101: Introduction to Genetics Terms and Concepts
for Understanding Rabbit Coat Color Genetics
lot of jargon and terminology used in genetics can
be confusing to someone who has not taken a genetics
class. This article is not a complete list of every
term and concept in genetics--my purpose is to give
rabbit breeders and hobbyists interested in rabbit
coat color genetics a basic, solid understanding
of the field.
can start this lesson with the most basic of basics.
Where do the sperm and egg come from that became
our favorite bunny? These cells come from the doe's
and buck's gonads.
In the doe the gonads are called ovaries and in the
buck they are called the testis. The doe produces
several eggs during her estrous cycle. For rabbits,
this occurs every 16-18 days, provided the doe is
not pregnant. Does usually become "receptive" to
bucks at about 3½ months of age and are capable of
conception at 4 or 4½ months.2 This
is when she is in "heat." The buck has a supply of
sperm in the testis all the time and is always ready
sperm and egg contain genetic material that when
combined form the genome of their offspring. You
have probably heard of the word genome by now. If
you have not, you have probably been on a desert
island--were you the last survivor? You could buy
many rabbits with that money! With all of the cloning
and genetics research being reported in the news
today, the word genome might be misunderstood. Genome refers
to the complete set of genes or genetic material
that makes an organism unique. A gene is
one of many discrete units of hereditary information
located on the chromosomes and consisting of DNA. DNA is a
set of nucleic acids that are usually localized in
a cell's nucleus and that form the molecular basis
for heredity in most organisms. A chromosome is
a long strand of DNA that is wound tightly around
little ball-like proteins called histones. Each of
us has our own personal genome, locked in to every
cell of our bodies, that makes each of us the people
that we are. The word genome can also be generically
used to refer to the unique set of genes that makes
all apes, apes, or that makes all rabbits, rabbits
(i.e., the rabbit genome).
rabbit genome contains 22 pairs of chromosomes; 21
pairs are autosomal
chromosomes. That is to say, they do not
determine the sex of the rabbit. The last pair is
chromosomes. The infamous X and Y or X and
X. A doe has two X-chromosomes, and a buck has an
X and a Y chromosome. If a gene is on a sex-chromosome,
then the gene is a sex-linked gene. If two genes
are on the same chromosome, they are called linked genes.
In some species, specific traits appear in only males
or in only females. The genes that control these
traits are mostly likely located on the X and or
each pair of chromosomes, one comes from the buck
(sire) and on comes from the doe (dam). This is where
the duplicate letters come from in genetics (i.e.,
AA, Aa, BB, Bb, or DD, Dd). Each pair of chromosomes
is known as homologous
chromosomes. Each letter represents one allele
for a gene from each parent. An allele is
each possible form of a gene.
homologous chromosomes are not identical! They each
contain the same number of genes in the same sequence
running along the strand of DNA and each has its
own mutations. They're like two parallel streets
with the same number of houses. The location of the
gene along the strand of DNA is called the locus,
or the physical address of the gene (i.e., 432 Fairmount,
432 Haven = gene A on chromosome 1A and gene A on
chromosome 1B). Each chromosome can contain different
alleles for each gene. The assignment of A and B
to a chromosome is random because we cannot determine
which one came from which parent under a microscope.
It's like different people living in the corresponding
houses on two different streets (the Jones live in
423 Fairmount and the Smiths live in 423 Haven =
having an at on chromosome 1A and an A
on chromosome 1B). The alleles for a gene are referred
to as a series (A
series = A, at, a; B series = B, b; C
series = C cchd, cchl, ch,
c; etc.). Therefore, if I want to talk about the
alleles for the C gene, I would say, "the C series." Later
we will talk about what each letter means in relation
to the coat color of the rabbit.
talking about genes, it is usually good to start
with a reference. In most cases, the gene of concern
is compared to the wild type. Wild type refers
to the allele that is the most common in the wild
for a specific gene. For rabbit coat color, this
is the chestnut agouti coat color. All of the alleles
for a gene arise from mutation.
Radiation, chemicals, UV light, free radicals from
within the cell, and many other sources can cause
these mutations. With reference to the wild type
and other alleles for a gene, an allele can be classified
as recessive, dominant,
E series for rabbit coat color is an excellent example
of mutations that are dominant and recessive to the
wild type. The allele for the E gene that produces
a steel coat is Es; this is dominant to
the wild type form for the gene E. The two other
alleles for the E gene, e and ej, are
recessive to the wild type, E.3 A
good example of codominance is found in the snapdragon.
The genes for flower color have several alleles.
If you cross a red flowered plant with a white flowered
plant, you get offspring with pink flowers.4 The
pink is not the result of a different allele, but
the expression of both the white and the red alleles
at the same time.
Gene Labeling System
genes are assigned letters to represent them, a capital
letter usually indicates a dominant allele and a
lowercase letter usually indicates a recessive allele.
The superscript letters (Es) are used
to differentiate between two dominant or two recessive
alleles. Superscript letters are also used to differentiate
between dominant alleles and the wild type (Ed,
Es, E). Remember that the terms "recessive" and "dominant" are
assigned to alleles in relation to the wild type
allele. Using a system that keeps all alleles for
a gene using the same letter makes it is easier to
keep track of genes when doing pedigree analysis.
It is true that the letter chosen for a gene is somewhat
random. The genes are usually assigned in the order
they were discovered. The first gene discovered for
rabbit coat color was labeled A, the second one B
, etc. Sometimes the letters are assigned to indicate
what the gene is responsible for. For example, Du
is the Dutch spotting gene or En is the English spotting
gene. If you are the one that discovers a gene, you
usually get the privilege of deciding the nomenclature,
or name, that should be used for the gene.
is hard enough keeping track of what an allele means--imagine
if each allele were assigned a random letter or number
to represent it. There would be a lot more people
on the planet who were bald from pulling out their
own hair! Figure 1 shows
an example of the ease of comparing two rabbits,
based on coat color, using the established system
of using the same letter for all alleles of a gene.
Figure 1 also shows how chaotic it would be, using
the same two rabbits, if a random system were used.
You can see that a direct comparison is much easier
using the established system of naming.
1: The effectiveness of gene labeling systems
of gene labels
using established system
of gene labels
using random system
in Rabbit Coat Colors
Table 1 shows
the common naming of the genes involved in rabbit
coat color and lists the alleles for each of the
genes involved in the coat color of rabbits. In the
table, they are listed in order of dominance with
the most dominant listed first. Four important terms
when talking about the genetic make up of an animal
are genotype, phenotype, heterozygous,
Heterozygous and homozygous refer to the combination
of alleles the rabbit has for a particular gene.
A heterozygous combination means the rabbit has two
different alleles for the gene and homozygous means
the rabbit has the same two alleles for the gene.
Genotype and phenotype have to do with the genetic
makeup and physical appearance of a rabbit. An easy
way to help keep track of what each means is to think
of them as thus; genotype = gene type, phenotype
= physical type.
genotype is comprised of all the genes in an animal,
including genes that do not show in the animal's
physical appearance. The phenotype is comprised only
of the genes that show in the appearance of the animal.
The phenotype comes from what we can tell about the
animal by looking at it. If you have a solid chestnut
agouti mini Rex, you know that the phenotype of the
animal is ABCDEEnDuVWSi. These are the alleles for
the genes that code for the chestnut agouti color.
Now, each of these dominant alleles could be masking,
or hiding, recessive alleles. Because we cannot tell
what the second allele is for some genes, we use
the underscore (_) to represent the alleles we do
not know. The genotype for this chestnut agouti mini
Rex would be A_B_C_D_E_EnEnDuDuVVWWSiSi.
find out what the missing alleles are in a rabbit,
we could breed this rabbit to others. (Yeah, I know, "Duh!")
The breedings can help to fill in the missing alleles:
when the offspring is born, you can look at the color
of their coats. A pedigree with
information about the rabbit's ancestors' colors
can help to fill in the gaps, too.
are times, though, that the pedigree will be of little
use. If the rabbit is a red-eyed or blue-eyed white,
it would be difficult using a pedigree to determine
the other alleles in the rabbit. We could, however,
find out what the missing alleles are in our chestnut
agouti mini Rex by breeding him to other rabbits--hopefully
other mini Rexs--that have recessive alleles. Ideally,
this would be done in one cross with an aabbccddeeenenduduvvwwsisi
rabbit. Realistically, this cross is achieved in
several different breedings.
the above cross, you can see that the phenotype of
the recessive rabbit is long and contains information
that we are not talking about. If the rabbit is solid
in coat color and does not posses the silvering gene,
we can shorten the phenotype to just the genes of
interest, aabbccddee. This helps save keystrokes
and ink. To shorten it even further, you can list
only the recessive alleles. For a chocolate mini
Rex, this would be aabb. In this case, the other
alleles would be assumed to be C_D_E_. This works
if you do not know the other alleles. If you do know
something about the other alleles, then you could
fill them in like this: aabbCc.
1: Alleles for Coat Color in Rabbits5,6
agouti pattern. Gray with white belly
and Tan coat pattern. This allele gives the
rabbit a white belly, black nonagouti on the
dorsal surface, whitish eye circles, and tan
on the foot pads, under the tail, and at the
edge of the white belly.
Solid black coat
type, black eumelanin
of black eumelanin with brown eumelanin.
|Notes: Agouti brown rabbits are cinnamon
colored, where nonagouti brown rabbits are
solid brown in color
pigmentation. Fully colored
pigment is reduced to white
pigment reduced to sepia brown
reduces black pigment to pale brown
Restricts all pigment to the extremities. This
allele is temperature sensitive.
Eliminates all pigment.
|Notes: Mutations in the C series reduces
pigmentation. The superscripted H for the Himalayan
gene indicates that it is dominant to the other
alleles and the lowercase c indicates it is
recessive to the C, normal pigmentation. Pigment
in the eye is also reduced by the alleles in
|D Series (Dilute locus)
pigmentation, black and yellow is intense
pigmentation. Black to blue and red to yellow.
|Notes: The homozygous condition, dd, results
in different pigmentation changes according
to other alleles present: black to a blue-gray,
brown to lilac, or yellow to cream.
or eliminate the agouti band of phaeomelanin
and darkens the belly/TD>
weaker version of Ed. Considered more codominant
with E than Ed
brindling. Mosaic distribution of black and
color. Coat is yellow with a white belly
|En Series (English Spotting)
|Du Series (Dutch Spotting)
Dutch. Minimal amounts of white
Dutch. Extensive white spotting
|V Series (Vienna White, BEW)
White. All pigment is removed from the hair.
Pigment is also removed from the anterior surface
of the iris, giving it a blue appearance.
|W Series (Wide Band)
agouti band double in width
allele - An alternative form of a gene. The word
allele comes from allelomorphs.
autosomal - A chromosome that is not directly
involved in determining sex, as opposed to the sex chromosomes.
chromosome- A long, threadlike association
of genes in the nucleus of all eukaryotic cells and most visible
during mitosis and meiosis. Chromosomes consist of DNA and protein.
codominant - A phenotypic situation in which
both alleles are expressed.
DNA - Short for deoxyribonucleic acid. Nucleic
acids that are localized in a cell's nucleus and form the molecular
basis for heredity in most organisms. DNA have a double helix of
deoxyribose and phosphate, held together by hydrogen bonds.
dominant - The allele that is fully expressed
in the phenotype.
gene - One of many discrete units of hereditary
information located on the chromosomes and consisting of DNA.
genome - The complete complement of an
organism's genes; an organism's genetic material.
genotype - The genetic makeup of an
gonads - The male and female sex organs;
the gamete-producing organs in most animals.
heterozygous - Having two different
alleles for a given trait.
homologous chromosomes - Chromosome pairs
of the same length, centromere position, and staining pattern that
possess genes for the same traits at corresponding loci. One homologous
chromosome is inherited from the organism's father, the other from
homozygous - Having two identical
alleles for a given trait.
incomplete dominance - A type of inheritance
in which hybrids have an appearance that is intermediate between
the phenotypes of the parental varieties.
linked genes - Genes that are located
on the same chromosome.
locus - A particular place along the length
of a certain chromosome where a given gene is located.
mutation - A rare change in the DNA
of genes that ultimately creates genetic diversity.
pedigree - A family tree describing the occurrence
of heritable characteristics in parents and offspring across as
many generations as possible.
phenotype - The physical and physiological
traits of an organism.
recessive - The allele that is completely
masked in the phenotype.
series - The alleles for a gene.
sex chromosomes - The pair of chromosomes
responsible for determining the sex of an individual.
sex-linked genes - Genes located
on one sex chromosome but not the other.
wild type - For a specific gene, the allele
that is the most common in the wild.
Campbell, Neil. Biology, Fourth Edition. The
Benjamin/Cummings Publishing Company Inc., Menlo
Park, California, 1996
2 Manning, Patrick J. Newcomer, Christian E., Ringler, Daniel H., The
Biology of the Laboratory Rabbit, Second Edition. Academic
Press, Inc., San Diego, California, 1994 p. 32
3. Castle, W. E. The Genetics of Domestic Rabbits: A manual
for students of mammalian genetics and an aid to rabbit breeders
and fur farmers. Cambridge, Harvard University Press, 1930
4. Campbell, p. 248
5. Manning P. 7
6. Manning P. 12