COAT COLOR GENETICS OF THE
FLEMISH GIANT
Mary F.
Mahalovich, PhD
INTRODUCTION
Why write this article since there are
a fair number of publications on coat color genetics in
rabbits? If you are like most breeders you may want to know
what the genetic basis is for coat color in your Flemish
Giant rabbits (abbreviated to Flemish). Many of the coat
color articles written are not specific to Flemish or they
don’t spell out all of the genes in the coat color series so
as to know what makes a sandy a sandy and not steel or a
light gray. Some breeders already have an intuitive feel
for what generates better-colored rabbits and by breeding
large numbers of animals they can effectively produce
winning animals on the show table. Others cannot afford a
large herd, so with a basic understanding of coat color
genetics, their goal is to produce winning show animals with
a smaller herd size.
For some rabbits it is possible to
figure out their coat color genetics from a three- or
four-generation pedigree and when in doubt, the color of
their kits helps solve the remaining unknown genes.
Knowledge of a rabbit’s coat color genetics helps make
better decisions about what rabbits to mate in the future or
in what kinds of rabbits to look for in other breeders’
stock to enhance one’s herd.
Some of the previous publications are
confusing or appear to be contradictory when evaluated
alongside recommendations among Flemish breeders as to what
produces good coat color. This review article is an effort
to sort out how researchers unraveled coat color genetics in
rabbits and what might be the genetic basis behind the
recommendations for better coat color among Flemish
breeders. As a result there may be more questions than
answers since some of those earlier publications do not
always cover a particular breed like the Flemish.
Lastly, based on what is currently
known on coat color genetics, I have added some of my own
thoughts as to what may be contributing to lighter or darker
colored animals within a color variety, specifically for the
light gray and steel varieties. A brief genetics overview
and summary of the major coat color series are offered
below, with more detailed information on the genetics behind
each color variety, followed by a special section on eye
color in the light gray and steel Flemish.
GENETICS OVERVIEW
The largest unit of inheritance is the
chromosome found within the nucleus of a cell. Each
chromosome is made of many genes. Genes are the basic unit
of inheritance which contains the information for making
proteins. The domestic rabbit, Oryctolagus cuniculus,
has 44 chromosomes or 22 pairs of chromosomes; 21 of which
are autosomal (not sex determining) chromosomes and the last
pair are the sex chromosomes, where XX = doe and XY = buck.
The known major genes for coat color are found on four of
the 22 chromosome pairs.
Each rabbit inherits one set of
chromosomes from each parent. When an egg is fertilized the
genes undergo recombination (mixing up of the genes). Not
all genes are transmitted independently, but travel together
as a group during recombination. These tight groupings of
genes are known as linkage. So linkage refers to genes
closely located on a chromosome that tend to be transmitted
as a whole block during the formation of eggs and sperm.
A locus (loci for plural) is a specific
location on a chromosome where a gene is found. An allele
is one of an array of genes possible at a certain locus on a
chromosome. Each gene site is made up of two alleles, one
inherited from its dam and the other from its sire.
Alternate effects on the same character are produced by
different alleles. Where a rabbit can only have two alleles
AA, Aa or aa in its genetic makeup,
there may be multiple alleles for that gene in the overall
population A, at, and a. A
dominant allele is represented with upper case letters and
recessive allele is represented with lower case letters. If
a rabbit has the same alleles for a gene AA or aa
it is called a homozygote, and if the alleles are different,
it is a heterozygote Aa.
There are two kinds of interaction
among genes. Complete dominance is an interaction among the
alleles within a locus, where the dominant
allele suppresses the expression of the recessive allele.
For example, what we see or the outward appearance of the
AA rabbit is the same as the Aa rabbit.
Incomplete dominance may also occur within a locus, where
the heterozygote is intermediate (or has no dominance) in
gene expression to either homozygote. For example, the
outward appearance of the AA rabbit is not the same
as the Aa rabbit, which in turn is different from the
aa rabbit.
When genes at one locus influence the
expression of genes at a different locus this
interaction is referred to as epistasis. Examples of
epistasis are the presence of the gene combination aa
with dd in blue Flemish, which results in a rabbit
having slate blue, not brown eyes and the gene combination
cc turns off the expression of all the coat and eye
color genes and results in a white Flemish with pink eyes.
The combination of alleles at each
locus or across all loci is referred to as the genotype
(G) of the rabbit. The genetic expression or outward
appearance is called the phenotype (P). The outward
appearance is based on a rabbit’s genetic makeup or
genotype, its environment (E) and the interaction of its
genetic makeup in the environment (G x E) and is represented
by the following formula P = G + E + G x E.
COAT COLOR SERIES
There are many major coat color genes
in the rabbit, but six are critical in Flemish (A-series,
B-series, etc.). The alleles for each color series
are ordered by their degree of dominance in Table 1. Coat
color genes are simply inherited (also referred to as
Mendelian inheritance) and are determined by genes with
major effects. These major effects can readily be seen in
the changes in coat color by substituting one allele for
another like an agouti rabbit A_ compared to a
non-agouti or self rabbit aa.
How do different alleles come about
within a color series? Mutation is the fundamental process
that gives rise to new gene variants or alleles. Mutations
are typically recessive in form and are noted in lower case
letters, such as the tan at and self a
alleles at the agouti locus (Table 1). Two exceptions to
this rule are the dominant mutations ED
and ES alleles at the Extension locus.
Mutations are rare, and the probability that a mutation
becomes fixed in the population, is even rarer. Mutations
usually set the rabbit apart from others making them easier
prey, or they don’t produce a needed protein or hormone and
the rabbit is less vigorous. Some mutations though not
lethal, do not last long in a population. But some
mutations are lethal, resulting in miscarriage, absorption
of embryos, or if carried to term, stillborn young or young
that die shortly after birth.
The total genetic information possessed
by the reproductive members of a population of rabbits is
called a gene pool. Several alleles are not found in the
Flemish gene pool (at, b, cchm, ch,
ED, and ej) (Table 1).
They could be introduced into Flemish either by mutation or
crossbreeding. Examples of crossbreeding in rabbits are a
Flemish by Tan mating, a Flemish by Himalayan mating, or a
Flemish by Harlequin mating. Crossbreeding is a useful tool
to: 1) introduce new genes into a breed’s gene pool, 2)
provide the raw material to create new breeds like the
Velveteen rabbit or Altex (McNitt et al. 2000), and
3) increase profits in commercial meat rabbit operations.
At this writing the Altex is a commercial sire breed and is
not a fancy breed, and like the Velveteen, it is not
recognized as such by the American Rabbit Breeders
Association (ARBA).
The original genetic makeup for the
major coat color genes of the wild type rabbit is
represented as AABBCCDDEE. The wild type rabbit
experienced mutations to other alleles over time, which were
fixed in the population if favorable to the rabbit’s
survival or were favored by selective breeding in domestic
rabbits. The genetic makeup of the domestic rabbit is more
recently represented as A_B_C_D_E_ to make room for
the other possible alleles at each color series (Table 1).
A blank ( _ ) represents the same allele or other alleles
lower in the series.
The agouti color is a consequence of
variations in shade of yellow pigmentation and band width.
The sandy, fawn, light gray, and steel varieties are
considered agouti rabbits and the A-series genotype
for these Flemish is noted as A_, so their genotype
is either AA or Aa for the agouti Flemish (and
the outward appearance of the AA rabbit is the same
as the Aa rabbit). The recessive homozygote aa
is a non-agouti rabbit and has the self-colored coat of the
black and blue Flemish varieties (no eye circles, no nostril
and chin markings, and no light-colored belly fur).
Some possible explanations for the
degree of a lighter agouti rabbit in the sandy, fawn, light
gray, or steel varieties is that the A allele may not
be completely dominant to the a allele, so that a
heterozygous rabbit Aa may be lighter in color than a
homozygote rabbit AA (Cleffmann 1953). The more
popular explanation is the presence of minor genes (also
called polygenes or modifying genes), which modify the width
of the band. This discussion of darker and lighter agouti
rabbits should not be confused with the “richness” of the
coat color, influenced by the w or wide-band gene
discussed below.
The next gene or the B-series
deals with pigment. The B-series is sometimes called
the Black locus by some authors and the Brown locus by
others. The dominant allele B infers the full
development of the black pigment in the coat of agouti and
self rabbits and is considered completely dominant in both
its homozygous BB and heterozygous Bb forms.
The homozygous recessive rabbit bb is brown in color,
referred to as seal, chocolate or nutria brown. The b
allele is not typically found in Flemish, so the B-series
genotype is noted as BB for all seven color varieties
in Table 2. A special group of minor genes are also thought
to darken or lighten the coat color of the alleles at the
B-series (Robinson 1958).
The expression of coat color or the
C-series is the full development of both yellow and
black pigments along the hair shaft. The C allele is
considered completely dominant in both its homozygous CC
and heterozygous C_ forms and the remainder of the
alleles in the C-series are considered recessive to
the C allele (Lukefahr 1986). The chinchilla alleles
cch_ are considered to be incompletely
dominant to each other (McNitt et al. 2000). The
substitution of other alleles in this series leads to a
progressive decline in the pigment produced. The yellow
pigment disappears quickly and black (cchd)
diminishes to sepia (cchm, cchl)
to just pigment on the extremities (ch) as
seen with the Himalayan allele. The cchm
and ch alleles are typically not found in
Flemish, but were included in the discussion to show how
moving down the C-series impacts pigmentation. It is
also unclear if the cchl allele is present
in Flemish, as it is somewhat temperature sensitive.
Lastly, the presence of the gene combination cc in a
rabbit suppresses pigment development altogether, regardless
of what alleles are present for the other color series. The
cc rabbit’s coat color is white and its eye color is
pink.
The E-series has three dominant
ED, Es, E
and two recessive alleles ej and e.
It is also called the Extension locus, as the black pigment
of the terminal band extends into the intermediate band
color. It is unclear if ED is represented
in Flemish. If so, the agouti black rabbit ED_
has the same color as a self black aa Flemish (Table
2). The next allele in the series Es is
believed to be responsible for producing the steel agouti
Flemish and was first obtained from the Steel Dutch
(Robinson 1958). The Es allele extends
the black ticking on the terminal band into the intermediate
band. Some yellow or white coloring may still be present in
the intermediate band. In combination with another Es
or e allele, results in an agouti-black Flemish. The
E allele is the normal extension of black in the
terminal band, found in the agouti rabbit. When a rabbit
inherits two copies of the recessive e allele (ee),
the black ticking is either limited or absent altogether as
seen in the fawn Flemish.
A mutation discovered by Sawin (1932b,
1934) increases the width of the banding so that the
intermediate band may occupy up to one-half of the band
width. The dominant allele W in the homozygote WW
or heterozygote Ww forms results in a normal
intermediate band width. A rabbit with the wide-band gene
combination ww would be desirable in the sandy and
maybe the light gray Flemish. The wide-band gene is thought
to improve the ring pattern in the light gray Flemish.
However, Chinchilla rabbits that posses the wide-band gene
AAcchdcchdww may show no black
surface color and are called ghost chinchillas because of
excessive white pearling throughout the hair shaft (McNitt
et al. 2000).
A Flemish with a normal intermediate
band width is sometimes referred to as a narrow-band rabbit;
both phrases refer to a rabbit with either the WW or
Ww gene combination. It is important to note that
there are no data to confirm the presence of the wide-band
gene in Flemish.
The presence of the w allele in
Flemish could be confirmed by designing a breeding test
similar to the one performed by Steuber and Lukefahr
(1991). The Belgian Hare and possibly the Rex, Silver, and
Tan breeds carry the w allele (McNitt
et al. 2000). The best crossing partner for the sandy
Flemish to test for the presence of the wide-band gene is
the Castor Rex variety, since their genotypes differ at only
the wide band and Rex fur genes, and their does would stand
a better chance of accommodating the larger size crossbred
offspring.
Basic Color Crossing Strategies
It is acceptable to cross sandies,
fawns, and whites as a color group, providing the whites are
derived from sandy and fawn lines. Utilizing whites from
crossing programs with the other varieties (black, blue,
light gray, and steel) can result in a sandy with very heavy
ticking. If the white rabbit carries a steel allele Es,
it can also produce the agouti steel A_BBC_D_EsE,
but in the absence of the chinchilla alleles at the C-series,
this agouti steel has gold ticking.
The second color group consists of
blacks, blues, light grays, steels, and whites, providing
the whites come from crossing programs in this color
grouping. Using whites from sandy and fawn lines in this
group are attributed to an increase in “smut” in the light
gray and steel surface fur and reports of a higher frequency
of shadow bars on the front legs. This strategy however,
has been a useful technique to introduce overall size and
bigger-boned qualities in rabbits belonging to the second
color group and the “smut” and shadow bars must be bred out
of the rabbits over time.
This second color grouping is sometimes
further subdivided into the black and blue subgroup and the
light gray, steel and white subgroup. For the black and
blue subgroup it is important that the black parent be a
self black, as the offspring of the agouti-black parent
(Table 2) will show light ticking on the cheeks, chest, and
flanks due to the presence of the steel allele Es.
Color Genetics of the
Flemish Varieties
Sandy and Fawn
The wild type rabbit in the Flemish is
considered the sandy color A_BBC_D_E (Table 2).
Searle (1968) considers the Flemish a dark, white-bellied
agouti and adds the w superscript to the Aw
allele to distinguish it from other agouti rabbit
breeds with dark belly fur (in this case, the w
superscript refers to the white-bellied agouti and
not the w allele used to represent the wide-band
factor). Schott (1990) covered the basic elements of what
produces a good sandy (presence of the wide-band gene
combination ww) and a good fawn (non-extension
alleles ee) (Table 2) and is not repeated here.
Sandy and fawn Flemish that are
presumed to have the wide-band gene combination ww
typically have a cream-colored belly and richer red, overall
coat color. An alternate explanation for the richer red
color is the action of minor genes. Sandy and fawn Flemish
with white belly fur are presumed to not have the wide-band
gene combination W_ and in general, have darker coat
color, due to the heavy ticking. The white belly fur and
heavy ticking in sandy Flemish are also attributed to
rabbits having ancestors from the German lines of Flemish
rabbits. Another desirable attribute of sandies descended
from German lines is the absence of shadow bars on the front
legs.
Black
One of the non-agouti or self-colored
Flemish is the black variety. The defining gene combination
for the black variety is aa (Table 2). When a self
black Netherland Dwarf inherits a steel allele ES,
the otherwise all-black Dwarf will show light ticking on the
cheeks, chest and lower sides (Schott 1989), a phenomenon
that may also occur in the black Flemish.
The black Flemish however, isn’t always
a non-agouti or self-colored rabbit. A common breeding
strategy to improve steel Flemish coat color is to mate
steels to steels (May 1990). This is mating increases the
likelihood of producing the agouti-black Flemish (Table 2),
which generally has the same outward appearance as the
non-agouti black Flemish. The agouti-black Flemish
occasionally has some lighter flecks around the back and
shoulders, which aids in distinguishing it from a self black
Flemish.
Blue
The other non-agouti or self-colored
Flemish is the blue variety. The defining gene combination
for the blue Flemish is aadd (Table 2). The recessive
gene combination dd is an example of epistatic gene
action resulting in the eye color being slate blue rather
than brown.
White
A white Flemish can either be an agouti
or self rabbit since the presence of the gene combination
cc suppresses pigment development in the fur and the eye
color is pink. There is another way to get a white rabbit.
The white chinchilla
(A_B_ cchdcchdD_ee
or A_B_ cchlcchlD_ee) may be
obtained when a fawn A_ee rabbit is crossed to either
a light gray A_Ee or an agouti-black steel A_Ese.
Both parents in the cross must also have at least once
chinchilla allele to obtain the gene combinations cchdcchd
or cchlcchl in their
offspring. The chinchilla alleles prevent yellow
pigmentation, the non-extension alleles ee prevent
black or dark coloration, and the result is no coat color
expression. However, the cchdcchdee
rabbit will have blue eye color and the cchlcchlee
will have brown eye color. Ticking is also common on
the lower part of the back and on the flanks since the ee
gene combination does not completely eliminate the black
pigmet from the tips of the hairs (Robinson 1978). The
genotype of another form of white chinchilla must also have
the dd condition and is represented as A_B_ cchdcchdddee
(Lukefahr 1986, McNitt et al. 2000).
References made to a “full-blooded”
white rabbit, regardless of how many generations of mating
white rabbits to other white rabbits, is genetically
ambiguous. All white rabbits possess color genes at the
A-, B-, D- and E-series, but the
presence of the cc gene combination does not permit
the expression of those other color genes. As an example, I
mated a light gray buck to a white doe and had a litter of
four light grays and three steels. The only way to obtain
steels in the litter is if the white doe carries the steel
allele ES (but it is not expressed in her
coat color because of her cc gene combination). If
the light gray buck had even one ES allele
it would be a steel and if two ES alleles
it would be an agouti-black and not a light gray Flemish.
Light Gray
In the earlier literature the light
gray Flemish was not considered and no specific genotypes
were noted for this variety (Table 3). Later, the light
gray was considered an agouti rabbit with the coat color
determined by another group of minor genes giving it the
chinchilla coat color (Robinson 1958). Some fancier
breeders thought the dominant steel allele ES
was responsible for light gray coat color; however, a
breeding test with a light gray Flemish doe and an American
Chinchilla buck confirmed that the gene responsible for the
light gray coat color was cchd (Steuber
and Lukefahr 1991). In later publications, the light gray
coat color was reported as the gene combination of cchdcchd
(McNitt et al. 2000). This genotype has potential
conflicts since this chinchilla gene combination results in
a marble-blue eyed rabbit. Most modern day iris color of
the light gray Flemish is brown, so selective breeding is
attributed to have broken the linkage of the marble-blue eye
color found in rabbits with the cchdcchd
gene combination. Marble-blue eye color is still
occasionally found in today’s light gray Flemish.
The presence or one or more chinchilla
alleles cchd, cchm, and
cchl in the Flemish could have the
following probable origins. First, it was always in the
gene pool, but just not recognized by the earlier authors
until they gained a working knowledge of this breed.
Second, the alleles could also have arisen through
mutation. The most likely explanation is that these alleles
were introduced into the Flemish by crossbreeding to other
rabbits carrying the chinchilla alleles to generate a new
coat color variety in the Flemish, and later, to improve on
the ring pattern.
The earlier records among the light
gray Flemish breeders do not mention the crossing of their
Flemish to other breeds to introduce the chinchilla alleles
into their herd. The first light gray Flemish were noted as
having a lot of “sandiness” in their coat color.
Higginbottom (1990) credits his light gray Flemish to
crossing a “full-blooded” white Flemish buck to a 7/8
Beveren doe and the subsequent back crosses of their
offspring to the white buck, producing light grays that were
agouti, but did not have a sandiness to their coat color.
Later crossing of a brother-sister mating of light grays
with no sandiness is attributed to helping to establish the
ring color.
Given the presence of the chinchilla
alleles in Flemish, it also seems likely that the chinchilla
alleles in the C-series are intermediate in dominance
to successive alleles in the series (Table 1). One
explanation for different shades of chinchilla from dark to
light is the presence of an intermediate chinchilla allele
cchm, but it is only mentioned in passing
that it isn’t found in North American gene pools or if it
ever was, has not been maintained over time (Lukefahr
1986). A second explanation, assuming incomplete dominance
and the presence of the cchd and cchl
alleles, rabbits with the cchdcchl,
cchdc, cchlc gene
combinations might be an explanation for the intermediate
chinchilla color rather than a specific allele cchm.
Lastly, the different shades of chinchilla from dark to
light could also be explained by minor genes.
Brown or rust backs are occasionally
seen in young, light gray Flemish. In the Chinchilla breed
the brown back may be seen in the young cchdcchdee
rabbit, but disappears with maturity (Robinson 1978).
This may be one explanation for brown backs in young, light
gray Flemish. The recessive gene combination ee is
not common in light grays, but mating fawn to light gray
Flemish would be one way to introduce the e allele
(the fawn would presumably be used to increase bone and body
size in the light gray Flemish). The brown or rusty back of
the young Chinchilla is also attributed to the cchl
allele, which removes most, but not necessarily all of the
yellow pigment. And with each new coat the “brown back”
slowly disappears (Robinson 1978). Knowledge of a breeder’s
mating practices would help sort out if there are fawn
ancestors in the pedigree of the light gray Flemish.
We do not know with absolute certainty
if the cchl allele is present in the
Flemish gene pool, but it is included in Table 1 given its
potential role in eye color, in differing shades of
chinchilla from dark to light, and in the occasional brown
or rusty backs in young, light gray Flemish. The presence
of the cchl allele in Flemish could be
confirmed by designing a breeding test similar to the one
performed by Steuber and Lukefahr (1991). Several breeds
carry the cchl allele (McNitt et al.
2000). The two breeds that are better suited as
crossing partners to a light gray Flemish are the American
Sable or the sable or seal Rex varieties, since their
genotypes differ at only the A- and C-series
and their does would stand a better chance of accommodating
the larger size crossbred offspring.
Another reason why the alleles in the
C-series show incomplete dominance is that it is
widely known among Flemish breeders that crossing a light
gray to a white “cleans up” the ring pattern and lightens
the overall appearance of a light gray Flemish. For this to
be true, the cch_ allele is not completely
dominant to the c allele, and produces the
intermediate light gray coat color cch_c.
In this example of incomplete dominance
the possible genotypes for darker and lighter colored light
gray Flemish may be explained by the following gene order
combinations from dark to light: cchdcchd,
cchdcchl, cchdc
to cchlc, with the latter two likely
contributing to the more desirable light gray Flemish,
assuming cchdcchd rabbits have
brown eyes, there is no cchm allele in the
gene pool, and the chinchilla alleles are intermediate in
dominance to successive alleles in the series. The more
common theory is that minor genes contribute to darker or
lighter colored rabbits. The other possibility for dark or
lighter light gray Flemish would be the presence of the
wide-band factor ww, which would increase the
intermediate pigment color along the hair shaft, and would
also give the rabbit the overall appearance of being lighter
gray in color.
Given the genotype of the sandy
A_BBC_D_E_ and the light gray A_BBcchd_D_E_,
why aren’t these two color varieties compatible in breeding
programs? Possible explanations are that different minor
genes contribute to changes in coat color (smut or
grayish-brown offspring) or the C allele itself is
not completely dominant after all and the presence of the
chinchilla allele cch_ modifies the coat
color of the heterozygote sandy offspring (Ccch_).
The action of minor genes is probably more popular
explanation.
Assuming the light gray Flemish have
the chinchilla alleles, the probable genotype is A_BBcchd_D_E_
(Tables 2 and 3).
Steel
The coat color of the different steels
has been referred to as black steels, sandy steels, and
chinchilla steels. Historically, the white-bellied steel
rabbit was considered an agouti rabbit, since the other two
types of steel coat colors, steel-grey (ESE)
and agouti-black “steels” (ESES,
ESe) all have dark belly fur (Table 4).
When the coat color genes were still unknown, the steel
Flemish was later considered to be modified agouti. Other
recognized steel varieties in angoras (English, French,
Satin), lops (Holland, English, French, Mini) and Netherland
dwarfs call for colored belly fur (ARBA 2001).
The steel Flemish wasn’t historically
represented as having the steel ES allele
since those rabbits had dark belly fur, so a group of
“umbrous” genes (meaning to darken) were thought to be
responsible for both the steel coat color and white belly
fur (Robinson 1958, Wilson 1941). While umbrous genes
darken the agouti to steel and behave in a polygenic manner,
the white belly fur of the agouti is usually unaffected in
their presence (Robinson 1958). (The umbrous genes are a
separate class of polygenes and are not the same as the
polygenes that impact darkening in the brown b and
blue d alleles.) In that case, only the steel
Flemish would carry the umbrous genes. These minor genes
are thought to act by decreasing the width of the normal
agouti band, in essence removing the ticking. Umbrous genes
have not been found in any breed other than the British
Flemish Giant Robinson (1978). Furthermore, the ESE
steel with dark belly fur is considered lighter in color
than a steel Flemish. White belly fur in the steel Flemish
is credited to rabbits derived from British stock (Wilson
1941).
Robinson (1958) was the first to
introduce the steel gene combination A_BBC_D_ ESE
and McNitt et al. (2000) later added the chinchilla
gene combination to the steel genotype A_BBcchdcchd
D_ESE. Based on the earlier literature this
genotype may have potential conflicts on two accounts: 1)
an ESE rabbit has dark belly fur, so it
unclear as to what contributes to the white-bellied steel
(presumably those umbrous genes (Robinson 1958)), and 2) a
rabbit with the gene combination of cchdcchd
would typically have marble-blue eyes (as would
the cchdc rabbit), unless the linkage
between the chinchilla alleles and eye color have been
broken through selective breeding.
The addition of the new alleles used to
describe the genotype of the steel Flemish (chinchilla cch_
and ES alleles) could have come
about as our working knowledge of the breed occurred over
time, by mutation, or by crossbreeding.
For this discussion on probable steel
genotypes focusing on the C-series, reserving
judgment on the cchdcchd and
cchdc gene combinations and eye color, that
successive alleles in the C-series are intermediate to each
other, and assuming light belly fur, the probable genotypes
for steel Flemish from dark to light are: A_BBcchdcchdD_ESE,
A_BBcchdcchlD_ESE,
A_BBcchdcD_ESE, A_BBcchlcchlD_ESE,
A_BBcchlcD_ESE, with A_BBcchdcchdD_ESE
rabbit having the darkest coat color of the five possible
genotypes. The author also assumes that the A_BBcchdcD_ESE
and A_BBcchlcchlD_ESE
rabbits are probably indistinguishable from each other in
coat color (assuming cchd = 3, cchl
= 2, and c = 1, the gene combinations of cchdc
and cchlcchl both
equal 4). Lastly, there may also be other modifying genes
further contributing to darker vs. lighter colored steel
Flemish.
For this discussion on probable steel
genotypes focusing on the C-series, assuming the gene
combinations cchdcchd or cchdc
still yield marble-blue eye color, that successive
alleles in the C-series are intermediate to each
other, and assuming light belly fur, the probable genotypes
for steel Flemish are reduced from five to three forms
ordered from dark to light: A_BBcchdcchlD_ESE,
A_BBcchlcchlD_ESE, A_BBcchlcD_ESE.
The author also assumes that the desirable genotype in
this example for steel Flemish is probably A_BBcchdcchlD_ESE
(since the gene combination cchdcchl
is likely being darker in color than the latter
two), leaving room for the possibility that there may still
be other modifying genes contributing to darker vs. lighter
colored steel Flemish.
Another possibility for color intensity
in steel Flemish may be the wide-band gene combination ww.
The ES allele causes a narrowing of the
band width while the w allele increases the band
width. In this case, the presence of the wide-band gene
combination ww in steel Flemish may not be
desirable. The steel with the wide-band gene combination
A_ESEww results in the agouti band
practically restored to the normal width; however, the top
color is lighter than a normal steel color, but still darker
than a narrow-band agouti (Robinson 1958). If the presence
of the wide-band gene is presumed to increase the length of
the intermediate band from ¼” up to ½” then this might also
explain the lighter colored steels. If this is true, then
the desirable genotype in this example for the steel Flemish
is A_BBcchdcchlD_ESEW_
(darkest chinchilla gene combination that yields brown eye
color and no wide-band gene combination).
The steel Flemish genotype is the most
complex of the color varieties due to two color series,
possible linkage between some of the chinchilla alleles and
eye color, complete and incomplete dominance among alleles
within the C-series, umbrous genes darkening the
agouti coat to steel and maintaining light belly fur,
modifying genes impacting darker vs. lighter coat color, and
the possible presence of the wide-band factor. Distilling
all of this information as it has developed over time
(Wilson 1941, Robinson 1958, 1978, Searle 1968, McNitt et
al. 2000), the most probable genotype for the
steel Flemish can be generalized to A_BBcchd_D_ESEW_
(Tables 2 and 4).
Historically, the steel to steel mating
and steel to white mating have been recommended for
producing the better steel Flemish (May 1990). A common
practice among some of today’s steel Flemish breeders is to
mate the black and white varieties to produce steels. These
offspring from a black-by-white cross tend to have darker
head and leg color, which can be improved upon in subsequent
generations by crossing these darker steels to other lighter
colored steels, but not to a light gray or black Flemish.
First, what is it about the white Flemish in this mating
that is so significant? The emphasis placed on its white
coat color is moot, but what is important are the major
genes it carries at the other coat color series, notably an
ES allele and its other minor genes. If
the white Flemish in this cross does not contain an ES
allele and steel offspring are produced in the
litter, then the second explanation is that the black parent
is actually an agouti-black Flemish (Table 2). Next, if the
steel Flemish has the chinchilla alleles, only the black
parent can contribute these alleles, since the white parent
has the recessive gene combination cc. The self
black parent would need to have the heterozygous gene
combination Ccch_ or the agouti-black
parent (which looks like a self black) could have either the
Ccch_ or cch__ gene
combinations. Careful analysis of the pedigrees and litters
produced from the mating of the black and white Flemish
would be needed in order to determine which parent is
contributing the ES allele and whether the
black parent is a self black or an agouti-black Flemish.
And without the chinchilla alleles present in the black
parent, the ticking of steels produced in this mating would
be gold in color.
May (1990) states it is possible to
obtain steels of varying shades in the light gray to blue
mating and in litters having even more intense color in the
light gray to black mating. Given the probable genotypes of
those three color varieties (Table 2) and assuming the black
Flemish was a self-colored rabbit, the presence of the ES
allele in the Flemish gene pool is unlikely, that
or it is not expressed because of some unknown interaction
with another gene. For example, if the blue parent had the
following genotype aaBBC_ddES_, something
would have to be acting to shut off the expression of the
steel gene combination ES_. There was no
discussion of the quality of the coat color in the blue and
black parents in these crosses, but if they had light
ticking on their sides, they could have had the steel
gene in their makeup. Given that a breeder hasn’t
accidentally substituted a steel or agouti-black Flemish in
these crosses and assuming the non-agouti parents had no
ticking, then the most probable explanation for the
appearance of steels in these litters are the umbrous
genes.
At this writing it remains unknown as
to whether the steel allele ES, umbrous
genes, or both are responsible for the steel Flemish
rabbit. Also, the earlier records don’t mention the
crossing of their Flemish with other breeds to introduce the
chinchilla alleles into the Flemish gene pool. As with the
light gray Flemish, it remains a mystery if the Flemish had
a mutation at the C-series to a chinchilla allele, if
there was crossbreeding to other breeds possessing the
chinchilla alleles, or if modifying genes are a further
explanation for the steel coat color.
MINOR GENES, MODIFYING GENES, AND
POLYGENES
Once a breeder has achieved a good coat
color, but notices the color of one rabbit is more desirable
than another, the next step to enhance coat color is
breeding for minor genes. These genes individually have
very small effects on coat color, but collectively work
together to change coat color by intensifying pigmentation.
Modifier genes behave in an additive manner and directional
selection can be practiced, much like strategies to improve
body size, body weight, and milking capacity, etc.
Strategies to improve minor genes may not be as rapid as
breeding for major genes, but deliberate and steady progress
over time can be made by mating parents with the desired
coat color. These modifier genes effects include the deeper
chestnut-red color in the sandy variety, the definition of
ring color in the light gray variety, and the degree of
steeling in the steel gray variety (McNitt et al
2001). Lighter colored steel Flemish coat color can best be
enhanced by mating those rabbits to other steel Flemish with
a darker coat color, rather than choosing another variety,
such as a black Flemish, to darken the coat.
EYE COLOR IN STEEL AND LIGHT GRAY FLEMISH
The continued presence of marble-blue
eye color from possible residual linkages with the
chinchilla alleles in the C-series is the most likely
explanation for eye color rather than assuming the rabbit
has a blue ancestor. Some will argue that it is accepted
breeding strategy to mate blacks, blues, steels, light
grays, and whites (providing the white is not from a sandy
or fawn breeding program) and that is how the blue eye color
is introduced into the steel and light gray rabbits. The
blue Flemish have slate blue eye color due to
the epistatic gene action of the gene combination aadd
at the A- and D-series. Even if there wasn’t a
distinction between marble and slate blue eye color,
consider for a moment the small number of blue Flemish
rabbits among breeders; their numbers in the breeding
population are not large enough to explain blue eye color in
both light gray and steel Flemish. Moreover, based on
common breeding strategies, other colors (black and white)
are crossed to improve the size and bone of the blue
Flemish, not the other way around (breeders don’t typically
use a blue Flemish to improve a trait lacking in another
variety of Flemish). Blue eyes coming from a blue Flemish
ancestor isn’t a likely explanation for the continued
presence of marble-blue eye color in steel or light gray
Flemish.
There are shades of brown iris color
and some litters will have kits with marble-blue eyes in
both steel and light gray Flemish up until 10-12 weeks of
age. These kits more often then not later develop the
medium-brown iris color. Other breeders prefer the darker
brown, almost a black iris color, as it is a “sure bet”
since the correct eye color is present early on and they can
cull sooner and not have to keep a rabbit beyond eight weeks
to verify eye color per the ARBA standard (ARBA 2001).
SUMMARY
The investigation of coat and eye color
in the Flemish breed has been a worthwhile exercise to
figure out both the probable genotypes for a given color
variety, but also, for what is the most desirable genetic
makeup to produce a winning show rabbit. Over the years it
has also been intriguing to investigate the genetic basis of
recommendations by various breeders as to what to look for
and breed for to produce the better show rabbit. Some of
these recommendations have a genetic basis and others may
remain old wives’ tales or urban legends. The author looks
forward to future studies in coat color genetics as new
mutations (although rare) become fixed in the rabbit gene
pools (most recently the lion’s head genetic mutation) and
as we gain more insight into the inner workings of
chromosomes and genes both through classical genetics
(selection and breeding) and by the newer molecular
techniques.
ACKNOWLEDGEMENTS
I’d like to thank the following
reviewers of this writing Dr. Steven Lukefahr and Mr. Tom
Orr. And special thanks to those Flemish breeders who first
got me started with this breed, Cal Harding, Lloyd D. Gregg,
and Bill Mairs. There has never been a more special
collection of rabbits than Buster, Papa Hoo Doo, Digger, and
Sweet Pea, well maybe, except for Vivian and Clarence, but
that’s another story.
