Chocolate and Chocolate Carrier
Dominant and Recessive Genes
The Mathematics of Genetics
Chromosomes and Genes
Chromosomes are strings of DNA. They act as blueprints or programs by which an organism builds itself. A chromosome is made up of individual points (or loci) called genes. An individual gene determines either the appearance or function of a body part. This gene may act alone or, usually, in concert with other genes to determine a particular appearance or function.
A chromosome string is linked together with one other chromosome string of the same type having the same gene locations. Thus, chromosomes and the genes on them occur in pairs in most of the cells of the body. There are 22 different chromosome pairs existing in each cell of the rabbit except the sex cells and red blood cells. Each chromosome pair controls different functions. For instance, the X and Y chromosomes determine sexual characteristics.
The same location on each chromosome of the pair controls the same characteristic. There are thus two genes involved - one on each chromosome at the same location. A particular gene location may allow only one type of gene to be present there; whereas, another location may allow different types of genes to occupy it. Those locations that allow only one type of gene on both chromosomes do so because any other type would cause either deleterious or fatal effects. An example of this would be the genes that control the shape of the teeth. If any other than the expected gene type is present at this location, the teeth would not grow in right, which could cause starvation.
Some locations allow different types of genes to stay there, causing different expressions of the same characteristic without deleterious effects. For instance, a hair color gene location may allow 2 genes of different types to occupy that location. One type of gene may be the gene that produces black hair, the other brown. Each chromosome may house either the black or the brown gene.
Some of the genes match their counterparts on the other chromosome exactly and some do not. When two genes match, such as, two black genes, this is called a homozygous pairing (homo- meaning the same). When the genes are not the same, such as one black and one brown gene, this is called heterozygous pairing (hetero- meaning different). Another name for heterozygous is hybrid. You've heard this term used in such common language as hybrid corn or hybrid tomatoes. These vegetables have one type of gene on one of the chromosomes, and a different gene on its companion chromosome.
When we speak of a gene location, we're really talking about two points - one on chromosome A and one on its companion chromosome B. Since there are two chromosomes involved, there are two genes involved - one from each chromosome. The points on each chromosome match up perfectly with the points on its companion chromosome. Thus, a location is made up of two genes.
Dominant and Recessive Genes
When two different genes occupy the same location, one of the genes expresses itself in the characteristic and the other either doesn't or does so to a lesser extent, modifying the effects of the other gene. When one gene expresses itself more than the other, it is called the dominant gene. The other gene is called the recessive gene. When the dominant gene expresses itself completely, to the exclusion of the recessive gene, this is calledcomplete dominance. Most dominant genes express themselves completely. When the recessive gene modifies the expression of the dominant gene in some way, the relationship of the dominant gene to the recesive gene is called incomplete dominance.
When a buck produces sperm or a doe produces eggs (both of these cell types are called gametes), the chromosome pairs in the cells that create these gametes divide, putting one chromosome of each type into the gamete. When the doe's egg is fertilized by the buck's sperm, the chromosome from the sperm unites with the same chromosome type in the egg and the chromosome pairing is once again restored. Whatever genes that came from the buck are now matched up with the genes of the doe. The expression of these genes in the resulting offspring depends on their dominance and how the other genes relate to each other.
The Mathematics of Genetics
Genetic determination is based on the law of probability. Without getting into the complicated aspects of gene mapping distance and linkage, I will present a simple concept of how you can figure what your offspring will look like.
It is common to represent a dominant gene by a capital letter and its recessive counterpart(s) as uncapitalized. Let's take the black/brown color location. This location is called B. The black gene, which is completely dominant, is represented as 'B'. The only known recessive gene that can occupy this location is brown, represented as 'b'. Since a location has two genes, there are 4 possibilities at this location: BB, bb, Bb, bB. Since black is completely dominant, if you have a rabbit that has at least one B at this location, the rabbit will have black in its fur. Thus, rabbits having the combination of genes: BB, Bb, or bB will have black in its fur. Rabbits having the combination of bb will have brown.
Note that it usually requires both recessive genes (bb) at a location for that set of genes to express itself. When the dominant gene is not completely dominant, the recessive gene will modify the expression of the dominant gene in some way. As far as the black/brown location, the black gene is completely dominant and will always express itself, if present, to the exclusion of the brown gene.
There are other genes that work with the B gene to determine that actual color of the rabbit. We will assume in this discussion that the other genes are set up to produce a solid black or a solid brown rabbit.
Let's take a black rabbit and mate it with another black rabbit. What are the possible offspring? We know that a black rabbit has at least one B gene. If we don't know for sure the other gene is B or b, we can represent that other gene as '_' (unknown). We can thus represent both genes in the black rabbit as: B_.
When it comes to dominant genes, we usually cannot know for absolute certainty that the other gene of the pair is also the same dominant gene. The exception to this is when the recessive gene acts with the dominant gene to produce a certain known characteristic, such as English spotting.
Getting back to our example, we have one black rabbit whose black/brown gene location is represented as B_. Let's assume the other black rabbit we are mating it to is also B_. Breeding the rabbits [B_ x B_], we combine the left gene of #1 with the left gene of #2 , giving BB - a black rabbit. Then we combine the left gene of #1 with the right gene of #2, giving B_ - another black rabbit. Then we combine the right gene of #1 with the left gene of #2, giving _B - another black rabbit. Then we combine the right gene of #1 with the right gene of #2, giving _ _ -unknown. We thus have:
How can we test to see if one of the black rabbits has a brown gene in it? Simply mate the black rabbit with a brown rabbit. If you get any offspring that are brown, you know for a fact that the black rabbit has a brown gene in it. The probability is that you should get about 50% brown offspring. Here is the formula:
Let's fill in the blanks. If one of the rabbits had both black genes (BB) all of the litter would be black, without exception, even if the other rabbit had one brown gene. It wouldn't matter how many times you bred these two rabbits together, you would always have black offspring. B_ x B_ = (BB, B_, _B, _ _)
BB x B_ = (BB, B_, BB, B_)Remember, if at least one black gene is present, the rabbit will be black.
Only if both rabbits had one brown gene would a brown rabbit (bb) be produced. It may take two or more breedings to see a brown rabbit because the probability is only 25%.
Bb x Bb = (BB, Bb, bB, bb)
In summary, to test for the presence of a recessive gene when only the dominant is showing, mate it with a rabbit that shows the recessive gene. If at least one of the offspring shows that recessive gene, you know positively that the rabbit you were testing has the recessive gene along with the dominant gene. If you do not get any offspring showing the recessive trait after several litters, the odds are that the rabbit in question does not have the recessive gene (but you can never know with absolute certainty). Remember that about half of the offspring should show the recessive trait. But if no rabbits in the litter expresses the recessive gene, we're still talking probabilities here. The failure of the recessive gene showing up in all of the litters does not prove the tested rabbit does not have the recessive gene, just that the probability of its having it is low. But if any rabbits in the litter show the recessive gene, it is proof positive that the tested rabbit has the recessive gene along with the dominant.
Filling the blanks, if the unknown gene is B, all of the litter will be black. If the unknown gene is b, about half should be brown. Since we are talking about probability here, the litter could still be all black or could be all brown. If you get an all black litter, you may still have to breed one or more times to see if at least one brown rabbit is produced. If a brown rabbit is produced, you know for a certainty that the black rabbit has one black gene and one brown gene. If you get all black rabbits from matings all the time, it is probable, but not certain, that the black rabbit has both of its genes black. B_ x bb = (Bb, Bb, _b, _b)
Hair color in rabbits is controlled by genes at several locations on the chromosomes. These genes act in conjunction with each other to produce quite a variety of different colors and patterns. Hair texture and length is controlled at other locations.
In this section, I intend to touch on only some of the basic color genes and their combinations. For a larger list and a thorough discussion, refer to the book, The ABC's of Rabbit Coat Colors, mentioned before.
The basic color genes in the rabbit are A through E, En, Du, Si, V, and W. Other genes act as color modifiers controlling the intensity of certain colors or patterns. They include the rufus modifiers, the plus/minus (blanket/spot) modifiers, and the color intensifiers. These modifiers are not single genes, but multiple ones that pool their effects.
A rabbit has two possible pigments that can be expressed in its hair - dark brown and yellow. The absence of both pigments results in white fur. All of the colors possible in rabbit fur are simply combinations of these pigments or lack thereof. The expression can appear on the same or different hairs, in certain patterns, and in different intensities.
In general, rabbits that have long hair, such as Angoras, have diluted color expression. Rabbits that have short hair, such as Rex, have more intense color expression. This is because, given the same genetic background, the number of pigment granules in the hair is the same. In long hair the pigment granules are spread further apart from each other, giving a pastel color. In short hair, the pigment granules are packed more closely together, making a more intense color.
Rabbits in the wild have a brownish fur color called agouti. Looking closely at this fur, you can see that it is made up of 3 to 5 bands of color. The hair closest to its skin is gray. This is followed by yellow, followed by black on the tips of the fur. These rabbits also have white bellies. This agouti pattern is found in some domestic rabbits today. They call this color chestnut. There are several variations of this agouti pattern in domestic rabbits. These variations are caused by the other genes and modifiers working together.
We can classify the color genes in two groups. First, the color pattern genes. These genes determine which pattern will be expressed: agouti, tan, or no pattern. All of the other genes are the color genes. These genes determine the placement and intensity of the color pigments on the hair.
The following table represents a list of the known genes, how they affect hair color, and some examples of rabbits having these genes. These genes are listed in the order of dominance within each group. Taking all of the genes together, there are thousands of color patterns possible. The American Rabbit Breeders Association has limited the number of color patterns it will accept for each breed in their Standard of Perfection..
Rabbit Hair Color Genetics
|A||Agouti pattern of banded hair||Has tan, fawn, or white at eye circles, triangle at nape of neck, feet, legs, and inside of ears. Has white belly.||Chestnut,|
|at||Tan pattern||Solid color instead of banded hair. Has tan, fawn, or white at eye circles, triangle at nape of neck, feet, legs, inside of ears, and belly.||Silver Marten,|
|a||Self-Color (non-agouti)||The hair lacks the banding, there is generally one color throughout.||New Zealand Black,|
|B||Black||In agouti (A_), produces the black band.|
In self (solids) (aa), produces solid black color.
|With A_: Chestnut and Chinchilla Netherland Dwarfs|
With at_: Black Silver Marten
With aa: New Zealand Black
|b||Brown (Chocolate)||In agouti (A_), produces a brown band instead of black.|
In self (solids) (aa), produces solid chocolate color.
|With A_: Chocolate Chestnut Netherland Dwarf.|
With at_: Chocolate Silver Marten.
With aa: Chocolate Netherland Dwarf.
|C||Full color development||Allows all 4 dark and all 3 yellow pigments to be present. Completely dominant.||New Zealand Black|
|cchd||Dark Chinchilla||Allows all 4 dark and only 1 of the 3 yellow pigments to be present. Area becomes white or pearl. Completely dominant over the following c genes.||Chinchilla|
|cchl||Light Chinchilla (Shading)||Allows 2 of the 4 dark and none of the 3 yellow pigments to be present. This lightens the color to sepia brown. Causes shading effects. Incompletely dominant over the following c genes. Shading is fine-tuned with the color intensifier genes.||Sable and smoke pearl Netherland Dwarfs|
|ch||Himalayan||Causes dark extremities (points) which include the ears, nose, feet, and tail. Produces red eyes with other ch or c. Incompletely dominant over c.||Californian, Seal Point.|
|c||Albino||Blocks the expression of all other color genes, producing a white rabbit with red eyes.||New Zealand White|
Ruby-Eyed White (rew)
|D||Dense coat color||Produces the full color shade. Causes the eye to be brown.||New Zealand Black|
|d||Diluted coat color||Changes black to blue, chocolate to lilac, chestnut to opal, orange to fawn. Causes eye to be gray-blue.||American Blue|
|Es||Steel||With agouti, covers the middle band with dark pigment. Darkens the agouti type landmarks: eye circles, triangle at nape of neck, feet, legs, and inside of ears. Leaves white guard hairs (ticking).||Black, Chestnut, and Chinchilla Steel|
|E||Normal Extension of dark pigment||Working with the C series genes, allows the complete expression of the dark brown pigment.||New Zealand Black|
|ej||Japanese brindling||Works with Agouti gene to cause the black and yellow colors to be arranged in areas instead of individual hairs in a mosaic pattern.||Rhinelander (with the Enen genes),|
|e||Non-extension of the dark pigment||Working with the C gene series and the rufus modifiers, this gene removes all or most of the dark pigment, leaving yellow, orange or white.||New Zealand Red,|
Sable Point Dwarf,
Frosted Pearl Dwarf
|En||English Spotting||Produces spots (broken patterns). Enen is normal spotting. EnEn causes spotting only in the head. enen causes no spotting. Works with the plus/minus modifiers to produce more or less spotting. Can also work with Du and V genes.||English Spot,|
|en||Self-Colored||Causes normal coloring without spotting.||New Zealand Red|
|Du||Absence of Dutch Pattern||DuDu causes no dutch patterns. Dudu causes partial white/colored patterns. dudu causes white belted dutch pattern. Works with the plus/minus modifiers to produce more or less color pattern. Can also work with the En and V genes.||DuDu: Solid color rabbits|
Dudu: Hotot (with EnEn and a lot of minus modifiers)
|du||White Belt Dutch Pattern||See Du for description||Dutch|
|V||Vienna White||VV: Normal coat color.|
Vv: Dutch type markings on colored coat and colored spots on white coat.
The V gene can work with the plus/minus modifiers and the Du and En genes.
vv: Causes no color to express itselt and produces an all white rabbit with blue eyes.
|VV: Almost all rabbits,|
Vv: no known breeds,
vv: Blue-Eyed White
|v||Vienna White||See V for description.||Blue Eyed White (bew)|
|W||Normal Width of the middle yellow or white agouti band||Normal coloring||Chinchilla|
|w||Doubles Width of the middle yellow or white agouti band||Colors the agouti pattern areas: eye circles, triangle at nape of neck, feet, legs, inside of ears, and belly.||New Zealand Red|
|Si||Normal Color||Si_: has no effect on color||New Zealand Black|
|si||Silvered Color||Produces silver white hairs and silver tipped hairs intermingled with regular hair throughout the coat.||Silver,|
Here are some samples of how the genes work together to produce certain varieties. Keep in mind that the modifier genes must be taken into account to produce the particular shades or patterns of colors you are looking for.
|Chocolate Silver Marten||at_||bb||cchd_||D_||E_||W_|
|Spotted - White/Black/Gold||A_||B_||C_||D_||ej_||Enen|
|Steel Siamese Sable||aa||B_||cchl_||D_||EsE||W_||silver ticking|
This concludes our discussion on Rabbit Genetics. The surface has only been scratched as far as the depth by which we could delve into the matter of genetics. If you want to experiment in improving your herd or to get just the right color you want, I refer you once again to The ABC's of Rabbit Coat Colors by Glenna Huffmon. Another good source of information is the specialty club for the particular breed you are interested in