Color Genetics

by Debra Sandoval

One of the attractions of Holland Lops to many people, especially new breeders is the wide variety of colors possible. Many new and even some established breeders want to know how to produce certain colors or what colors are possible from certain breedings. In order to make these kinds of predictions there are a few basic genetic principles which must first be understood:

1. The gene is the basic unit of inheritance. Every physical characteristic such as coat color, size, head shape, body shape, ear length and shape, etc. is controlled by either a single gene or a group of genes acting together.

2. All genes occur in pairs. One gene in each pair came from the animal's mother and the other came from its father.

3. When an animal forms eggs or sperm for reproduction the gene pairs separate independently of each other so that each egg or sperm receives only one of the two genes in a pair. At the time of conception, the egg and sperm unite to once again form an organism which has a pair of genes, one from each parent.

4. Within each pair of genes there may be several forms of that gene and depending upon which form is present a different physical appearance will result. For example, in people, if one form of the gene for eye color is present the person will have brown eyes, but if a different form is present the person will have blue eyes.

5. When there are two different forms of the gene present within the gene pair, one may mask or hide the effect of the other. For example, with eye color, if both the brown and the blue gene are present, the person will have brown eyes since the brown gene is dominant and hides the effect of the recessive blue gene. However, even though the effect of this recessive blue gene is hidden in the parent it may be seen in the offspring if it is combined with another blue gene. Therefore, two parents both of whom have brown eyes can produce a child with blue eyes, if they both carry a hidden blue-eye gene within their gene pair. From this you can see that the effect produced by the dominant form of a gene is seen whenever there is at least one gene of this form present, but in order for the effect produced by the recessive form to be seen both of the genes must be of this form.

6. In order to make it easier to discuss genes they are represented by a pair of letters with an upper case letter representing the dominant form and the lower case letter representing the recessive form. So, the gene pair of a person with brown eyes that carries a blue-eye gene would be represented as Bb while the gene pair for the person with blue eyes would be shown as bb.

Genes for Coat Color

There are many genes which work together to produce the exact color which you see when you look at a rabbit. In this article the five most important genes for coat color will be discussed but it is important to remember that there are also many other genes which are capable of modifying the effects of these five genes. It is these modifying genes which cause the variations within a color and are responsible for the difference between a light tort and a dark tort, for example.

A/a: Agouti / Non Agouti Gene

The dominant Agouti gene (A) results in the characteristic agouti type coloration of black and yellow bands on the hair shaft with a light colored belly, light eye circles, nostril and chin markings. All rabbits which show this typical pattern of light belly, eye circles, nostril and chin markings carry the dominant (A) agouti gene even if they don't have the typical banding on their hair shafts.

The recessive (a) gene results in a single color being produced on the whole hair shaft and everywhere on the body of the rabbit. This produces solid or self colored rabbits such as black, blue, chocolate and lilac.

B/b: Black/ Brown Gene

The dominant (B) gene causes a dark black pigment to be produced such that a rabbit which also has an Agouti (A) gene will be a chestnut agouti, or a rabbit which also has a pair of the recessive non-agouti (aa) genes will be a black rabbit.

The recessive brown gene (b) causes the production of a brown pigment to make a chocolate self rabbit (with aa), or a chocolate agouti rabbit (with AA or Aa). Note: This gene appears to be quite rare in Holland Lops and I have seen some rabbits which were called chocolate but were actually a dark sable. A true chocolate should be a uniform milk chocolate colored brown with no shading.

C/c: Full color/ Albino Gene

This is one gene which has more than two forms. There appear to be at least 5 different forms of this gene with a range in dominance from most dominant to most recessive. Also the dominance of some forms of this gene does not appear to be as total or complete as in other genes so that the second gene of the pair may influence the appearance of the rabbit as will be discussed below.

The most dominant form of this gene is the full color gene (C). When this form is present the color of the rabbit is determined solely by the other color genes. (i.e. C is non-contributory)

The next most dominant form is the Chinchilla gene (cchd) or sometimes written as (cch3). This gene prevents the formation of yellow pigment in an agouti rabbit which results in a chinchilla colored rabbit which has white rather than tan intermediate rings and white belly, eye circles, nose and chin markings. When this gene is found in a non agouti rabbit (aa) the results is a dark sable or seal colored rabbit which is almost black but shows some shading to a slightly lighter color on the flanks, chest and belly.

The next most dominant form is the light Chinchilla gene (cchl) or sometimes written as (cch1) This gene functions similar to the dark chinchilla gene but produces less pigmentation so that when found with an agouti gene (A) it produces a light or faded Chinchilla colored rabbit. When found with the non agouti gene (aa) this gene produces a Siamese Sable colored rabbit. The effects of this gene are additive so that a rabbit which has two of this gene (cchlcchl) will have the same appearance as a rabbit which has a dark Chinchilla gene (cchd).

The next form of this gene is the Himalayan gene (ch). This gene causes the production of pigment which is heat sensitive so that color is only produced on parts of the body that have a cooler skin temperature. The result is a rabbit with Himalayan markings of dark ears, nose, feet, and tail but no other body color.

The most recessive form of this gene is the Albino gene (c). This prevents any pigment formation so as to produce a red eyed white rabbit when both genes of the pair are in this form (cc).

D/d: Dense/ Dilute Gene

The dominant form (D) produces complete or dense pigmentation as determined by the other genes present.

The recessive (dd) form dilutes whatever pigmentation would be produced by the other genes so that it changes black rabbit (aa B- C- D-) into a blue rabbit (aa B- C- dd); an agouti rabbit (A- B- C- D-) is changed into a blue agouti or an opal (A- B- C- dd); a chinchilla rabbit (A- B- cchd- D-) is changed to a blue chinchilla or a squirrel (A- B- cchd- dd); a chocolate rabbit (aa bb C-) is changed to a lilac (aa bb C- dd); a Siamese Sable rabbit (aa B- cchl- D-) is changed to a smoke pearl rabbit (aa B- cchl- dd); etc.

ES/E/e: Extension / Non-Extension Gene

The most dominant form (ES) causes extension of the black band in agouti rabbits so as to hide the other bands on the hair shaft. It also causes the production of black pigment in areas where it is otherwise not produced in agouti rabbits (i.e. belly, eye circles, nostrils and under the chin). The result is a rabbit which appears to be a self (or solid) colored rabbit such as a black but which actually carries the agouti gene (A). Unlike other dominant genes the only way to determine whether this gene is present in a rabbit is by the offspring which it produces. For example, if a black rabbit is bred to another black or a tort (all of which appear to have aa) and an agouti marked rabbit is produced then it is known that one of the parents has an ES gene.

The ES gene is incompletely dominant over the form for normal extension (E), so that when these two are combined (ESE), the black is widened but does not completely cover the hair shaft which results in a steel colored rabbit. Therefore, a rabbit with A- B- C- D- ESES or A- B- C- D- ESe would appear black while a rabbit with A- B- C- D- ESE would appear steel.

The recessive form (e) for non-extension prevents the formation of the black pigmented band so that it is replaced by the yellow-orange pigment. This effect appears to occur mostly over the back of the rabbit with less effect on the sides and belly and the least effect on the face, ears, feet and tail. The result of this effect would be as follows:

An agouti rabbit (A- B- C- D- E-) would become an orange colored rabbit (A- B- C- D- e). (Note that in Holland Lops this color is often called a Fawn although it has the appearance of a bright orange and would be more correctly referred to as an Orange as is done in the Netherland Dwarfs.)

A black rabbit (aa B- C- D- E-) would become a Tort rabbit (aa B- C- D- ee). Note that the color some breeders refer to as Madagascar is a tort which continues to have some dark pigmentation over the back. This is most likely the results of various modifying genes.

A blue rabbit (aa B- C- dd E-) would become a blue cream rabbit (aa B- C- dd ee). This color is also sometimes called Isabel or blue tort.

A Chinchilla rabbit (A- B- cchd- D- E-) would become a Frost point or Ermine (A- B- cchd- D- e).

A Sable Siamese rabbit (aa B- cchl- D- E-) would become a Sable Point rabbit (aa B- cchl- D- ee).

The above genes are combined in the following table to show the genotypes of colors commonly found in Holland Lops. Note that the - indicates that the second form of the gene pair is not known.

TABLE OF GENETIC COLORS

Chestnut Agouti:  A- B- C- D- E-
Chocolate Agouti: A- bb C- D- E-
Opal (Blue Agouti):  A- B- C- dd E-
Lynx (Lilac Agouti):  A- bb C- dd E-
Orange:   A- B- C- D- ee
Fawn: A- B- C- dd ee
Black: aa B- C- D- E-
Blue: aa B- C- dd E-
Chocolate: aa bb C- D- E-
Lilac: aa bb C- dd E-
Tortoiseshell: aa B- C- D- ee
Blue cream (Isabel): aa B- C- dd ee
Chinchilla: A- B- cchd- D- E-
Squirrel (Blue Chin): A- B- cchd- dd E-
Frostpoint (Ermine): A- B- cchd- D- ee
Sable: aa B- cchd- D- E-
or: aa B- cchl chl D- E-
Siamese Sable: aa B- cchl - D- E-
Smoke Pearl: aa B- cchl- dd E-
Sable Point Siamese: aa B- cchl- D- ee
Blue Point Siamese: aa B- cchl - dd ee
Himalayan: aa B- ch- D- E-
Steel: A- B- C- D- ESE
Solid "agouti": A- B- C- D- ESES
or: A- B- C- D- ESe
Red Eyed White: -- -- cc -- --

Footnote: In Holland Lops the color which is often referred to as fawn is actually genetically orange in the above chart, and the color often referred to as lynx is most likely the dilute of orange or fawn.

In order to predict the colors possible from certain matings it is first necessary to determine the genotype of your breeding pair as much as possible. To do this, first find the basic genotype of your rabbits in the above table based on their color. Then try to fill in as many of the blanks as possible by examining the colors of the rabbits’ parents and ancestors and the colors of their previous offspring. For example, if one of the parents of a rabbit in question showed a recessive trait like non-agouti (aa), dilute (dd), white (cc), or non-extension (ee), then you know your rabbit must carry one of those recessive genes even if it is not expressed.

Similarly, if the rabbit in question has ever produced an offspring which showed a recessive trait then you know that your rabbit must carry that gene. Obviously, the more completely you know your rabbit’s genetic makeup, the more accurately you will be able to predict the colors resulting from certain matings. It is usually impossible to know each rabbit’s entire genotype, but to simplify things some assumptions can usually be made. For example, in Holland Lops the brown gene (b) appears to be quite rare, so unless a true chocolate or lilac rabbit appears in the background you can assume a rabbit has BB. Also for simplicity, any remaining blanks should be assumed to be both dominant forms until proven otherwise.

Next, list the possible combinations of sperm or eggs (only one form of each gene present) produced by each parent. For example, a black buck with the genotype of aa BE CC DD Ee would produce two different possible sperms: aBCDE and aBCDe. While an agouti doe with a genotype of Aa BE CC DD Ee would produce four different eggs: ABCDE, ABCDe, aBCDE and aBCDe. Notice that you can determine the total number of different sperm or eggs using the formula of 2n= the number of different forms, where n = the number of heterozygous gene pairs (those which contain two different forms of the gene). Once the possible combinations of genes in the egg and sperm are known the genotype of the offspring can be predicted by making a chart with the gene combinations of the sperm along the top and the gene combinations of the eggs down one side then filling in the possible combinations of the offspring in the center as follows:

BLACK BUCK:

    aBCDE aBCDe
  ABCDE AaBBCCDDEE AaBBCCDDEe
AGOUTI DOE: ABCDe AaBBCCDDEe AaBBCCDDee
  aBCDE aaBBCCDDEE aaBBCCDDEe
  aBCDe aaBBCCDDEe aaBBCCDDee

As you can see there would be eight possible combinations of genes in the offspring which would produce a ratio of three agouti rabbits, to one orange (or fawn), to three blacks, to one tort.

Here is another example, a tort doe who is out of a tort and a blue cream, has a genotype of aa BB CC Dd ee is bred to an agouti buck who is out of an opal and a tort, and has a genotype of Aa BB CC Dd ee. There are two possible eggs from the mother (21=2) and eight possible sperm from the rather (23=8).

 

TORT DOE:

    aBCDe aBCde
  aBCDe aaBBCCDDee
tort
aaBBCCDdee
tort
  ABCDE AaBBCCDDEe
agouti
AaBBCCDdEe
agouti
  aBCDE aaBBCCDDEe
black
aaBBCCDdEe
black
AGOUTI BUCK: ABCDe AaBBCCDDee
orange
AaBBCCDdee
orange
  ABCdE AaBBCCDdEe
agouti
AaBBCCddEe
opal
  ABCde AaBBCCDdee
orange
AaBBCCddee
fawn
  aBCdE aaBBCCDdEe
black
aaBBCCddEe
blue
  aBCde aaBBCCDdee
tort
aaBBCCddee
blue cream

 

This mating would produce a ratio or three torts, three agoutis, three blacks, three orange (or fawn), one opal, one fawn (or lynx), one blue and one blue cream (or isabel). Of course these ratios are only statistical predictions so that in a given litter it is possible to get only three blue creams, for example. I hope that this article has helped to clarity some of the questions people may have concerning rabbit color genetics.


HLRSC Official Guidebook - 5th Edition 2002