What is a Gene?
When we think of the definition of the word gene, we think of discrete units of DNA that have a very specific purpose. These units are the basis of heredity in living organisms. Genes are generally located on DNA, a chain made of similar but subtly different nucleotides abbreviated as A, C, G, and T. DNA never leaves the nucleus of the cell where it serves as a type of reference book that is used to provide information on how to make whatever a cell needs or uses. Like a reference book that cannot leave the library, a copy must be made. The copy is called RNA. RNA has the capacity to leave the nucleus, find a ribosome that eventually results in the formation of the appropriate end product. The process is complex and many factors that affect it are still being worked out.
Any given strand of DNA (or gene) can be modified by factors such as other strands of DNA, RNA transcription, or micro RNA. Interactions can be both cumulative and/or variable. However, RNA and micro RNA effects can be subtle and the results can be extremely difficult to fit into a Mendelian pattern. Many of the effects seen in RNA/micro RNA action can vary depending on the genetic background and the interaction of many factors.
For the layperson, this might all be fascinating, but it can be very difficult to apply in a practical, real-world breeding project. With many of these factors, we simply cannot apply a simple numeric formula. This can lead to a lot of confusion, even disagreement, among various persons within the hobby.
Of the generally described poultry genes, a great many do appear to be very straightforward. This is especially true of many of the autosomal recessives that we see in color and form in fowl. Of the dominant genes described, some seem to be simple, yet others appear to be multi-factoral and variable with modification from any possible background factor(s). Complex gene traits that have no clear dominance or recessive nature, such as eggshell color or earlobe color, seem to be difficult to describe in any Mendelian fashion. Their genetic basis is difficult to hypothesize based upon a strictly Mendelian approach. They are considered quantitative, meaning that the extremes seem to be arrived at through a cumulative effect rather than through the usual Mendelian segregation of extremes of phenotype expression.
In some instances, a group of cumulative factors can be very homozygous and appear to be an autosomal dominant when outcrossed with a non-carrier, but when the F1 is backcrossed to the non-carrier, “less-than-autosomal dominant” actions are seen. Further, each generation that a back-cross to the non-carrier is done, more factors seem to segregate and one sees less and less expression of the so-called “dominant gene”. However, for the hobbyist this may be of little concern. If one is outcrossing a line that fully expresses such a quantitative, multi-factoral effect, two procedures are usually undertaken. First, that F1 would be bred back to the fully expressed line and the “unraveling” would not be witnessed, as the cumulative direction would be toward concentration of the trait rather than toward fracturing and separation of the many factors. Secondly, if the hobbyist has outcrossed to the seeming dominant line but wants to eliminate the factor through using the backcross to the non-carrier, then the fracturing of the multifactoral trait may seem odd and even be inconvenient, but still allows for selection back to the original and desired, non-carrier phenotype.
The hobbyist is usually only outcrossing to bring in new blood and perhaps combat inbreeding depression, so they will rarely work to take a factor apart or see such segregations. The general outcross procedure in the hobby tends to be an outcross to a related phenotype to make an F1, then the F1 is backcrossed to the breed that the hobbyist is working on, generally to directly related individuals from the F1 parent of said breed. In such instances, you would not even be in a position to perceive the “gene” in question as displaying any aberrant behavior. In such an instance, the Mendelian epithet of “dominant” would seem to be so.
It is the person who is creating a new variety or breed that may find the multi-factoral and quantitative factors extremely frustrating and thus conclude that the given genetic formulas or statements are “wrong” or “don’t work”. They are often right too, when only considering the Mendelian context. Breeders working in this area are often left with little in the way of good choices as to materials for varietal or breed development. One may require one specific gene to make the new variety, but be unable to find the material (actual birds) where the one gene can be easily extracted. Such outcrosses will tend to bring a host of unwanted factors that then have to be selected out while the desired factor is selected for. This can present a very challenging and difficult situation and these people are likely to encounter a host of aberrant segregations that are hard to reconcile in a Mendelian fashion. Pity the breeder who reports such aberrant segregation in any forum only versed in Mendelian segregation and solely familiar with “genes” as only discrete units of perpetual absoluteness!
So how does the hobbyist apply this bewildering range of possibilities? Generally, we attempt to find some Mendelian basis upon which to give a standard operating procedure with the advice that there is modification, variable expression and/or variable penetrance. In truth, such statements give very little real information, but they do give the breeder some idea of how to approach his or her breeding goals. In the most direct language, any such statement about a given factor translates to the same practical application; breed in numbers in order to secure the desired resegregants. Further, it also means that if you have not been able to produce the desired phenotype through selecting from large numbers of your own stock, you will need to find stock expressing the desired phenotype and take that material into your own stock through outcrossing your line with them and selecting for the resegregants with the proper background genetic combination. Especially in the case of complex DNA interactions, there may be no way to reproduce the desired phenotype without the proper interaction genes.
A well-known instance of the interaction of several genes is the various expressions of the Pattern gene (Pg) in fowl. Each visually different pattern is a recombinant of several specific gene-traits. Most of these seem to be very straightforward, though some levels of refinement of patterns do seem to indicate more complex actions at work. The variation between a thin, “refined” lace and a thick, “over-colored” lace may show modification factors beyond the typical genes that make the laced pattern. However, for the layperson it is generally enough to know that lacing is a combination of the three genes, Columbian, Melanotic, and Pattern gene.
Further, most any hobbyist is going to know, or will soon be told, that an over-colored line of laced birds that can’t be “cleaned up”, even after generations of trying, needs to be chucked or outcrossed to a clean patterned line. The breeder who simply wants to understand the practical aspects of genetics does not necessarily need to be burdened by a host of very complex and cutting-edge, post-Mendelian quantitative genetics knowledge. Selection has gone on for millennia with no genetic knowledge whatsoever so it is certainly possible. Yet, I would suggest that the more one understands of these processes, the less uncertainty there is about how to proceed.
I would like to point out that each breeder is likely to see instances where the information seems to work only marginally, at best. There is a wide range of variability when one takes the entire
