each of their genes. And for this same reason, the
most usual scenario is that each individual should
be a combination of the most characteristic traits
of their dam and sire.
GENES ARE REGULATED BY THE ENVIRONMENT
If all this were to work in a simple linear manner,
what would happen is that animals with the same
genome would be exactly alike.
However, as we have seen this year during the
high-goal season in Argentina, individuals that
are genetically identical, as is the case of clones,
could present small phenotype differences.
And this occurs because the genes of each horse
interact with the external environment which
molds the way in which genetic combinations are
expressed in its phenotype. In order to explain this
situation, we must bear in mind the environment
in which the animals have developed, pointing out
that when geneticists speak of environment, we
are not referring to climate only, but to all external
stimuli that may have a bearing on the animal’s
development. Among this group of stimuli we
include, for example, the way in which it was tamed;
the type of training; what nutrition it received; if
it underwent diseases or injuries; the climate in
which it was raised, and an almost interminable
list of small etceteras. And it is also important to
emphasize the fact that these interactions include
those that the offspring receive during gestation
and during those first years in which animals are
most sensitive.
Even taking into account all these factors, we
still cannot reach a round figure, because it has
been determined that adding all the effects of
an individual’s genome and the environment that
surrounds it, it is still impossible to explain its
phenotype by 100%.
This is the reason why a few years ago the
term “EPIGENETICS” was coined (“epi” means
“above”), to describe DNA alterations that occur
without changing the nucleotide sequence. This
new section of genetics is what is used to try and
close the circle that would explain the mechanisms
that “mold gene expression” so as to produce a
certain phenotype.
EPIGENETIC REGULATION
If we go back to classic genetic theory, presumably
no phenotype change that has been produced by
the environment is heritable by its descendants.
This is so because these observable changes
were not produced by an alteration in the genetic
sequence of the horse, but by the way in which the
environment caused this genome to express itself.
For example, a foal that has been underfed as it
was growing up, and has therefore grown less than
expected, should not necessarily produce smaller
offspring because its DNA has not undergone any
change.
In order to understand this we must know that the
horse’s genome is expressed through a process
in which the DNA is read in each cell and turned
into proteins in charge of producing the rest of the
molecules that make up the organism.
Control of this reading process is given by cellular
mechanisms that enable or make difficult the
access to the machine that reads DNA strands in
which the genetic codes that shall regulate the life
of the animal are found.
Thus, when a coded protein in a gene is needed for
a metabolic process as, for example, bone growth
in a colt during its development, these regulating
factors make reading of certain specific genes
possible, allowing an increase in the amount of
bone cells produced. Once development has
concluded, these growth genes are “silenced” and
are no longer used by the cells of the adult horse.
Likewise, the expression or inhibition of certain
genes is the cause of having such different cells,
such as a neuron or a muscle fiber, even though
all of them maintain identical DNA nucleotide
sequences. These processes are known as
“epigenetic regulation”.
One of the most important epigenetic mechanisms
is DNA METHYLATION, which consists of a
chemical mark added to specific regions of the
genome, which make them less accessible and
therefore diminish the expression of these genes,
to the point of even being able to silence them
completely.
Therefore, depending on what tissue we are
dealing with, or the age of the animal, or external
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