Introduction to Genetics and Inherited Traits

In the previous chapters 1 and 2 we covered chromosomes and genes for inherited traits.  In so doing I presented many genetic terms which were possibly new to you and I attempted to give their definitions as I went along.  Today I will review some of the same terms and try to give them a more explicit definition.   In the process we will also be covering some new ground.  With that said, put your feet up, lean back with a fresh drink and lets get started.   

As you read about genetics you quickly discover certain terms are flashed about which are not always explained.  It’s almost as though the writer expected you to know their genetic meanings.  Words such as, autosome; codominance; incomplete dominance; non-dominance; complete dominance and simple dominance.  Or terms like, homozygous; heterozygous; hemizygous; express; expressivity; factor; locus; allele; gene pool; modifier and of course recessive just to name a few.  These terms are used excessively and their meanings need to be completely understood to follow any genetic conversations or writings. Sometimes their use, in a clear simple sentence, becomes self-explanatory.  However that is not always the case so lets review their meanings.  

We’ll begin with the term’s autosome and sex-linked:  In birds the gender or sex chromosomes are labeled Z and W; in mammals they are X and Y.   All other chromosomes are classified as being autosome chromosomes. For our pigeons, this would be the majority of their chromosome makeup or 78 of the total 80 chromosomes present.  It follows then that an autosome gene is defined as a gene found on any autosome chromosome and a sex-linked gene is found on the sex or gender chromosomes.   It is important to understand why this distinction is made between these two types of chromosomes.    

If you will recall, in the first two chapters we discussed how the combination of sex chromosomes resulted in the bird’s gender.  We learned that a cock always inherited a Z chromosome from each of his parents resulting in a sex chromosome set or pair of two Z’s.  A hen on the other hand always inherited 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.  Keep in mind, genes which are recessive in nature require they be present in a pure state to be expressed.  That is, there must not be another allele present which is more dominant at the other gene locus. 

Oops there go two of those strange genetic terms (allele and locus).  If you will recall from the two previous articles (chapters), an allele is another gene possibility for the same genetic function found at a locus or location point on the chromosome.  Example, grizzle verses non-grizzle or the color blue verses the color ash-red.  There is an allele for every gene mutation and both the normal and or its mutant will reside on that same locus but only one allele or gene possibility can reside there at any given time. 

Now back to why it’s important to understand this distinction between autosome and sex chromosomes.   If a cock always has two Z sex chromosomes then he will have two gene possibilities for every set of genes present while a hen with only one Z would only have one gene possibility for each gene type.  As a result she is always pure for these sex chromosome genes.  Her recessive ones will show their expression, just as her dominant genes would.   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 express.  His dominant genes of course would always be expressed regardless if present in a pure or impure state.   

Autosome chromosomes always come in pairs so there is no such distinction between a cock and a hen in respect to autosome genes.  Autosome pairs operate is much the same fashion as a cocks Z chromosome pair. 

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 show or express.   

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 in respect to her single Z chromosome’s genes.  It sets up an additional term for her known as hemizygous where a pure condition exists with only one gene being present.  In humans and other mammals it is the male with his X/Y combination that is hemizygous.    

This imbalance in the number of sex chromosomes between the sexes along with the various gene mutations there gives us a mechanism for sex linkage between the youngsters produced.  We can say, without any doubt, that all sex-linked genes found on any hen come 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 locus.  It will change a black to appear dun, an ash-red to appear yellow and a brown to appear khaki. 

Since this dilute gene is recessive to wild type color intensity, all cocks when they are 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 would be no second option to override its function.  She would be pure for the condition in her hemizygous state.  This knowledge is extremely useful to any color breeder in their selecting of potential breeding partners for such phenotypes as almond, dilute, reduced, pale and other such sex-linked genes. This sex linkage simplifies our tracking of Z chromosome gene mutates in the inheritance process.  Next month we will go into this sex linkage in more detail.   Since this is a Racing Pigeon newsletter I will show how we can identify cocks from hens while still in the nest.  This can be very helpful if you want to identify your future widowhood cocks or celibate hens. 

Wait just a minute.  I’ve gone and dropped two more terms, Wild Type and phenotype, without defining them first.  Soooo I guess I had better do it now. 

A phenotype is what you see, while a genotype is what it is genetically.  Take an almond for example.  A classic almond with its many colors of yellow, bronze, black, and white and other color shades in-between are simply not the result of one single gene.  Sure the gene for almond is present but to be a classic almond other genes such as t-pattern, kite as well as recessive red are also required.  Therefore a classic almond is a phenotype or simply the total effect of the genes which you can see.  There are several phenotypes or ways for almond to be displayed, classic almond only being one of them.  These various differences in phenotypes are the result of varying genetic makeup’s or genotypes. 

A Wild Type is the standard phenotype and or genotype of the species Columba livia commonly known as the Rock Dove.  It is the ancestor of all our domestic breeds of pigeons.  The species Columba livia is found in the wild with a phenotype that has no mutations.   For us genetic nuts this form of pigeon is simply a starting point used to measure the change brought about by gene mutations.  In other words, the term Wild Type is basically a base line or reference point to identify new alleles.  For our domestic pigeons it is the common blue bar or rock dove Columba livia phenotype and or genotype. 

Now lets examine recessive autosome genes. Autosome genes when recessive will only express themselves when in the homozygous or pure state.  The term hemizygous can not be applied to autosome genes, as the autosome chromosomes will always exist in pairs and hemizygous is a single state.   

Recessive autosome genes can be very difficult to track through the inheritance process.  Often they will lay hidden in the family tree for several generations.  As a result they are passed along from generation to generation without ever being seen.  Then one day, out pops a youngster with an autosome recessive gene in full display.  When this happens we know that each parent is a carrier (heterozygous) and that they have each passed a copy of this gene on to our youngster.  This youngster is now pure or homozygous and as such is displaying the recessive gene’s effect.  We also know that any protégée (offspring) from this youngster will always receive a copy of the gene.   This too is useful information in selecting potential breeding partners.  It is useful when working with recessive opal, pearl eye, aberrant wing, crest, drumming, Davis syndrome, and Dutch scraggly just to name a few of the many autosome recessive genes. 

Okay what happens when one of the two autosome genes is dominant; complete dominance; simple dominance; codominance; incomplete dominance and or non-dominance? Well for starters their presence will all be expressed.  When dominant type genes are present their effects can be clearly seen.  Unlike the recessive genes which can lay hidden for generations, dominant genes can not lay hidden. They can be tracked by simply noting their presence.  What you see is what it is, so to speak.    

Let’s start with the definitions of dominant and or complete dominance.  Dominant being the preferred term.   These two terms both refer to a gene that produces the same phenotypic effect, full expression, whether inherited in a homozygous or heterozygous condition.  In other words, it is displayed in the same way regardless if it is pure or not.  Autosome genes such as spread, and dirty are examples of dominant genes.  With only one gene for the factor present, you will see its complete expression displayed.  Spread, which changes the way pigment, is displayed on a bird is a typical example.   A blue pigeon with the gene for spread will appear as solid black.  It would look the same whether it was homozygous or heterozygous.  Thus the term dominant or complete dominance is used. 

Codominance, incomplete dominance and or non-dominance are dominance’s of a lesser degree. Codominance being the preferred term.   They are defined as being a condition in which single alleles of a gene pair in a heterozygote state are only partially expressed.   One gene for their effect will be displayed to some degree while an enhanced or improved phenotype will be expressed in the homozygous state.  Grizzle, almond, indigo and dominant opals are examples of codominant genes.   

Lets use grizzle to demonstrate what is meant by codominance.  A single gene for grizzle on a blue bar based bird would give the typical salt and pepper looking phenotype with black bars and black salt and puppetry head, body and wing tips. This same gene in its pure or homozygous state would be almost completely white with only a small amount of black peppery effect around the bird’s head.  Only the very ends or tips of its wing feathers would show color.  In other words, it would look more stork marked than grizzle.   The codominance, incomplete or non-dominance here is in reference to the fact that the heterozygous state is lesser in expression than its pure homozygous condition.

Codominance, is sometimes used when two completely different mutation’s are each displayed.  Indigo and grizzle being one example.  When both are present you see an indigo grizzle phenotype.  One will not suppress or mask the other as in epistasis.     

Epistasis is a condition where one gene suppresses the effects of another non-allelic one.  In other words two different dominant mutations are present but only one will show   When this happens the gene is said to be epistatic.  Spread which makes a blue pigeon black is epistatic to the gene for pattern since the solid black color will mask the black markings of the pattern gene.  Recessive red is another good example since it will mask most other color and or pattern genes.  White which is the absence of pigment is not effected by either the effects of spread or recessive red. 

The following terms are more common or at least self explicit. 

Express: To have or show an effect.   A dominant gene is able to express itself while a recessive gene remains hidden if paired with a dominant gene.  

Expressivity: The amount of effect. Usually refers to less then a full expression due to co‑dominant genes or modifiers.  

Factor: also Gene: A gene. A part of a chromosome that effects a certain characteristic, for example the gene or factor for feathers.  

Gene Pool: All the genes in a set of birds. Examples, all the pigeons in your loft that are available for breeding are a gene pool.  

Modifier: Secondary genes that modify the expressivity of a co-dominant or dominant gene.  

Recessive: Of relating to a trait that is expressed only when the determining allele is present in the homozygous condition.  A gene that is not visible when paired with its wild type or more dominant allele.    A gene that is not visible when paired with other genes.  The condition which is opposite of dominant.    Recessive red and recessive opal are examples of recessive genes.

Wild Type: The standard phenotype and or genotype, which are used to measure all gene mutations or changes from the standard.  It’s basically a base line or reference point to identify the mutations. 

Okay, between this article and the two preceding ones I think we covered all the basics.  I know this one was very dry but we had to go through it to get onto other more interesting areas of genetics.  If you have made it this far you should have a good understanding of the terms used.  Remember next month and we will go into the sex-linked mattings that will allow us to pre identify the gender of the young by their color.  This is one area where genetics can be beneficial to us racing enthusiast.  It can also be useful in reviewing a pedigree for its truthfulness and I’ll show you how its done.