Understanding Basic Rat Genetics

         Ok, so you've got your rats.  You've done some research, and you think you have a market for babies.  Or maybe your just curious about the makeup of your little friends.  Genetics!  There's a lot of info out their, and it's not all easy to understand.  Believe me, I've read some of it.   Some of it doesn't even agree with what the last thing you read says.  I've wanted to put together a simple collection of some basic information for a while.  My goal here is to create a starting point for people that are interested in genetics, and/or would like to breed their rats.  That said, I'll try and keep it as easy to understand as possible.

Basic Genetics   Basic Colors     Simple Markings   Simple Types
Basic Genetics:

    Before we can delve right into the genetic make up of your rat, it's necessary to understand a little about genetics.  Thankfully, a great deal of the basic traits are expressed through simple dominate and recessive genes.  That's great, but what does that mean?
    DNA is the basic material that composes who and what we are.  DNA is wound into chromosomes, and chromosomes are found in pairs inside our cells  Rats have 21 pair of chromosomes.  When sex cells are produced only  one half of each pair go into each cell, so a sperm would have 21 individual chromosomes, and an egg would have 21 individual chromosomes as well.  When breeding occurs, the sperm and egg fuse, and the resulting embryo has 21 pairs of chromosome.  One half of each pair the father, and one half from the mother.
    One each chromosome there are many genes.  The location of each gene is called a locus (loci plural) and is named in the scientific world with a letter such as "x".  There multiple types of possible gene for each locus, and the different types of gene are called alleles.  For example you may have blue, brown or green eyes.  They are all caused by the gene for eye color, but there are 3 different alleles.
   Fortunately, as I have said before, many rat traits are composed of genes that only have two alleles. . When you breed two rats, the babies get two alleles of each gene, one from the mother and one from the father. Each allele may be either dominate or recessive.  With a dominate trait, you only need one allele present in order to see that trait, however with a recessive you must have two of the recessive alleles.  We use capital letters to represent the dominate trait (X) and lower case letters to represent the recessive one (x).
    When you talk about what a rat looks like your referring to it's phenotype.  This is actually how the rat looks.  When you talk about the genes he possesses your referring to the genotype.  This is what he is genetically.  Some rats can be phenotypicly the same while genotypicaly different.  For example an agouti rat can be either AA or Aa.  These rats look exactly the same, but one of them carries the recessive gene for black.
    The actual genotype of a rat is very long as it contains an allele for every trait even if it is not expressed.  A portion of a solid black rats genotype would look something like aaGGDDBBCCMMPPRRPePeHH.  In order to simplify things we usually only show the genes that we are interested in; therefore, when talking about color and referring to a black rat that didn't carry any other colors we would call him aa as opposed to that mess above even though they are the same rat.
    Ok, lets look at it with some examples and see if it makes sense.

Basic Colors:
    First of all, all rats are black or agouti based.  That means that if you were to take away all the color influencing genes, every rat would be either black or agouti. Most other colors are caused by recessive genes, that act on or "dilute" the base color.  Because they are recessive, both parents must have at least one of the recessive alleles in order for the babies to inherit that color.  This is why, when you breed two rats of unrelated colors (i.e. blue and Siamese), you often get black or agouti babies.
    Because of this, it is necessary to understand the basics of the colors you wish to work with BEFORE you start breeding.  Otherwise, you could end up with lots and lots of black rats that nobody wants.  (I personally have a soft spot for solid black rats, but I have found that they are harder to place.)
    All other colors build on these base colors.  Agouti based rats give us our ticked colors (such as fawn and cinnamon), where as black based rats give us our non ticked colors (such as beige or mink).

Agouti and Black  American Blue    Russian Blue  Mink and Cinnamon   Fawn and Beige
  Champagne and Amber     Pearl     Platinum    Siamese and PEW

Agouti and Black:
    These traits are represented by the letter "a".  Agouti is the dominate form (A), and black is the recessive (a).  A rat with at least one "A" will be agouti, where as a black rat must be "aa".  When we have a rat with the dominate phenotype (in this case he's agouti) but we do not know whether the second allele is dominate or recessive it may be shown as "A*".  The "*" just states that the other allele is unknown.
     This said, when you breed two black based rats together you will always get black based babies (or non ticked colors).  When breeding agouti rats, it depends on the second allele.  An "AA" rat will produce nothing but agouti based (or ticked colored) rats, because it can only donate the dominate allele to it's offspring.  An "Aa" rat could donate either allele and could potentially have either ticked or non ticked babies.

A* (AA or Aa) = agouti
aa = black

 Back to Basic Colors

American Blue (Slate Blue):
    Ok, now working on the base color, other genes are added and act to dilute the color in some fashion.  In this case, the genes that cause American blue are represented by "g".  (Don't ask me, I'm not the one that chose the letters...lol).  The "G" allele is the dominate one, however this is not the one that causes the American blue coloring.  It is in fact the recessive allele, "g", that dilutes the base color to American blue.
    Now we know that the base color is either agouti, A*, or black, aa.  When you add two g alleles to these base colors you get American blue agouti and American blue, respectively.  Because American blue is recessive, a rat can carry the g allele without expressing it.  We would call this animal an American blue carrier.
    There are several ways you can get American blue when breeding. (In this section when referring to American blue, I am also including American blue agouti.  The difference is only relative to ticked or non ticked coats as stated above.  When inheriting the g alleles both will act the same).
      The most sure way to get American blue babies is to breed two American blue parents.  Because both parents are gg, there is no other allele to be donated, and therefore all babies will be American blue.  (one g comes from the mother and the other g comes from the father).  gg x gg = gg.
    You can also breed an American blue to an American blue carrier.  Since the American blue rat is gg and the carrier is Gg you will get both American blue rats and rats that aren't.  Gg x gg = Gg and gg.  Statistically each baby has a 50% chance of inheriting the American blue allele from the carrier and therefore being American blue.  The babies that do not inherit the g allele from the carrier will be carriers themselves as they must inherit it from the parent that is gg (American blue).
     Finally the third way to get American blue is to breed two carriers.  Each parent my donate either the G or the g to the offspring.  Gg x Gg = GG, Gg, gG or gg.  Therefore, statistically, there is a 25% chance the baby will be American blue, a 50% chance it will carry American blue, and a 25% chance that it will neither be nor carry American blue.  The only way to know which of the non blue babies carries the g allele is to breed them to a gg rat, Because phenotypicly they will look just like the GG ones.
    When breeding an American blue rat to a rat that does not carry the g allele, all offspring will be carriers.  gg x GG = Gg.

A*gg = American blue agouti
aagg = American blue
A*Gg = Agouti carrying American blue
aaGg = Black carrying American blue
A*GG = Agouti
aaGG = Black

 Back to Basic Colors

Russian Blue:
    Russian blue and American blue are genetically different, and therefore will not  produce blue babies when bred together  (unless of course the parents are carriers of the other type of blue).  Unfortunately, not everyone knows this, and will try to cross the two blues and be confused when all the babies turn out to be black or agouti.  Again this is because both are just dilutions of the original base color.
    Russian blue is also a simple recessive, and it is represented by  "d", where  D is the non dilution allele and  d  is the one causing the dilution.  It works much the same as the gg blue does.  A dd rat is Russian blue, a Dd rat is a carrier, and a DD rat neither carries Russian blue nor is Russian blue.
    The breeding possibilities and statistics would be the same as if breeding American blue.  In fact all simple recessive traits will behave in this same manner.  dd x dd = dd,     Dd x dd = Dd, or dd,
Dd x Dd = DD, Dd, dD, and dd, and DD x dd = Dd, and so on.

A*dd = Russian blue agouti
aadd = Russian blue
A*Dd = Agouti carrying Russian blue
aaDd = Black carrying Russian blue
A*DD = Agouti
aaDD = Black

 Back to Basic Colors

Mink and Cinnamon
    Cinnamon is simply the agouti based version of mink.  Mink also acts as a simple recessive, and is represented by the letter "m".  M is the non dilution allele and m is the one causing the base color to be diluted to mink.  Again you can get mink by breeding two mink rats (mm x mm), a mink rat to a carrier (mm x Mm), or by breeding two mink carriers (Mm x Mm).

A*mm = Cinnamon
aamm = Mink
A*Mm = Agouti carrying mink
aaMm = Black carrying mink
A*MM = Agouti
aaMM = Black

 Back to Basic Colors

Red Eye Dilute

    Another very common simple recessive is the red eye dilute (RED).  This is what gives us the colors of fawn (agouti based) and beige (black based). R is the letter used where R is dominate and r is recessive.  It is the recessive allele that causes the dilution.

A*rr = Fawn
aarr = Beige
A*Rr = Agouti carrying RED
aaRr = Black carrying RED
A*RR = Agouti
aaRR = Black

 Back to Basic Colors

Pink Eye Dilute
    Just like RED, the pink eye dilute (PED) is a simple recessive that dilutes the base color.  We represent PED with the letter "p" where P is the non diluting form, and p causes the dilution.  PED is responsible for amber (agouti based) and champagne (black based).

A*pp = Amber
aapp = Champagne
A*Pp = Agouti carrying PED
aaPp = Black carrying PED
A*PP = Agouti
aaPP = Black

 Back to Basic Colors

    Pearl works differently than the colors we've looked at so far. It is multifactorial, meaning that you need more than just one thing in order to get that trait. Pearl is expressed in the dominate form, but only on mink based colors.  Using Pe to represent pearl, a rat must be Pepemm in order to be pearl.  When both dominate pearl alleles are present it is a lethal combination, and the baby will die before it is born.  Because pearl is dominate, a mink or cinnamon rat cannot carry pearl.  It will either be pearl or not have the gene at all.  This said, because pearl only shows up on mink, a non mink rat could carry the pearl gene.
    When breeding there are several combinations that could give you pearl.  Your best bet is to breed a pearl rat to a mink based rat (mink or cinnamon).  This would be Pepemm x pepemm.  The possible outcomes of this cross would be Pepemm and pepemm.  Each baby would have a 50% chance of inheriting pearl, and all of them would be mink based.  The only other option would be to breed a non mink based rat that carries mink and the pearl gene (PepeMm).  This rat would need to be breed to a mink based rat (mm), or a rat carrying mink (Mm).  When you breed two pearl rats together (Pepe x Pepe) there is a 25% chance for non pearl (pepe), a 50% chance for pearl (Pepe), and a 25% chance of lethal (PePe).

A*mmPepe = Cinnamon pearl                               A*mmPePe = lethal
aammPepe = Pearl                                               aammPePe = lethal
A*M*Pepe = Agouti rat carrying pearl                  A*M*PePe = lethal
aaM*Pepe = Black rat carrying pearl                    aaM*PePe = lethal
A*mmpepe = Cinnamon
aammpepe = Mink
A*M*pepe = Agouti
aaM*pepe = Black

 Back to Basic Colors


   Platinum is also a multifactorial color.  In this case, the color is caused by the combination of two simple recessive dilutions.  You can get platinum one of two ways, and both involve diluting American blue farther.  This can be done either with mink or with RED.  A mink based platinum would be ggmm while an RED based platinum would be ggrr.  Breeding a mink based platinum  (ggmmRR) to a RED based platinum (ggMMrr) would simply give you American blue rats that carried both mink and RED (ggMmRr).  Therefore in order to get platinum, you must know the type of platinum that your rat is, and breed to a compatible rat.  For instance, you could breed a ggmm platinum to an American blue that carried mink (ggMm) and have a 50% chance of getting platinum and a 50% chance of mink.
A*ggmm = Platinum Agouti                                       A*ggrr = Platinum Agouti
aaggmm = Platinum                                                  aaggrr = Platinum
A*ggMm = American blue agouti carrying mink       A*ggRr = American blue agouti carrying RED
aaggMm = American blue carrying mink                  aaggRr = American blue carrying RED
A*Ggmm = Cinnamon carrying American blue          A*Ggrr = Fawn carrying American blue
aaGgmm = Mink carrying American blue                  aaGgrr = Beige carrying American blue

A*GgMm = Agouti carrying both mink and American blue            A*GGMM = Agouti
aaGgMm = Black carrying both mink and American blue               aaGGMM = Black
A*GgRr = Agouti carrying both RED and American blue
aaGgRr = Black carrying both RED and American blue

 Back to Basic Colors

Siamese, Himalayan, and Pink Eyed White (Albino)
These colors are a bit more complicated.  All three are cause by  alleles on the same locus, but in this case there are three alleles, not just two.  C, the non diluting dominate allele,  c, the recessive allele for albino, and c(h), the recessive allele for Siamese.  Ok, but then where does Himalayan come from?  Genes on the C locus exhibit co-dominace.  This means that when they are present together, neither one is dominate over the other, but the blend and share their traits.  A Himalayan would be a mix between Siamese and albino (cc(h)).  The two recessive alleles are still inherited the same way as all other recessives, cc being an albino, c(h)c(h) being a Siamese.  The only difference here is that when one of each recessive allele is inherited, cc(h), the resulting baby is Himalayan.
   So, if you breed Siamese to Siamese, c(h)c(h) x c(h)c(h), you will have all Siamese offspring.  If you breed albino to albino, cc x cc, then you will have all albino babies.  Now, when you breed Siamese to albino, c(h)c(h) x cc, you will get all Himalayan offspring, cc(h).
   One more thing to note.  Albino will completely mask all other traits being carried.  Even if the rat were A*ggmmcc, he would only appear to be albino, the platinum agouti traits would be entirely hidden.  Siamese can be effected by other color dilutions giving color point Siamese, such as blue point (American), or Russian blue point.  Himalayans can also have color points.

A*cc = Albino                               A*c(h)c(h) = Seal point Siamese
aacc = Albino                                aac(h)c(h) = Seal point Siamese
A*Cc = Agouti carrying albino      A*Cc(h) = Agouti carrying Siamese
aaCc = Black carrying albino        aaCc(h) = Black carrying Siamese
A*CC = Agouti                            A*cc(h) = Himalayan
aaCC = Black                               aacc(h) = Himalayan

Back to Basic Colors

Simple Markings

    Coming Soon!!!

Simple Types

Coming Soon!!!