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Coat Color and Structure Genetics

Abstract

The study of rabbit coat and fur structure genetics was conducted over a two year period making use of field research as well as web research. Many of the topics found on reputable organization material were then tested on live animals through selective breeding and record keeping. The information was recorded over many decades through the use of pedigree programs. As a result of field research, I was able to support the evidence found online. The percentages of specific phenotypes in litters were also recorded using breeding pairs with known genotypes furthering the support for order of dominance of alleles.

                Keywords: Rabbit, Phenotype, Genotype, Genetics


                Originating in Europe, rabbit breeders have been utilizing selective breeding for desired traits for hundreds of years as a source of food, fur, and fancy. Hobbyists use genetics to help breed in desirable traits into their lines. Among those traits, fur color is the basis of weather the animal is disqualified off the table or a best in show champion. All creatures originating from two parents inherit one gene from each parent per allelic pair. Modern technology has allowed scientist to map out genotypes and phenotypes of almost any animal empowering rabbit breeders to anticipate what the kits will look like prior to them ever being born. By understanding the genotypes and their meanings, one can use a Punnett square to find the percent chance of the possible outcomes. From that point forward, one can use the cumulative percentages to find all possible outcomes in a given litter. There are a total of 17,010 different color combinations that are possible using only eleven different genes; A, B, C, D, E, En, Du, V, Si, W, and P mutation. And in addition to those, there are six coat structure genes; F, L, R, Sa, Wa, and M. The genes interacting with each other “(called epistatis) is what produces all of the different breeds [and varieties] of rabbits” (Huffmon, 1999, p. 27). To determine the possible phenotypes, one must consider all the allelic pair combinations for an accurate prediction. There is a mathematical formula used to calculate and predict the probability of getting different variety and fur structure combinations in a given rabbit litter by utilizing the established alleles and their dominance or lack thereof.

                 There is some important terminology linked to genetics that is critical to understand in order to successfully calculate possible genetic outcomes. First, it is important to understand that “there are only two actual ‘colors’ involved in the animal coat colors – these being black and yellow” (Huffmon, 1999, p. 2). Each of the alleles epistatically modify the amount, pattern, and density of each color. Genotypes are characterized by various uppercase and lowercase letters. An allele is an alternative form of a gene. For example: On the black gene, uppercase B stands for black. However, there is also the lowercase, more recessive b allele which stands for chocolate. There can be more than two different alleles for one gene. The allelic pair is both genes together. A phenotype is the characteristic that is observable. For example, you can observe that the color of a particular rabbit is black. A genotype is the letters that describe the animals’ characteristics. When one is referring to homozygous and heterozygous, they are referring to weather the allelic pair is the same (homozygous) or different (heterozygous). A heterozygous allelic pair is always characterized with a capital letter followed by a lowercase letter. A homozygous rabbit’s genotype shows that both the genes are the same, but it could be two lowercase letters or two uppercase letters. A modifying gene is a locus that changes the expression of the other. For example, dilute modifies the phenotype by making the color on the hair shaft less dense giving it a lighter shade. Allelic pairs must consist of either a homozygous or a heterozygous combination to determine the phenotype and genotype of the rabbit.

                The dominance of a gene is one of the most important genetic concepts to understand. A dominant trait is a phenotype or characteristic of a rabbit with one or both alleles being present. A dominant trait is characterized with a capital letter and is always listed in order of dominance. It is impossible to get a dominant trait in the offspring that isn’t present in one or both parents. An incompletely dominant trait is dominant over the more recessive trait or traits, however it is only completely expressed when it is homozygous. An example of a common incompletely dominant trait would be “cross breeding red and white sweet peas. The first generation of offspring would be pink as the two would blend” (Huffmon, 1999, p.23). Because red is incompletely dominant over white, it cannot be fully expressed when crossed with white. A recessive trait is expressed with a lowercase letter and is only a phenotype when the recessive traits are homozygous. Recessive traits can be hidden by the more dominant traits without showing up for many generations, if at all. If a rabbit is hiding a recessive trait, they are called carriers. In order for offspring to express a recessive trait, both parents must carry one or more recessive alleles. The dominance, or lack thereof, of an allelic pair determines the phenotype and genotype of a rabbit.

                A genetic mutation is a result of any event that changes the genetic structure. Genetic mutations are a common occurrence, but most of the time they are an insignificant change that is harmless and rarely change the phenotype of the rabbit. Generally, Mutations are “recessive to the original gene, but not in all cases” (Huffmon, 1999, p. 26). The first wild European rabbits were all agouti, so even though it is rare, every alternative form of a gene is a result of a mutation. Mutations are acquired one of two ways: “they can be inherited from a parent or acquired during a…lifetime” (Genetics home reverence, 2014, para. 2). However, mutations that occur in the parents must be in the egg and sperm cells in order to pass it on to the offspring. A mutation is the only mechanism of evolution that can add alleles. There are instances when they do have a significant change in the phenotype. One of the oldest mutations is the at allele eliminating the agouti ring color. This mutation did occur in the sex cells, so therefore, it was passed on to the offspring. There are four different ways a mutation can occur. They include insertion, deletion, substitution, and frame shift. Insertion is when a base pair is added to the DNA. Deletion is the exact opposite, where part of the DNA is lost. Substitution is ultimately when one part of the DNA is replaced by another. Lastly is a frame shift, where the patterns of three are shifted down losing or changing the meaning of the entire strand. Most naturally occurring mutations result when the cell makes a copy of itself inaccurately. All of these types of mutations stated above are rarely significant in changing anything other than the spelling of the DNA strand.

The first gene that modifies the color of the coat is Agouti on the A gene. These genes affect the pattern of the coat and “there are only three pattern setting groups involved with rabbits” (Huffmon, 1999, p.24). The most dominant allele, A, is Agouti. Agouti is also known as the Wild Rabbit gene with ticking and bands on the hair shafts. There can be as few as three bands and as many as five. It is completely dominant over the other two alleles, so any combination including A results in an Agouti phenotype. The “Tan Pattern” gene is on at. A Tan is also a breed recognized in the American Rabbit Breeders Association standard; however, it is also a recognized variety, or color, in some other breeds. It creates a partial Agouti pattern with an orange triangle at the base of the neck, lacing up the ears, and on the belly but is free of any ticking or ring color. The Rufus modifier gene works to darken (or lighten) any orange markings on a Tan marked rabbit. The at gene is also completely dominant over a, which means atat and ata are both the more dominant tan pattern. The most recessive form of the gene, a, is self, or absence of agouti pattern or ring color.

                The B gene is the only color gene, or pigment gene. There are only two alleles: Dominant B is black and recessive b is chocolate. Chocolate looks like a uniform dark brown color on the pelt. It is never called brown, though because in some breeds, brown means chocolate based agouti. This gene works with color intensification modifiers that control how dark the pigment of the color is.

                The C gene, known as the Chinchilla gene, has five alleles making it one of the most complicated genes. It works to remove certain pigments from the coat. In order of dominance, there is C, c(chd) (or c(ch3)), c(chl), c(h), and c. C is normal coat, or no pigment modification. c(chd) stands for dark Chinchilla which removes all but one yellow band in the coat. Dark Chinchilla is completely dominant over the more recessive alleles in the gene. The last three genes interact with each other causing different varieties and qualities of those varieties. That being said, c(chl), or light Chinchilla, is the most dominant of the three, but it is incompletely dominant. It removes all yellow pigment in the coat which lightens the coat. When the gene is homozygous, it is called seal, which looks similar to a smutty black, or lighter black. Heterozygous c(chl) (c(chl)c(h) or c(chl)c) is Sable. c(h) is the Himalayan/Pointed White gene. The c(h) gene is responsible for removing pigment from the eyes and the warmer parts of the body. As a result, it is very important to keep pointed whites in a climate controlled barn to avoid smut. Smut is the resulting color on the usable part of the pelt (the body). Although there are true pointed whites that are homozygous, heterozygous pointed whites (c(h)c) can usually still be shown, just may not do as well due to the lightened coat pattern. c is the most recessive allele and is known as the REW (red eyed white), or albino gene. This gene removes all pigment from the eyes and coat. REW can have the genotype of any variety of rabbit that isn’t already on the C gene. Many breeders call a REW a colored rabbit with a white sheet over it. This means that the varieties of the babies are more or less a surprise when bred to another variety. A REW can hide any allelic pair except those that are on the C gene.

                The D gene is simply the dense/dilute gene. With two alleles, it modifies the intensity of the color of the rabbit. The words dense or dilute is referring to the amount of pigment is in each hair shaft. Dominant is normal intensity, or the dense version, of the color. For example: black is dense to blue. Recessive dilute is a lighter version of that variety. Blue is the dilute of black.

                The E, the extension gene, has four alleles; Es, E, ej, and e in order of dominance. The Es gene is the steel gene. It is the only mutation that is more dominant than the original gene, normal extension, and it is the only dominant gene that has the ability to hide. This gene works to extend the black pigment to the ends of the hair shaft, however, is known to play games. The variety, steel, has the genotype EsE because it is incompletely dominant. Homozygous EsEs is identical as the more recessive E gene, and so is heterozygous Ese. This makes working with steel tricky because when you breed two genetic steels together, 50% will be self and 50% will be steel, but out of the selfs, you don’t know which are homozygous normal extension (EE), and which are homozygous steel (EsEs). Normal extension, E, is most rabbits. The black band extends the normal amount. It is the original gene on the E series. The ej gene is the brindling or random extension gene. It extends or partially extends dark pigment in random patterns. It then uses modifying genes to create more extension then non-extension or vice versa. There are two breeds which use this gene to create a specific pattern as defined in the ARBA standard of perfection. It can come in any combination of the following: black, blue, chocolate or lilac and either orange (Japanese) or white (Magpie). Lastly, there is non-extension on the e gene that removes most or all dark pigment leaving all yellow pigments. This produces varieties like tort, fawn, and red. The smut seen on some individuals of these varieties is from modifiers which leaves some dark pigment.

                The En gene is named after the English Spot because broken pattern originated in this breed. The English spot is a marked breed that has characteristics such as a nose butterfly, colored ears, a Herringbone (a jagged stripe starting at the base of the neck to the tail), eye circles, cheek spots, and a chain (spots forming a sweep) increasing in size down the sides. Most brokens share many of these traits, but they all must have a nose marking and color on the ears in order to be shown. Broken (any variety and white) is dominant, however it is incompletely dominant. There are three patterns associated with this gene: charlie, broken pattern, and self (or sport in most marked breeds). A showable broken is typically heterozygous because the broken pattern on most breeds must stay between 10-50% in order to be considered show quality. A true charlie is homozygous dominant, and in most cases are much less than 10%. They are so named after the famous actor, Charlie Chaplin, because many times they exhibit a small mustache instead of the typical butterfly. Recessive en is simply solid or no pattern. Many breeders believe scattered white hairs on a solid pattern originates from the broken gene. That is a myth. Scattered whites are an entirely separate gene, but breeders will breed their scattered whites to broken so that the pattern can hide them. Unfortunately, that means many broken rabbits carry and throw the gene. In addition to hiding scattered whites, the broken pattern also hides the severity of it. There are plus and minus modifiers that control the amount of color on the pelt of a broken. A minus modifier is responsible for false charlies. A false charlie is a heterozygous broken with a lot of minus modifiers causing them to have less than 10% color. Plus modifiers are responsible for booted brokens, which would be a rabbit with over 50% color (a disqualification for most breeds.)

               The Du gene is known as the Dutch gene. Du is normal pattern, but it is incompletely dominant over the other two alleles, du(d) and du(w). It is found in only three breeds: Blanc De Hotot, Dwarf Hotot, and Dutch. The more dominant of the two genes, du(d), stands for Dark Dutch, found primarily in the Dutch breed. Homozygous recessive creates a dark saddle, a white belt, white stops on the back feet, Cheek circles, and a white blaze. Being a marked breed, most of the points are on the markings, and being so specific, they are second only to the harlequin in weight on the markings. The du(w) is found primarily in the Hotot. This gene causes most of the animal to be white except for an even band of color around each eye. The main issue with this gene that du(d) and du(w) are linked 87% of the time, and passed independently only 13% of the time. This means a traditional Punnett square does not apply. Although it is still possible to get properly marked babies with the du(d) gene present, the percentage of marked babies is greatly reduced, so it is important to try and breed it out. The Du gene also works with the broken gene to create the Hotot markings. Unfortunately, since broken is incompletely dominant, you can get sports (absence of the broken gene) and charlies (homozygous dominant broken), and is therefore, difficult to get a Hotot with proper markings.

                The V gene is the Vienna gene. This gene works to remove the pigment from the coat and most of the pigment from the eyes causing them to be blue. The dominant is normal color, but it is incompletely dominant. A heterozygous allelic pair is often abbreviated as VM (Vienna marked) or VC (Vienna carrier). The second allele doesn’t always express itself in a non-showable pattern, but in most cases when it does, it often looks like a Dutch, or is missing required markings to be shown under the broken class (too much or too little color, absence of color on the nose, or the complete absence of color on one or both ears). Homozygous recessive is abbreviated as BEW (Blue eyed white). The gene that predisposes rabbits to seizures is also linked it the Vienna gene causing the likelihood of a BEW or VC to experience epilepsy.

                The Si gene is the silvering gene. The dominant allele is completely dominant over the recessive allele. Silvering is when the tips of the coat are white and there are white hairs throughout a self pattern. It is found in only a handful of breeds with varying degrees of silvering required. For example, the Silver Fox has more black in the coat than silvering. The D’Argents, however, have a very large percentage of their coat with silvering.

                The W gene is known as the wideband series. This is important for fawns, reds, and oranges because it doubles the width of the middle yellow or white band in agoutis coloring the markings of an agouti. The Rufus modifier gene works with the wideband series to darken the color on reds or lighten the color on fawns with this gene.

                The P mutation series is not known in any ARBA recognized breeds or varieties, but it is recognized in some European varieties such as the Lutino and Shadow. This gene partially dilutes the coat and removes all pigment from the eyes, making them pink. The color looks similar to that on a fawn without the agouti markings. Prior to this mutation, the only gene capable of making a pink eye were the c(h) and cc genes.

The first of the coat structure is F is the furless series. The furless gene is recessive and unfortunately, usually fatal. Babies born with this gene are often born with fur on their stomachs and extremities. In 1999, Texas A&M University-Kingsville had a mini Lop named Fuzz who was born to one of their professors with this rare genetic mutation and he lived beyond breeding age. Even compared to other furless rabbits, Fuzz had less fur on the stomach limbs and head. When 100 of his offspring out of New Zealand White does reached breeding age, “matings of his offspring (half-brother to half-sister) resulted in about 1 out of four furless rabbits, suggesting a recessive gene mode of inheritance” (Department of Animal Science, 2013, para. 1-3, 6, 7). This was the first study of the stability of the hairless gene. Later, they sold out of their stock to another research facility in Missouri.

The L coat structure is the Long hair series which controls wool. It is a recessive trait which exists in six different ARBA (American Rabbit Breeders Association) recognized breeds. It is characterized by a long coat free of most guard hair. The long undercoat has a desirable crimp to it allowing it to be used. The wool on production animals such as the angora can be spun into yarn. There are also small fancy wool breeds not designed for production.

                The next coat structure is seen in only two ARBA recognized breeds called the Rex and the Mini Rex. This is a recessive trait in the R series known, as the Rex gene. It causes a short dense coat that is extremely plush. The plushness is caused when the under coat and guard hairs are the same length.

                The S coat structure is called the Satin series. What it means to be Satinized is the diameter of the hair shaft is incredibly small causing a slightly transparent look to the coat. This gene is recessive to the normal coat. Currently, it is only found in three breeds: Satin, Mini Satin, and Satin Angora.

                Wa or the Waved series also known as the Astrex gene is a recessive trait, however, it is very unstable. The Astrex Rex is an extinct breed that exhibited this trait. The instability of the trait was what caused the extinction, however, there are breeders working on breeding in more stability. Rabbits born with this trait have very curly or wavy coats as babies, but most of the time they grow out of it for several months before growing back into it. It is not to be confused with the F gene, because many times babies will go through a molt that will leave little to no fur on their bodies, which will then grow back.

                Lastly is the M Maned series. This was a fairly recent mutation that caused rabbits to be born with wool on their head and skirt area. The only ARBA recognized breed with this gene is the Lionhead which was passed its final showing in the 2013 ARBA convention. This is the only dominant coat structure gene and it is incompletely dominant. That means that both alleles affect the phenotype of the rabbit. A single mane is a heterozygous genotype, and a double mane is homozygous dominant. But since it is a dominant trait, it is also possible to get a Lionhead with no mane.

                A Punnett square is a way to calculate the chances of getting certain allelic pair. It is structured in a way that is similar to lattice multiplication. The first row symbolizes the dam, and the first column symbolizes the sire. For example, if you breed two animals, one who is heterozygous agouti (Aa) to a self (aa). In this scenario, 50% are selfs and 50% are heterozygous agouti (self-carriers). You can multiply two or more results to find the percent chance of getting certain combinations. For example, a heterozygous dense(Dd) bred to another heterozygous dense (Dd) will have 75% show the dense phenotype and 25% the dilute. In this case, the sire is heterozygous agouti/heterozygous dilute, paired with a self/dilute. For example:

75% dense X 50% self = 37.5% self/dense

75% dense X 50% agouti = 37.5% agouti/dense

25% dilute X 50% self = 12.5% dilute/self

25% dilute X 50% agouti = 12.5% dilute/agouti

When those four percentages are added together, they equal 100%, elimination any chance for any other combinations that include dilute or agouti. Furthermore, more genes can be multiplied in to add to the possible outcomes. For example, if you breed a heterozygous dominant on the Satin gene to another heterozygous dominant furred animal:

37.5% dense/self X 75% normal fur = 28.125% normal/dense/self

37.5% dense/agouti X 75% normal fur = 28.125% dense/agouti/normal

12.5% dilute/self X 75% normal fur = 9.375% dilute/self/normal

12.5% dilute/agouti X 75% normal fur = 9.375% dilute/agouti/normal

37.5% dense/self X 25% satinized fur = 9.375% dense/self/satin

37.5% dense/agouti X 25% satinized fur = 9.375% dense/agouti/satin

12.5% dilute/self X 25% satinized fur = 3.125% dilute/self/satin

12.5% dilute/agouti X 25% satinized fur = 3.125% dilute/agouti/satin

When adding another allelic par, all possibilities regardless of likelihood must be explored. By multiplying another allele, the number of outcomes possible is doubled. This only works if you know the genotype of the parents. If unsure, substitute the recessive gene with a question mark. This symbolizes the percent chance of any of the given possible allelic pair combinations with the same level of dominance as the other allele, or lower.

                When calculating the genotype, you must adjust the number of possible outcomes accordingly by accounting for whether the animal is heterozygous or homozygous. Using the example above, with the agouti and dense genes, you have:

0% Homozygous agouti X 25% Homozygous Dense = 0% Homozygous agouti/ Homozygous Dense

0% Homozygous agouti X 50% Heterozygous Dense = 0% Homozygous agouti/ Heterozygous Dense

0% Homozygous agouti X 25% Homozygous Dilute = 0% Homozygous agouti/ Homozygous Dilute

50% Heterozygous agouti X 25% Homozygous Dense = 12.5% Heterozygous agouti/ Homozygous Dense

50% Heterozygous agouti X 50% Heterozygous Dense = 25% Heterozygous agouti/ Heterozygous dense

50% Heterozygous agouti X 25% Homozygous Dilute = 12.5% Heterozygous agouti/ Homozygous Dilute

50% Homozygous self X 25% Homozygous Dense = 12.5% Homozygous self/ Homozygous dense

50% Homozygous self X 50% Heterozygous Dense = 25% Homozygous self/ Heterozygous Dense

50% Homozygous self X 25% Homozygous Dilute = 12.5% Homozygous self/ Homozygous Dilute

In this instance, since there are three possibilities for each allelic pair, the number of possible outcomes is tripled (or quadrupled if the parents carry two different recessive traits) for every allelic pair added. It cannot be proven if the animal is heterozygous or homozygous (unless out of a homozygous recessive parent) until bred to a rabbit who has the same recessive trait. Even then, if they do not throw the recessive trait, there is no guarantee they don’t carry it.

                In the methodical process of calculating the fur outcomes of rabbit litters, it is necessary to understanding the genotypes, how to find them, and what the alleles mean. From that point, one uses the Punnett squares and mathematical formulas to calculate the possible outcomes and their likelihoods. Understanding the genotypes provides breeders with the tools to figure out the percent possibilities that are needed to plug into the mathematical formula. By knowing all the possibilities and genes, one can figure out the cumulative percentage of possibilities from that given point forward. Breeders can use this tool to calculate to not only what they might get using a specific breeding pair, but serve as a guideline as to what varieties are acceptable to breed together to minimize unrecognized varieties.


Works Cited

American Rabbit Breeders Association. (2011). Standard of Perfection.

Department of Animal Science. (2013, October 14). Basic genetics. Retrieved on March 10, 2014 from Cornell University:                                                            http://www.ansci.cornell.edu/usdagen/codominance.html

Genetics Home Reference. (2014, April 21). What is a Gene Mutation and how do Mutations Occur?. Retrieved on April 27, 2014 from The U.S.                         National Library of Medicine: http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/genemutation

Huffmon, G. M. (1999). Rabbit Coat Color Genetics. (5th ed.). Howell, MI: Independently Published.

Lukefhar, S. D. (2014, February 26). The Rabbit Research Program at TAMUK. Retrieved on March 10, 2014 from Texas A&M University- Kingsville: http://users.tamuk.edu/kfsdl00/rabb.html

Stewart, E. (2012, November). Rabbit Color Genetics. Retrieved on April 25, 2014 from American Rabbit Breeders Association: https://www.arba.net/district/8/am-11-12-ericstewart.html