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Genetics 000
Dark Colors

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"Genetics  000" : the Easy Way to learn Horse Colors

Genetics is an incredibly complicated science. But you only need to grasp a simplified concept of genetics in order to understand horse colors. This simplified concept of genetics is easy to learn.

A Little Background:

Most people 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.  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 our bones are made of bone cells!  These all come into existence as the result of the original fertilized cell dividing, over and over, into more and more cells.  So... how does each cell know what to become?

Here is a picture of an imaginary cell, "magnified" to make some of its parts visible to the naked eye. The dark line around it represents the cell membrane, which holds it together.

The yellow shape represents the nucleus of the cell.  This can be broadly compared to the head of a body; it's "command central".

The brown shapes in the cell represent some of the various structures that we're not going to study, because they don't apply to horse color.

The genetic material that we need to learn about, to understand horse colors, is contained in the nucleus of every cell.

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 their time "crumpled up" into tiny X shapes called chromosomes.

The "X" shapes inside the nucleus (see drawing above) represent chromosomes.

Squashed into each chromosome are 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 to form the sperm (in a stallion) or egg (in a mare) cells.  Thus, 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, like those pictures, above, though;
the pairs (almost always) split independently from each other.

The process might be better imagined like this:



The foal 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 for every single trait 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.


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.)


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.)

Extra credit:   say  "HO-mo-ZY-gus" and  "HET-erro-ZY-gus"

Here is a 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 being 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.  We'll start by studying the pair of genes that controls PIGMENT.

This first specific pair of genes 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 ! )

  • To continue learning simplified Horse Color genetics,
    please click Pigment.

To follow the educational, logical progression of this web site, click "Next", below.

Pigment Agouti

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