes, only one person can be at home.

As in all families, each gene-family has their own traditions and rules, and each family member has their own personality.

Unrelated genes are neither dominant nor recessive towards each other, only towards other variations of the gene at their specific loci.

Think of a gene as having a particularly shaped hole in a child's shape sorter.  Only blocks that are shaped to fit the hole can fill that spot, although they could be red/blue/pink/green/purple polka-dotted.  These blocks are the alleles for that gene.   

In most cases there are two identically shaped holes, and each will hold one block of the correct shape.  Dominance refers to the way these two identically shaped blocks, which may or may not have different colours/patterns translates into the appearance (phenotype).

If one of the blocks is dominant to the other, it will override the more recessive block.  If incompletely dominant, the appearance will fall in between the dominant and recessive blocks, and if recessive, it may show in down or first feathering, but will not show in normal adult plumage (when a more dominant allele is present).  There is also a variation called co-dominance, wherein both blocks are fully expressed, but I am not aware of any genes relating to chickens that have co-dominant alleles.

tadkerson
Subject: Re: Recessive and dominant "white"
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 by tadkerson (12/16/2009 9:09 am)
L0rraine wrote:
Sonoran Silkies wrote:
I think of recessive white as an OFF switch--it turns OFF the plumage colours and patterns that are genetically present in a bird.  Since you cannot see them, you cannot select for or against them, and after a few generations you have no idea what any individual bird is underneath the white.
So white is independent of color?  So someone mentioned earlier in the thread, I think, that sometimes it does create unpredictable lighter variations (or that's how I interpreted what was said).  Is there an unpredictable 'dimmer switch' action
No, white does NOT dilute.  There are two different white genes.  Recessive white is indeed an offswitch.  Tim can explain it better, but at the cellular level it prevents the formation of all pigment.  It can be leaky, allowing a bit in occasionally on a feather here or there, but overall is pretty non-leaky.  Dominant white, on the other hand works differently, and is very leaky.  It is more like a filter than an offswitch--it doesn't prevent pigment from forming, but it does prevent it from being placed in the feather (Tim, if I got any of this wrong, please correct me).  One copy prevents black pigment from entering feathers; two copies also prevents red pigment.  Exchequer leghorns are leaky dominant white.  Red pyle is dominant white.



My understanding is that black acts similarly in ducks, but that it's incompletely dominant letting some of the original color come through (but that's not what causes the partridge color right?).  Does it allow for the lavender variations?  OOOOhhhh my head always  hurts when I try to go here. 
I don;t know anything about duck genetics, so I cannot relate how chicken genes relate to duck genes--Henk does know both, so maybe he will chime in.  Partridge is a hobby name that can mean a couple of different things.  If you are talking about a penciled bird such as a partridge rock, wyandotte, cochin or silkie, it is the interaction of a specific
black allele, e^b (brown) combined with Pg (pattern gene).  Lavender is caused by a double dose of lav (lavender).

Chicken plumage can contain only two pigments: black (eumelanin) and red/gold (pheomelanin).  There is a basic pattern of which of these two colours appear where on the body of a male vs female.  Various genes modify which colours go where (extending or restricting the colours to/from these areas), dilute or deepen pigment tint/shade (which may also alter the hue), or create patterns on individual feathers.

kathyinmo wrote:
OK, these are great tips! Thanks so much! I think I was just spending too much time figuring out where the allele is, in the overall picture. I was focusing on how many and where, etc.  So, now I am this far:
A gene is a piece of DNA that carries information about a specific trait. 

A chromosome is a string of genes connected together (although most of the chromosome is DNA that has no known function or no genetic activity). 

An allele is a gene that is a member of a set of genes that all belong to the same locus, or location, on a chromosome.  These genes are often thought of as being related to each other through mutations (one allele could be a mutation of another allele) or they could be mutations of an ancestor gene. 

Chickens, like people, usually have two of every chromosome.  The chromosomes in a chromosome pair are not identical, since one comes from each parent.  A gene is said to be dominant when only one gene (rather than two) is sufficient for the expression of that trait to which the gene corresponds. Some genes are referred to as incompletely dominant. The expression of these genes is inhibited by (usually unknown) modifying genes. When the inhibiting, modifying genes are not present, the incompletely dominant gene expresses. This interaction with modifying genes is responsible for the seemingly random nature of the expression of incompletely dominant genes.


I like to think of a chromosome as a child's shape sorter toy.  Each locus corresponds to a differently shaped hole, the alleles for that hole are all the blocks that are the correct shape, but they can be any colour (indicating that they have a different affect).  The gene is usually named for the affect it has on a bird.  For example, the blue gene dilutes black pigment, the mahogany gene deepens the colouring of gold pigment, the extended black gene causes areas of the bird which would be gold to be black instead, etc.

Your definitions are correct except for incompletely dominant.  It is an intermediate expression between the alternative alleles.  Modifying genes have no particular bearing on an incompletely dominant gene any more than they do on a dominant or recessive one.  If the modification is to inhibit expression, it will occur with dominant or recessive as well.  The blue gene is an excellent example of an incompletely dominant gene.  One copy of blue (Bl/bl+) dilutes black to blue; two copies of blue (Bl/Bl) further dilutes the plumage to splash. 

To your definition of recessive, I would add that in a hen, one copy of any sex-linked gene is sufficient for the gene to express, as there can be no alternative more dominant version.
I think of recessive white as an OFF switch--it turns OFF the plumage colours and patterns that are genetically present in a bird.  Since you cannot see them, you cannot select for or against them, and after a few generations you have no idea what any individual bird is underneath the white.

Breeding to a white is completely unpredictable unless you already have reliable data about the  non-white background of the individual white bird or have non-white offspring from that individual bird, and can therefore extrapolate.  Silkies carry recessive white.  It is not very leaky--unless both parents supply a copy, offspring will NOT be white.  Recessive white is an OFF switch that prevents the formation of pigment--genetically any colour or pattern genes can be present in the bird, and will NOT display because there is no pigment available.  Excluding the recessive white genes, any two recessive white birds could have completely different genetic makeup in regards to plumage colour/pattern.

Dominant white is leaky, and without modifiers does not create a completely white bird.  Red pyle is a good example of where dominant white "leaks."  Only if a dominant white bird has two copies of the same dominant white allele will all offspring be white (with or without leakiness).  If their is only one copy, then only half the offspring will receive that copy.

Recessive white X black does not yield black--it all depends on the genes the white is hiding.  Likewise with dominant white.   If carrying blue or splash, the offspring may/will inherit those dilutions.  If carrying pattern genes, those may be inherited.  If melanizers are missing, their lack will be inherited.  Etc. 

Since white is an OFF switch to the colours and patterns genetically present, trying to determine the variety of the offspring is kind of like saying "if I mate my black (or whatever variety) to an anonymous bird, what colour will the offspring be?"  You only know part of the equation, and without test breedings do not have sufficient information to provide anything but guesses.

Independent genes are not recessive or dominant to each other.  Recessive or dominant relates to different alleles of the same gene.  Black is the E-locus gene.  White is c-locus for recessive white or I-locus for dominant white.  All chickens have two copies of alleles for each of these genes.

Recessive white (c) is recessive to "not white (C)," not to any of the black alleles: E, E^R, E^Wh, e^b, e+.  Each of those black alleles has a hierarchy of dominance.

A single set of data, based upon one or a related small group of individuals does not give a generalized answer to all occurances.  Recessive white in silkies bred to another colour often, but not always results in partridge.  With my white, bred to a splash, I got chocolates.  That does not mean that recessive white X splash always results in chocolate.  It did with that particular breeding pair.  Should I expect that your jersey giants would have the same result if bred to any splash?  Of course not; it depends on the particular genes each half of the breeding pair donates to the offspring.

Breeder & Exhibitor of fine silkies in recognized and project varieties.
adult and started pairs occasionally available;
   No eggs or chicks. 
Support your local poultry clubs, breed clubs, ABA & APA!

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Breeder & Exhibitor of fine silkies in recognized and project varieties.
adult and started pairs occasionally available;
   No eggs or chicks. 
Support your local poultry clubs, breed clubs, ABA & APA!

Reply
post #2 of 2

Now all you need is to put it all into picture form, so I can keep up.  smile

Seriously though, this is great.  I love it.  Maybe you can highlight & "quote" the sections for organization & readablilty?

Laree 
Giving you the stink-eye.  Yeah, you.   

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Laree 
Giving you the stink-eye.  Yeah, you.   

Reply
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