Concept of breed, strain, variety, type:

we define a breed as a collection of individuals belonging to the same animal species, which are possessed with the same hereditary formula and distinguish themselves from other groups within the same species for a number of morphological and functional characteristics which can be transmitted onto their progeny" (Borgioli E., 1978).
The morphological and functional characteristics of a breed, though, are not immutable but are subject to variations as a consequence of human selection and environmental conditions in which they are bred.
A breed, therefore, is not a static entity, but a continuously evolving one. In fact, in consequence of selection (whether natural and/or induced by humans), of the appearing of mutations, and of environmental interaction, there takes place the so called genetic differentiation by which we observe a progressive variation of the original characteristics of the population to which the same breed belongs.
Domestic animal breeds, in fact, however genetically pure as far as a number of characters are concerned, are at all effects more or less numerous populations that present within themselves a relevant level of genetical variability and therefore a series of different genotypes, even though they are similar as far as the manifestation of characteristics is concerned.
In general, therefore, to the same phenotypes may and will correspond different genotypes (and vice versa). This "constitutes the greatest obstacle to obtaining the genetic improvement of breeds, because of the difficulty in identifying and isolating those specimens which not only present valuable characteristics, but are also genetically pure, and therefore able to transmit to their offspring the characteristics that are desirable to breeders " (Borgioli E., 1978).
"As a rule, one cannot claim that any breed is superior to all others, because this supposedly superior breed will provide full proof of its great breeding value only in ideal environmental and especially nutritional conditions; whereas, by less favourable environments, food scarcity, poor anumal husbandry, or the transmission of infective or parasitary diseases will cause the same breed to be economically less productive and useful than local breeds." A more modern concept of breed has been later associated to that of breed standard, that is to say to a reference model where the morphological features as well as the productive capacities specific to the breed are established. This concept of breed is inspired by a certain established set of ideals and does not always reflect the genetic reality.
The different breeds can be classified as:
· Primitive or traditional breeds (i.e., populations derived from wild species by means of environmental selection). Such breeds are constituted by specimens which have not gone further than the first post-domestication phases, and are therefore characterised by a great morphological variability.
· Secondary or standardised breeds (i.e., deriving from the precedent ones by selection of certain types or characteristics). Such breeds have been created more recently and present therefore a more reduced variability due to the reproductive isolation which they have undergone.
· Artificial breeds (i.e., derived from the combination of secondary or of secondary and primitive breeds). These breeds possess great productive potential, but reduced variability.
· Mendelian breeds (i.e., selected on a single gene basis).
(Fortina R., 2000)
Within a breed, in fact, there can be groups of specimens having in common one or more secondary hereditary characteristics, which have evolved as a result of environmental factors (climate, food, animal husbandry, etc.) and enhanced by selection.
These are the so called strains.
The following example will help clarify this concept:
The Leghorn, a poultry breed, has undergone selective actions based on different orientations, i.e. carried out aiming at certain specific results. Such actions, therefore, took place in consideration of the various morphological and functional characteristics in the selected individuals, and within different breeding environments.
This has brought about the creation of a number of strains:
· Canadian Leghorn
· English Leghorn
· German Leghorn
· American Leghorn
· Production Leghorn
· Italian Leghorn (Livornese)
· Dutch Leghorn.
we have employed the term "strain" as per the above definition. However, we have to remember that according to past animal production sciences, this term indicated the group of primitive breeds, from which breeds at large had been created.
Within a single poultry breed it has become customary to operate a distinction in sub-breeds or varieties, which distinguish themselves exsclusively by the colour of the plumage and/or the form of the comb, maintaining at the same time unaltered the remaining phenotypical characteristics they have in common.
It is finally necessary to clarify the term type, that represents the specific characteristics of an individual specimen's construction. Such characteristics reflect the peculiar productive functionality of the aforesaid specimen.
The definition of type rests upon the observation of morphological and physiological characteristics as are defined, for instance, by the relationship between length and width, by the measurement of muscular mass, by bone and skin refinement or lack of it, by abdominal volume, etc. and by an evaluation of the estimated functional abilities of the examined specimen (i.e., by its estimated productivity). We are thus able to distinguish between poultry breed which present a meat type , or a laying type, or a double purpose type, along with a certain number of types that, once their original purpose ceased to exist, are classified among ornamental poultry breeds (game, bantams).
The classification of the progeny obtained within a determined breed is based upon the expectation that every single specimen displays the production and performance abilities which are specific to the breed in question. In the practice, however, we find that every single individual may present a fluctuation of performances, due to the more or less developed homogeneity of the population constituting the breed itself; to due, also, to the extent the breed characters are fixed (and therefore transmitted in higher or lower degree to the offspring); and finally, to the specific characteristics of the individuals and to the reaction of their genetic inheritance to environmental conditions.
In particular, as far as the ability is concerned for every specimen to transmit the breed characteristics to its offspring, the individual qualities of the same specimen play a primary role.
The complex of characteristics an individual is potentially endowed with, as being transmissible by the genes contained in its own chromosomes, constitutes its genotype.
Such genetic information, however, is not inheritable as such by the offspring; it can however determine the reaction norm within which it manifests itself, based on the environmental conditions the same individual is born, grows, and produces.
Therefore, what we can ascertain in a specimen by means of observation and analysis is its phenotype, that is to say, that complex of charactericstics which are deeply rooted in the same individual as being transmitted by its parents (hereditary or genetic inheritance) and which have come to display themselves through interaction with the environment in which the same individual specimen is raised. In fact, the genotype simply determines the answers, or reactions of an organism to the environment; and as the variety of living organisms on the earth is extremely large, it derives that the same genotype, undergoing different environmental conditions, can display a variety of phenotypes (Borgioli E., 1978).

Texts by Dr. Alessio Zanon, Ph.D.; Translation by Dr. Stefano Bergamo

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