Text 482, 121 rader
Skriven 2004-10-22 21:42:00 av Michael Ragland (1:278/230)
Ärende: Crossbreeding Pacific Oys
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http://hmsc.oregonstate.edu/projects/wrac/whatisgenetics.html


What is Genetics? 

>From a general perspective, the discipline of genetics is nothing more
than the study of heredity and variation. Similarities between
individuals (or families) is caused by heredity, differences between
individuals (or families) is caused by variation. Genetics simply
attempts to explain the mechanisms and causes for the differences or
similarities between individuals.
However, the general field of genetics is actually a composite of
several unique, historically distinct disciplines, each having the same
goal of explaining the reason for why something is the way it is, but
attempting to do so from a different viewpoint or organizational scale. 

Molecular genetics, for example, places its emphasis on the cellular
level, specifically focusing on the structure of DNA and those
mechanisms that directly affects that structure. DNA (deoxyribonucleic
acid) is the "blueprint code" that tells each cell what to do and when
to do it. Within that code is all the information required for life (for
example, it contains information on how to synthesize, monitor, and
release proteins). DNA also contains information that is not directly
necessary for life but that can make the individual uniquely different
(such as hair or eye color, or how fast they grow). Now, here comes the
neat part. Each nucleated cell possesses an entire copy of all this
information. Different cells may have different parts of the total
blueprint turned on or off based on their cell type, but the information
is still all there. When we refer to the animal's genotype, we are
referring to the genetic makeup of the DNA for the alleles of the gene
of interest.

Two genetic disciplines focus their attention on the next "higher" level
of organization, the individual. They remain separate fields, even
though they are commonly presented concurrently in many college courses,
because their emphasis is based on two distinctly different kinds of
genetic traits. 
Qualitative traits are those traits controlled by one or two (and rarely
a few) genes, none of which are ever directly influenced by the
environment. The effect of each gene is typically "large" and
discernable in nature and overall, will result in discrete, observable,
phenotypic classes. For qualitative traits, the individual's phenotype
(what you see) is a clear representation of its genotype. The genetic
discipline that focuses on qualitative traits is known as Mendelian
genetics. Mendelian genetics is mathematically characterized by
probabilities (including meiotic expectations), gene and genotypic
frequencies. 

Quantitative genetics focus on those traits that are under the influence
of many (hundreds) of loci (plural for locus-the site of a gene on the
DNA), most of which have a very "small" effect on the total phenotypic
value. In addition to the genetic effects associated with the
quantitative trait, the phenotype is directly influenced by
environmental factors. This results in the observed phenotypic variation
appearing continuous in nature. The observed phenotype for a
quantitative trait is the sum of all the genetic effects across all
involved loci and the environmental effects. It is very possible to have
an exceptional genotype raised under poor environmental conditions
appear phenotypically worse than a truly genetically inferior individual
raised under better environmental conditions. In quantitative traits,
individual genotypes are usually not identified, but the expressed trait
is measured for all individuals and genotypic values are then assigned.
Examples of quantitative traits include all growth, performance,
survivability, viability and fertility traits and are characterized
mathematically through means and variances. 

There are two historical fields in genetics that built the foundation on
which quantitative genetics resides. Statistical genetics derives all
the mathematical background for analyzing quantitative traits.
Biometrical genetics (Biometrics) relates the measurement of
quantitative traits through specific crosses (such as backcrosses) that
were commonly used to study Mendelian genetics. These two disciplines
played a much larger role before the advent of the personal computer and
even though they are rarely discussed today, we must appreciate the fact
that we would not have quantitative genetics today (and therefore little
understanding of the genetic improvement we are able to achieve today)
without either field. 
The third organizational tier of genetics is at the population level and
hence, is called population genetics. It focuses on the flow of genes
through a population or differences of gene or genotypic frequencies
between populations. Population genetics does not attempt to assign a
genotypic or numerical value to each individual in the population but
instead utilizes sampling techniques to estimate the means, variances
and covariances that are used to characterize the population. Population
genetics can be loosely thought of as an extension of Mendelian genetics
to the population level.

Until now, all of these genetic disciplines have been "spatially"
oriented. If we add a temporal context and look at how the genetics of a
population change over long periods of time, we have evolutionary
genetics. To the prospective breeder this has no impact on their bottom
line and hence, will not be discussed further.

Even though each field may be conceptually viewed as a "distinct"
discipline, there is in reality substantial, growing overlap among all
fields. Advances in technology over the last 20 years have resulted in
the creation of "wide" informational conduits, effectively connecting
the once distinct fields into the more homogenous field of genetics we
are familiar with today. These relationships are depicted in the linked
diagram.

Modern breeding theory utilizes concepts that originated from all three
genetic-oriented spatial-organization levels. As a result, to be
successful in our business decisions, we need at least a cursory
understanding of the basic principles involved in molecular, Mendelian,
quantitative and population genetics.

"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well with it."

Stephen Hawking
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