Introduction to Genetics and Inherited Traits 

Chapter 4 - "The Ten Commandments of every Pedigree". 

 

One of the things I talked about in the previous chapter was sex-linked matings.  I promised to show how this information can be used to check the accuracy of a bird's pedigree.  This will be our primary focus here.  Many times, I have been asked to review pedigrees to see if they were genetically possible.  “Why?”  you might ask.   Well primarily, to insure it was possible for the parents and or grandparents to produce the bird in question.  Since you, the buyer, have paid your hard-earned money for offspring from certain stock, you're entitled to get what you were promised.  Should the pedigree have obvious genetic errors then there is a high possibility that the bird is not the product of the stock listed.    Many times, while conducting these pedigree checks, I have found discrepancies that were without question, genetically incorrect.  Two blue bars producing a red check or two non-grizzles producing a grizzle are examples.  Such discrepancies bring the legitimacy of the pedigree into question.   Either these birds were bred in an open loft and are the product of a non-monogamous coupling; or they are the result of some new mutation.  The first possibility or adulterous chance mating is very common in open lofts while the odds for the second, a mutation, is more like winning the lottery.  Possible yes but highly unlikely.     Since the hen is the one who lays the eggs, we tend to accept her as being legitimate while the cock may or may not be.  Well this is normally the case but not always.  Some cocks will attract more than one hen into their nest.  The first to lay her eggs is not always the one to set them.  Many older hens will accept eggs placed in their nest as being their own.  Once accepted she can become very possessive and may drive the true mother away.  If she had not begun the process of coming down on eggs herself, she simply goes into the brood condition and begins the incubation.   Thus she becomes a foster parent.    Flyers simply cannot live in their lofts 24 hours a day so they're not aware of everything that goes on.  Basically, any youngsters produced in an open loft should be suspect as to its true lineage.  The only way to be sure of parentage is via breeding in separate pens.   So how do we go about checking to see if the seller is telling the truth when he says that his birds were hatched in individual breeding cages?  If they are truthful then there should be no discrepancies. I say this on the assumption that the breeder can classify the bird's phenotype correctly. In racing homer circles, this is one of our biggest weaknesses.  Most flyers simply have no basic understanding of the different phenotypes and this can cause confusion when reading their pedigrees.  Terms like chocolate, slate, self, silver and mosaic all have different meanings to different flyers.  It is best to learn the proper terms for these phenotypes and avoid the confusion and or embarrassment when misunderstood.  Example, the term silver when used by US flyers to describe a mealy is incorrect.  The majority of the English speaking racing world including the US Show Fancy uses the term silver to describe a dilute blue bar.  Now there is a world of difference between an ash-red bar (mealy) and a dilute blue bar (silver).  We as racing types should learn to speak the same pigeon language as the rest of the world.  Should you not be familiar with the proper terms or need to brush up on them, I suggest you start with Wendell Mitchell Levi's two books The Pigeon and the Encyclopedia of Pigeon Breeds. If you are the reader of the pedigree check with the breeder to learn his understanding of the terms he used.  If you are the breeder, avoid the use of local terms and use the more excepted ones for clarity.   But I digress so let me come off my soapbox and get back to the subject at hand: Checking the validity of a pedigree.    So how does all this work?  There isn't enough time to go into all the details but let me hit the high spots and give you the tools you need to conduct a check on your own.  The tools we will use are the rules governing dominant, co-dominant and recessive genes both when sex-linked and non sex-linked.  We have already discussed many of these in our previous discussions so this shouldn't be to difficult to follow.  Let's begin with the non sex-linked genes.   If you recall these are the ones found on the autosome chromosomes.  The rules here are not associated with the bird's gender.  In other words, they apply to cocks and hens in the exact same manner.  These rules are simple to follow.  They are in essence applying the principals of dominance over less dominant or recessive.   With pedigree in hand, begin by looking at all the birds listed.  Note they're marking patterns such as bar, checker etc..   We know that every pigeon carries two genes for pattern, having inherited one from each parent.  These pattern possibilities are t-pattern, dark checker, checker, light checker, bar and barless.  There is an order of dominance associated with these.  It starts with the t-pattern markings (also known as t-check or velvet) as being the most dominant in the pattern series and runs down to barless being the least dominant or most recessive. The simplest way to remember this order of dominance is to say that the more pattern shown the higher it is on the pecking order.   Nature when faced with the problem of two gene possibilities solves the dilemma by always selecting the most dominant as the phenotype.   Let's put that last statement to the test.  A bird is listed as being a light checker.  For this to occur, one of its parents must be at least a light checker or darker in pattern.  Example: a dark check cock with one gene for dark check and one for light check mated to a dark check hen carrying one gene for bar.  Thus you have two dark checker pigeons producing a light check.   Is this possible?  Yes, all that is necessary would be for the sire to pass along his light check gene while the dam passed along her gene for bar.  The outcome would be a light checker-carrying bar.  Remember the order of precedence.  The more pattern shown the higher on the pecking order.  The light check would be displayed while the bar would not.  During future reproduction, the gene for either light check or bar will be selected at a rate of 50/50.  Should this light checker/bar be mated to a bar/bar partner then 50% of the young will be light checker/bar and the remainder bar/bar.  The light checker/bar will appear as a light check in phenotype where as the bar/bar will be a bar in phenotype.  Color does not enter into this equation even though color can also be used in sex-linked identification.  We will get into that in just a few minutes. From the pedigree, we can't tell what the unseen pattern gene is.  We only know the one, which is shown or listed on the paper, so we must follow the first rule of what I call………… "The Ten Commandments of every Pedigree".  Rule #1:  A darker pattern can produce a lesser pattern but never the other way around.  Okay so what happens if some other modifier such as grizzle or spread is masking the pattern?  For starters, when a bird only carries one gene for grizzle, we will still see some pattern.  Bar for example when present indicates it's a bar under the grizzle effect.  Dark grizzle indicates the presence of checker but it is hard to distinguish which type of checker it is.  Its much easier to break it down with a bird in the hand than from words or general terms on a piece of paper.    Whenever you see a grizzle noted on the pedigree you should forget the pattern and turn to the fact that grizzle is dominant over non-grizzle.   Rule #2:  A dominant modifier can produce a lesser dominant but never the other way around.  Rule #3: Any dominant type gene will always be seen and on a pedigree it should be traceable back in an un-broken chain to its origin.     However, this only works in one direction so we have rule #4. Rule #4: Since dominant genes may not always be inherited, the chain of inheritance may be broken in direction from the older to younger.  In other words, a dominant characteristic such as grizzle must come from another grizzle parent without any break in the lineage back through the oldest grandparents; but should the chain of inheritance be broken then it is gone forever and can not magically reappear.    Rule #4: Since dominant genes may not always be inherited, the chain of inheritance may be broken in direction from the older to younger.  In other words, a dominant characteristic such as grizzle must come from another grizzle parent without any break in the lineage back through the oldest grandparents; but should the chain of inheritance be broken then it is gone forever and can not magically reappear.    Rule #5: There is no such thing as a throw back in dominant genes.    However there are times when out of the blue we have something appear from past ancestors.  This is the result of a recessive gene being expressed.  Autosome recessive genes must reside on both sides of the family tree to be passed along and recombined in the offspring.  Sex-linked recessive genes operate in the same way as autosome genes in respect to cock birds only.  In other words it still takes two to express, however; for a hen it only requires one recessive sex-linked gene to express due to her hemizygous state.    Rule #6: An autosome throw back only occurs when two autosome recessive genes are reunited.  Rule #7: A sex-linked throw back only occurs in hens.  It is the inheritance of a recessive gene from her sire.  This only applies to recessive genes.  Every grizzle pigeon must have at least one parent, one grandparent, one great grandparent and so on which are all listed as also being a grizzle on the pedigree.  This is in keeping with rule # 3.  A pigeon carrying this dominant gene will always display it unless it is being masked by white.  Should white be listed for one of the parents then you must look further up the pedigree to see where the grizzle originally came from.  Should there be no further reference to white or grizzle then the pedigree comes into dispute.    Two non-grizzle birds can not produce a grizzle.  However two grizzles can very possibly produce a non-grizzle.   This is only possible when both parents are carrying a single gene for non-grizzle and each has passed it along to their offspring.  Again we are dealing with the order of precedence and the possibilities of inheritance.  This entire process applies to any dominant type gene and not just grizzle.  Some of the more common types of dominant genes are Spread, Almond, Indigo and Dominant Opal.  There are others but they are not common in racing homer stock.  If you are not familiar with any of these, you can see examples of them on this  web site. Okay what about using sex-linked colors in checking the validity of a pedigree. This is only possible through the process of dominant over non-dominant as well as the sex-linked mechanism put into play by the hemizygous condition for all hens.  Here the process is just a little more complicated.   I guess what we should first do is review some of the material and terms covered last month.

In birds, the gender or sex chromosomes are labeled Z and W.   All non-sex chromosomes are classified as autosome chromosomes just as they are in other beings.  It follows that an autosome gene is defined as a gene found on any autosome chromosome and a sex-linked gene is one found on the sex or gender chromosomes.  If you recall, the combination of these two sex chromosomes results in the bird’s gender.  A cock always inherited one Z chromosome from each of his parents resulting in a sex chromosome set or pair of two Z’s.  A hen inherits a single W from her dam and a single Z from her sire.  The thing that is significant is that there are no know genes on the W chromosome.  Therefore a hen will always receive fewer genes in her genetic makeup than will a cock.    Since cocks always have two Z or sex chromosomes they will have two gene possibilities for every set of sex-linked genes.  Hens with their single Z chromosome can only have a single gene possibility for each of their sex-linked gene types.  Hens are never homozygous nor heterozygous for genes found at their single Z or sex chromosome.  Instead they are hemizygous meaning they are pure for each of their single sex linked genes.  This in turn means that all hemizygous genes, regardless if they are classified as recessive or dominant will be expressed as pure.  Keep in mind, genes which are recessive in nature, require they be present in a pure state to be expressed.  Since hens do not have a matching Z chromosome, for them, there is no competition for the order of dominance in expression between alleles.  Rule #8:  A hen receives her color from her sire.  It is impossible for an ash-red hen to have a blue or brown sire; or a blue hen to have a brown sire.  Why??  Well think about it.  A blue cock must either be pure for blue or split for blue and brown.  If he had any ash-red genes then they would be dominant and show but since they do not we can rule them out.  If he doesn't have ash red then he certainly can not pass it along to his progeny.   A brown cock must always be pure for brown to be displayed.    Did you pick up on the fact that hens will always be the same pigment color as their sire or one which is of lesser dominance than her sire?  Her color will never be greater in order of dominance than her sire, regardless of what else is found in the her family history.  Just like the dominant genes, there are no throw backs in color.  It either exists to be passed along or it doesn't.  Now lets examine this process for a cock colors inheritance.  Since he will receive his color from both parents and if his dam is a more dominant pigment color than his sire then he will be the color of his dam.  If his sire is the more dominant color of the pair then he may be that same color or less.  Note I said "may" and not "will".  The reason for this is due to the fact that his sire has two gene possibilities and either can be passed along.   So in the case of a cock, it will first be the color of his dam if she is the more dominant of the two.  Should she be the same as the cock then this would still apply.    However, should her color be lesser in dominance then the son will be the color of his sire or less, depending on what the sire's two gene possibilities are.    Just like a hen, no cock will be more dominant in color that either of his two parents.  Again there are no throwbacks associated with these three pigment possibilities, ash-red, blue/black and brown. We find that on the Z chromosome, in addition to the three basic pigment colors, we also have both dominant and recessive color modifiers.  These too play a part in evaluating pedigrees.  We find a complete series of genes know as the Almond series which consist of Almond, Qualmond, Faded, Hickory, Sandy and Frosty which are dominant to their non-almond alternative.  For cocks and hens these all operate in a fashion similar to Rules # 2 through # 5 and # 10.    Basic Rule #9:  A hen receives all her sex-linked color modifiers both dominant and recessive from her sire and these will always be expressed or displayed.  A cock on the other hand would need for both his Z chromosomes to have the same recessive gene present on both Z chromosomes for them to be expressed.  His dominant genes of course would always be expressed regardless if present in a pure or impure state. Perhaps a simpler way to say all this would be; a hen will always express her single sex linked recessive genes when present since there would be no dominant alternative.  All other recessive genes, both the sex-linked ones of a cock and all the autosome recessive genes, regardless of gender, would still require both be present to be shown or expressed.    Okay, that means there is a major difference for recessive gene expression between the sexes. Terms like homozygous, two of the same type or being pure for that condition and heterozygous, two which are different from each other or non-pure for that condition would simply not apply to a hen due to her single Z chromosome.  It sets up an additional term for her known as hemizygous where a pure condition exists with only one gene being present.  This imbalance in the number of sex chromosomes between the sexes along with the various gene mutations that can be found there gives us a mechanism for sex linkage between a youngsters produced and its opposite sex parent.  We can say, without any doubt, that all sex-linked genes found on a hen came from her sire even though her sire may not have shown evidence of such genes.     Let’s take the gene for dilute as an example.  This mutant gene will cause color intensity to be much lesser than the intensity of a non-mutation gene normally found at this gene locus point of the chromosome.  It changes a black to appear gunmetal dun, an ash-red to appear yellow and a brown to appear khaki.  Since this dilute gene is recessive to normal color intensity, all cocks when heterozygous (impure) cannot display their dilute factor expression.  To do so they would have to be homozygous or pure for the gene.   However, a daughter if she were to inherit it would show its effects.  There is no second option to override its function.  She would be pure for the condition in her hemizygous state. The term hemizygous can not be applied to autosome genes, as the autosome chromosomes will always exist in pairs while hemizygous is a single gender chromosome state.  Now lets put some of this info into practice as it relates to color and see if it works.  In pigeons there are only three pigment colors.  These are all associated with the Z chromosome at the b locus.  The three pigment possibilities are ash-red, blue/black and brown.  Blue/black while sounding like two separate colors is (in pigeons) only one and that is black.  The blue we see is produced by the way black pigment is distributed.  So while we often refer to a blue bar as being a blue, it is in reality, a black and not a blue.  Just as there is an order of dominance in autosome pattern marking genes, there is an order of dominance between these three pigments.  Ash-red is the most dominant followed by blue/black and then brown.  What is different between the sex-linked genes is that every cock bird will have two gene possibilities for all genes found on the sex chromosomes including color while the hen only one.  The cock when not homozygous or pure will be governed by this order of dominance, whereas the hen will be whatever she is.   This does not apply to the autosome chromosome genes for which there is no difference between the sexes.    Since we know a hen will receive her single Z chromosome from her sire, we therefore also know that her pigment color was without doubt from her sire as well.  This being the case, it becomes impossible for an ash-red hen to have a blue or brown sire; or a blue hen to have a brown sire.    Rule #10:  A cock will never be more dominant in color or color modifiers that either of his two parents.  It is impossible for an ash red cock to have two blue or two brown parents or any combination of the two; or a blue cock to have two brown parents.  A brown cock must have a brown dam but his sire may be a blue or ash red if these are split for brown.  In addition to the dominant Almond series of sex-linked genes we also have some recessive sex-linked genes.  These are pale, dilute, reduced and rubella.  Each of these will have an effect on color intensity for any of the three basic colors.  Dilute for example turns an ash red into yellow, a blue into silver dun and a brown into khaki.   Since these are recessive and sex-linked all cocks must be pure or homozygous to display their effect.  A hen on the other hand being hemizygous or having only one Z chromosome will always be pure for these conditions and regardless if recessive or dominant these color modifiers will display their effect.   Thus we have rules number seven and nine.  Rule #7: A sex-linked throw back only occurs in hens.  It is the inheritance of a recessive gene from her sire.  Rule #9:  A hen receives all her sex-linked color modifiers both dominant and recessive from her sire and these will always be expressed or displayed.  To recap I will list these ten rules or what I like to call "The Ten Commandments of every Pedigree".    So to put it simply, when you are checking a pedigree, none of these ten rules may be violated.  All the birds listed must pass muster on each and every rule.  If any do not then the pedigree is in question and another look needs to be given to the bird and or breeder to learn why.  Rule #1: A darker pattern can produce a lesser pattern but never the other way around.  Rule #2: A dominant modifier can produce a lesser dominant but never the other way around.  Rule #3: Any dominant type gene will always be seen and on a pedigree it should be traceable back in an un-broken chain to its origin on the pedigree.   Rule #4: Since dominant genes may not always be inherited, the chain of inheritance  may be  broken in direction from the older to younger.    Rule #5: There is no such thing as a throw back in dominant genes.    Rule #6: An autosomal throw back only occurs when two autosome recessive genes are reunited.  Rule #7: A sex-linked throw back only occurs in hens.  It is the inheritance of a recessive gene from her sire.  Rule #8:  A hen receives her color from her sire.  It is impossible for an ash-red hen to have a blue or brown sire; or a blue hen to have a brown sire.  Rule #9:  A hen receives all her sex-linked color modifiers both dominant and recessive from her sire and these will always be expressed or displayed.  Rule #10:  A cock will never be more dominant in color or color modifier that either of his two parents.  It is impossible for an ash red cock to have two blue or two brown parents or any combination of the two; or a blue cock to have two brown parents.  A brown cock must have a brown dam but his sire may be a blue or ash red if these are split for brown. Ok, that's more than enough for now.

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