Quantitative Traits and Selection Methods
By
Brian Reeder
Quantitative traits are common in the phenotypes of domestic fowl. Unlike qualitative traits that produce the classic 1:2:1 pattern of inheritance, quantitative traits vary over a continuous range and are the result of alleles of two or more genes. Large numbers of birds are needed to select for traits of a given preferred combination. In observing a group of birds, one should note the range of expression of a given trait. As an example, let us consider the single comb for a moment. When I say single comb, we all have a basic agreement about what that word means. It is a blade comb, flattened with triangular teeth or points at the top row. In this basic regard, the description is exact, but we all know from observation and experience that the expressions of the single-comb can range tremendously. Combs can be huge and tall and very thick, or they can be large and thin, flopping easily and very susceptible to frostbite. There are tiny single combs that are thick and tiny single combs that are very thin in width.
Some single combs are rough while others are smooth. The numbers of points varies widely as does the exact size and shape of the points and the blade section on the back. Folds, lines, creases and many other strain-specific traits are also seen on this comb type. Some lines of single comb birds are deeply homozygous for their phenotype expression and their single comb expression breeds true and may be very prepotent in outcrosses. Other lines are segregating for a given number of traits and so their single combs are not of one consistent form. Consistent gene expression in the phenotype implies homozygosity for the alleles in question.
As an example, let us say you want to make a small thick comb that does not get frostbite. You have set out a parameter for a trait that may represent more than one gene. You begin by selecting those birds that express the individual traits you want to combine as well as those that are coming closest to the ideal expression.
As you blend each generation, you are looking for intensifications of traits as well as further recombination, working to bring all traits together as homozygous in one population. In each generation you will be looking for an increase of percentage in the given areas of selection, with an eye toward a total increase of multi-trait expressing individuals.
In our single comb example, you would note and select those with rough combs, small combs and thick combs. Those that had two traits combined and those that had three traits combined would also be noted and they would be given some level of preference. Multiple mating schemes could then be employed for both blending traits to get the multi-gene recombinant homozygotes and for intensifying the expression of homozygosity in recessives in general. Each generation should show an increase in the desired traits if your matings are well planned and you know what you are looking for. Each population or line within the over-all group is scored for every trait in the set of traits being selected for. In this way, the percentages of increase for any trait can be gauged in each line of the population.
To manage quantitative selection you need to pay attention to trends in the population. Those birds that show the greatest expression of desired traits are the most likely candidates to further express the trait and for enhancing expression into a more extreme (homozygous for a very specific combination) expression. This is easily done when the background genetics support the expression of the desired traits, especially if many or all of those traits happen to be dominant factors, making their early expression more obvious. Selection for major phenotype groups of factors may actually be practicing some level of selection on many, many more alleles than the simple explanation of single gene traits would imply.
With recessive genes or when both dominant and recessive traits are involved, the production of homozygotes is necessary to see the recessive effect. This can make selection more difficult. In the case of a recessive trait, pedigree is much more important, as recessives cannot be seen in the phenotype. Thus, you may find yourself frequently working with generations that do not express some or all of your desired phenotypic expressions.
The recombination of phenotypic expression in a multi-gene recessive scenario is difficult and requires a multi-pronged approach, patience, good record-keeping, large numbers of birds, and a focus on homozygotes. In this instance, we may only see very small incremental increases in gene expression for the total expression of all involved alleles for several generations. Yet, as later generations reach high expressions of homozygosity, the numbers will tilt and the population expression will begin to be set and express in high percentages.
In summary, quantitative selection is picking those that look the most the way you want them to look and selecting in that direction each generation. You may need to be patient if you are working toward expression of a large number of recessive traits. With dominant traits, you may get faster results due to being able to visually identify heterozygotes. Select for those birds with the most traits you want and as you see some increase (even if just a five or ten percent increase per generation) then you are heading in the right direction.
Hello! My name is Brian Reeder. I have been interested in breeding domestic plants and animals since early childhood. My main focus of experimentation and research over the last two decades has been domestic fowl. I also work with guppies and plants including Hemerocallis, Iris and Paeonia. I hope to document some of my projects here.
