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Coat Color Genetics
Have you ever wondered how we can predict what color puppies will be born in a litter of Labradors? Or questioned  how two black
parents can produce yellow or chocolate offspring? This is the page that will answer your questions. First, let's talk a bit about basic
genetics.
(Click here to skip to the Color Prediction Charts.)

A "gene" is a specific combination of DNA on a chromosome that codes for a certain trait. It may control the size of your
earlobes, the thickness of your hair, or in our case, the color of fur. Some traits are controlled by more than one gene - they are called
polygenetic ("many gene")  traits.

Each gene is made up of two alleles. One is inherited from each parent. There may be many different alleles for each gene that exist
in a species. For example, there is an allele each for blue eyes, brown eyes, green eyes, hazel eyes, etc - but you only end up with two -
one from each parent.

In most cases, the expression of a gene is determined by the dominance of the particular alleles. When one allele expresses even in
the presence of a different second allele, it is considered dominant. For example, brown eyes are dominant over blue eyes - so if you
inherit a brown eye allele from your mother, and a blue eye allele from your father, your eyes will be brown. The blue eyed allele is
considered 'recessive' because it requires two copies in order to be expressed. For some traits, expression is more complicated than
that, but for the purpose of our conversation, this is all you need to know.

Coat color in Labradors is controlled primarily by two genes. One gene contains alleles for either black or chocolate fur (and
pigment).  The other gene is what controls the expression of yellow fur color.  First, let's consider the two genes separately -then we
will talk about how they work together. The gene that controls black or chocolate is called the "B" locus. Since we already know that
each dog can have only two alleles for each gene, we can figure out that, at the B locus, a Lab will  have alleles that are either black,
chocolate, or one of each.  Black is dominant to chocolate, so that means if he has one chocolate allele and one black allele, he will
appear black in color. Labs require two chocolate alleles in order to express chocolate colored fur. Labs with only one recessive
chocolate allele are said to 'carry' chocolate, or be 'chocolate- factored' , because they can pass it on to their offspring, even though
they do not appear chocolate themselves.

At the site of the yellow gene, known as the "E" locus, the dog will have alleles that are either 'on' for yellow, or 'off' for yellow. There
are no other color alleles associated with this gene. The dog must have two 'on' yellow alleles in order to
express yellow colored fur.  Because two 'on' alleles are needed,  they are considered recessive to the 'off' alleles. (By the way, yellow
ranges from white all the way to fox red. These are all still considered yellow and are controlled by the yellow gene. The darkness of
yellow is actually controlled by other genes called amplifiers, but we won't get into that. It's enough to know that yellow varies in
shade, but it's technically still yellow). Labs with only one 'on' yellow gene are said to be carriers of the yellow gene, because they can
pass it on to their offspring, even though they do not appear yellow themselves.

Wait a minute! Hold on here. How can a dog be black or chocolate AND yellow at the same time???

They can't. It turns out that the yellow gene is a 'masking' gene- that means, when it is expressed, it over-rules the expression of the
other gene (black or chocolate). So your dog could have 2 chocolate alleles, but the two yellow alleles will be expressed instead.
However, if your dog has only one  or zero 'on' yellow alleles, the color from the other gene (black or chocolate) will be what you see.

In genetic terms, each allele is given a letter. Lower case letters are used to indicate recessive alleles, and upper case letter are used
for dominant alleles. In labs, B is used to denote dominant black, and b is used for the recessive allele of chocolate (B of course was
chosen because the gene is at the B locus).Yellow, at the E locus, is denoted by  E  for 'off' yellow, which is dominant to e - 'on' yellow.  
Every lab will have two alleles at the B gene and two at the E gene. For example, let's consider Ruger - his genetic color coding would
look like EEBB. He has two 'off' yellow alleles, and two dominant black alleles. He appears black in color. Onyx is another black Lab,
but her color coding is a little more complex - EeBb. She has one 'off' yellow allele, one 'on' yellow allele, one Black allele, and once
chocolate allele. She is black in appearance because B black is dominant to b chocolate, and the yellow masking gene is not activated
because only one allele is 'on'.

So why do we care? Knowing what the genetic code of your dog is allow you to predict what color puppies they will have when mated
with another dog of a known genetic code. The genetic coding (ie. EeBb) is called the 'genotype' of the dog -how they actually look
(black, chocolate or yellow) is called the phenotype. If you know the genotype of your dog, it can help you predict what the phenotype
of the puppies will be. It will also give you a clue as to what the phenotype of his parents might have been, if this is unknown.

Below are the different colors of labradors and their possible genotypes. You should be able to see why I abandoned art in favor of a
career in science.
Pigmentation refers to the color of the dogs skin, particularly on their nose and around their eyes. Black labs always have black
pigment. Chocolate Labs always have chocolate pigment. But yellow labs may have either, depending on the alleles they carry at the B
gene site.

Yellow labs with at least one Black allele will have black pigment (because it is dominant). Yellow labs who have two b chocolate
alleles will have brown pigment. Yellows with brown pigment are often referred to as 'dudley' labs.  In our diagrams, yellow Labs with
chocolate pigment will have brown squares on their nose for the pur oses of the diagrams above to denote their brown pigment. There
are no health or behavior issues linked to coat color or pigment of the three proper Labrador colors (although there are some health
issues linked to certain mismarks such as grey colored labs),   but  Dudley labs may not be shown as it is a disqualification in the
breed standard.

Carriers are dogs who 'carry' a recessive allele but do not express it themselves. In the diagrams above, we picture carrier dogs by
having a box of a different color located in their belly.

The Punnett Square

The Punnett Square is a simple tool used to predict in what combinations alleles will be inherited. Let's look at an example. Keep in
mind that each puppy gets ONE allele for each gene from each parent, for a total of two alleles for each gene.

In our example, we will breed a handsome tri-factored black lab to a yellow lab. The genotypes for each parent would be EeBb and
eeBB respectively. The first thing we need to do is figure out, for each parent, the different combinations of alleles that could be
inherited. Let's start with dad.









Puppies could get the yellow 'off' allele paired with the Black allele ------- EB
They could get yellow 'off' paired with chocolate allele -------------------------Eb
They may end up with yellow 'on' and Black ---------------------------------------eB
or they could get yellow 'on' and choclolate ----------------------------------------eb

Mom's turn - she is yellow, with a genotype of eeBB







As you can see, there is only one possible combination of alleles from mom --------------eB

Now, we build our Punnett Square. It's like a multiplication table for geneticists. Along  the top, let's plug
in the alllele combinations that could be inherited from the stud dog. The left hand column is for the
dam's alleles.








Next, we figure out what will happen when each puppy inherits their alleles from mom and dad.










So this represents the odds of each puppy coming out a certain color. We also use this to represent the expected ratios of puppies in the
litter- in this case, we will be expecting 25% Black puppies who carry yellow, 25% Black puppies who carry yellow and chocolate,    
25% yellow puppies, and 25% yellow puppies who carry chocolate. Naturally, every litter does not come out in these exact proportions-
it does usually average out over time. Notice that even though the sire is tri-factored, he did not produce any chocolate puppies because
the dam does not carry a chocolate allele that can be passed down.

Click
HERE to see a  comprehensive matchup of all possible litter combinations, and their results. This took a long time to produce, so
please do not copy it onto your website. Feel free to link to our page if you would like to show your visitors this information.
THIS is a
link to Dr. Sheila Shmutz coat color page. She is THE professor of coat color whom I was lucky to study under, and who has done
much of the genetic research to determine the information I've shared above.
  EB
Eb
eB
eb
eB
       
  EB
Eb
eB
eb
eB
EeBB
EeBb
eeBB
eeBb
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