A Little Background
Most people, these days, have heard of cells.
Most living creatures are made up of small units called cells.
The smallest of these creatures only consist of one solitary cell,
but creatures like humans and horses are made of of millions, or
billions, of cells. And they are so small that a microscope is
needed to see them.
Each cell takes on the characteristics it needs to become the part of the body it makes up.
Our brains are made up of brain cells; our livers are made up of
liver cells; etc. Even bones are made of bone
cells! How does each cell know what to become?
Here is a picture of an imaginary cell,
"magnified" to make it visible to the naked eye:
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The yellow shape represents the nucleus of the cell.
The brown shapes in the
cell represent some of the
other various structures that are inside any given cell.
The nucleus of a cell can
be broadly compared to the head of a body; it's command central.
The genetic material that we need to learn about, to understand
horse colors, is contained in the nucleus of each cell of a horse. |
Cells grow and reproduce according to "blueprints"
that are embedded inside them. The blueprints are made up of
proteins, and those proteins are arranged in incredibly long
strings, which spend most of the time "crumpled up" into tiny X
shapes called chromosomes.
The "X" shapes in the nucleus (see drawing above) represent
chromosomes. (But you don't need to remember that to
understand horse colors.)
Squashed into each chromosome are
the incredibly long strings of protein-blueprint-commands, arranged in
pairs. To grasp this, we can imagine that they look something
like this, but much longer:
You don't need to memorize all of the above. Just remember this summary:
The "blueprint commands" for how the
parts of a mammal develop
are arranged in
PAIRS, which we call GENES.
Pairs of Genes
Genes exist in pairs because mammals, such as horses (and
humans), need two sexes
-- male & female -- to reproduce.
When horses (and humans) reproduce, the X-shaped chromosomes unravel
in the cells used for reproduction, and split in half. Each parent gives half of its
"string of commands" to the foal (baby):
+
... and when the half-strings join together to
make a new foal (or baby), the result is PAIRS of GENES again, or
...
=
This is how each
parent horse gives their foal half
of its genetic material, or
GENES.
The foal will always
receive
half of its dam's genes,
and half of its sire's genes.
The chains don't split
neatly down the middle, though;
the pairs (almost always) split independently from each other.
The process might be better imagined like this:
Notice that the foal
still gets one gene at each location from each parent,
ending up with two at each location.
[Note to those
knowledgeable in genetics:
I've adopted the common usage of "gene" instead of "allele" for this
extremely simplified presentation.]
Each
pair of genes
determines one characteristic, or trait.
Each cell of the new
foal, from the fertilized egg until the foal matures and dies,
takes
its directions from these blueprints.
Warring genes?
Q. If the foal's
blueprints come from the genes from both parents, how does it decide
which parent to "take after"?
A. You've probably heard
the terms "dominant" and "recessive". This is how order is
made of possible contradiction, and it is explained below.
HOMO-
Sometimes the directions
from the mare and the stallion for a characteristic are identical. In that case,
there is no question that the foal will have that
characteristic. There is no conflict.
IDENTICAL genes in a
pair is called being HOMOZYGOUS for the trait
(HOMO means same, ZYGOUS refers to the fertilized egg.)
HETERO-
However -- and this is
where we learn the most important part of the equation -- when the
directions from each parent are DIFFERENT, the cell has to be able
to tell which blueprint to follow. (See how, below.)
DIFFERENT directions for a
trait are called HETEROZYGOUS
(HETERO means other.)
Here is an imaginary diagram to
help us picture homozygous and
heterozygous genes:
Homozygous
genes, where the pair is like identical twins,
are represented by the two ovals in a pair having the
same color (circled in blue).
Heterozygous genes,
where the two genes in the pair are different from each other,
are represented by the two ovals in a pair being
different colors (circled in fuchsia).
When there are two
different genes in a pair:
the gene/blueprint that gets
followed
is the DOMINANT
gene.
the blueprint that gets
ignored
is the RECESSIVE
gene.
Dominant genes are written in upper case: E, A,
Cr, D, Ch, Z, etc.
Recessive genes are written in lower case of the SAME LETTER: e, a, cr,
ch, d, ch, z, etc.
The same letters are
always used at the same location, that is, within a pair;
variations are shown by upper or lower case,
or by adding superscript to the same letters (Aa, aa, AAt)
Note: The dominant gene is always written before the recessive gene,
no matter which parent it comes from, such as Ee or Aa
(eE or aA, for
example, are not used)
Coming up next: looking at ONE
ACTUAL PAIR
of our imaginary string of genes: PIGMENT
This first pair we'll
study will be the pair of genes that determine what pigment
is in the horse's hair. (We've just chosen any old pair for
this imaginary picture. The good news is that we don't need
to know WHERE on the string the pair is located in order to know
how they work ! )
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To continue learning simplified Horse Color genetics,
please click Pigment,
below.
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