Basic Genetics and the Australian Shepherd Part One: Inheritance of Color and Pattern
First printed in the Aussie Times
by George P. Johnson
The following article is the first in a series which will appear periodically
in the Aussie Times. The intent of these
articles is to review the
fundamental concepts of genetics as they pertain to the Australian Shepherd, answer
questions about genetics from the ASCA membership, and inform the membership about
discoveries in the field of genetics research and how these discoveries will affect our
Aussies. It is my hope that these articles will in some small way encourage the membership
to learn more about the genetics of the Australian Shepherd.
The study of genetics is often divided into three specific areas: transmission,
population, and molecular. While somewhat artificial, these divisions make the subject
easier to discuss and roughly approximate the development of the discipline; therefore, we
will follow them in this series. The oldest of the three divisions is transmission
genetics which deals with the passing of genetic information from parent to offspring
which we see in our dogs' pedigrees. This area is also called classical or Mendelian
genetics after Gregor Mendel, who discovered the basic principles of inheritance.
Unfortunately, Mendel worked in relative isolation and his ideas, although published, were
not widely seen by others. Several years after his death the principles of inheritance
which he discovered were independently re-discovered by others. Although the concept that
"Like Begets Like" is an ancient one, Mendel was apparently the first person to
accurately record results of experimental crosses and make predictions concerning the
outcomes of others. Much of Mendel's work with pea plants can be directly applied to
Australian Shepherds.
In the process of reproduction, dogs pass genetic information contained within sperm
and eggs to their offspring. The combination of the sire's and dam's information is
responsible for the characteristics of the puppies. Examples of such characteristics would
be color, pattern, presence of white and copper trim, and such genetic diseases as Collie
Eye Anomaly and Progressive Retinal Atrophy. Each parent contributes 39 chromosomes to
each offspring, making a total of 78 chromosomes per dog. A chromosome is composed of a
linear sequence of units of genetic information; each unit is known as a gene. In the
simplest of situations, each gene is responsible for a single characteristic or trait. In
the following discussion, the term dog is used to refer to both sexes unless specifically
stated otherwise.
Inheritance of Color in The Australian Shepherd:
The trait of flower color that Mendel studied in pea plants is controlled by the
activity of a single gene with two variant color forms. The Australian Shepherd parallels
this as the base color of Aussies is also controlled by a single gene (at a location known
as the B locus) with two variant forms (alleles),
black and red. The appearance of the dog is referred to as its phenotype and an Aussie is
either phenotypically black or red. Merle versus solid pattern is controlled by a separate
gene and will be discussed later in this article. A merle is either a black or a red dog
because color and pattern are separate traits. The black color phenotype is due to the
presence of at least one copy of the dominant black allele (B) while the red
phenotype is due to the presence of two copies of the recessive red allele (b). By
convention, dominant alleles are always represented by "UPPER" case letters and
recessive alleles are always represented by "lower" case letters. Due to the
ability of the black allele to overshadow or "dominate" the red allele, a
phenotypically black dog may be a carrier of a hidden red allele; such dogs are described
as red factored or red carriers.
While the appearance of a dog is known as its phenotype, the genetic makeup of the dog
is known as its genotype. Each dog actually carries two alleles for color - one
contributed by its sire, and one contributed by its dam. Because of the dominant nature of
the black allele (B), a black dog's genotype may be either BB or Bb;
therefore, it is impossible to tell the genotype of a black dog by looking (B?).
On the other hand, due to the recessive nature of the red allele (b), a red dog is
always known to have the bb genotype. The solution for the problem of determining
whether a dog with the dominant black phenotype is homozygous dominant (BB) or
heterozygous (Bb) was solved by Mendel. (Note: the prefix 'homo' means same. The
prefix 'hetero' means different.) The solution is known as a testcross in which a dog with
the dominant black phenotype (BB or Bb) is mated to a dog with the recessive
red phenotype (bb); the goal is to determine if the black dog carries the recessive
red allele (b).
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Parent 1
Black (BB) |
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B (Black allele) |
B (Black allele) |
Parent 2
Red (bb) |
b (red allele) |
Bb (Black)
(Red Carrier) |
Bb (Black)
(Red Carrier) |
| b (red allele) |
Bb (Black)
(Red Carrier) |
Bb (Black)
(Red Carrier) |
Figure 1. Homozygous Black (BB) Mated to a Homozygous Red (bb) |
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Figure 1 shows the outcome of a testcross involving
a homozygous dominant black dog (BB) when mated to a red dog (bb). All pups
will be black but red factored (Bb). All the pups are heterozygous or carriers of
the red allele. This representation of a mating is known as a Punnett Square and
represents the genotypes of the parents and the possible genotypes of the offspring. While
each individual dog carries two color alleles, during the process of sperm and egg
formation, a separation of the alleles occurs so that each parent contributes only one
color allele to each pup. Each egg and sperm that combine to form a pup will contribute
one allele for each trait, in this case giving the pup its two copies of the allele for
color.
In a situation where we are considering only one trait, known as a monohybrid, the
Punnett Square will have four cells. The number of cells of a particular genotype will
give the probability or chance of having that genotype in the offspring. In the case of a
testcross involving a homozygous dominant (BB) black dog, four out of four cells
are heterozygous and all offspring will exhibit the dominant phenotype. The probability of
getting a black dog from this mating is 4 out of 4 cells = 1.0 = 100%. A red pup cannot
result from this mating.
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Parent 1
Black (Bb)
(Red Carrier) |
| B (Black allele) |
b (red allele) |
Parent 2
Red (bb) |
b (red allele) |
Bb (Black)
(Red Carrier) |
bb (red) |
| b (red allele) |
Bb (Black)
(Red Carrier) |
bb (red) |
Figure 2. Heterozygous Black (Bb) Mated to a Homozygous Red (bb) |
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Figure 2 is the Punnett Square for the testcross of
a heterozygous (Bb) or red-carrier black dog. Because the B and b
alleles will separate in forming either eggs or sperm, two combinations are possible in
the offspring. If a pup receives the B (black) allele from the red-carrier parent
it will have the Bb genotype and the black phenotype. But, if the pup receives the b
(red) allele from the red-carrier parent it will be matched with the b (red) allele
from the red parent, giving the pup the bb genotype and the red phenotype. This is
where chance dictates what genotype/phenotype a pup will have. Two of the four cells of
the Punnett Square have the heterozygous genotype (Bb) and two cells have the
homozygous recessive genotype (bb). There are 2/4 chances for one genotype or the
other. On average 50% of the pups will be of one color and 50% will be of the other.
Unfortunately, in any testcross, the appearance of black pups tells you little about
the genotype of the individual in question. After the production of seven black pups there
is better than a 99% chance that the dog was indeed homozygous dominant. The birth of
additional offspring will increase this confidence level but you can never be 100% sure of
the dog's genotype. But, the appearance of a single red pup immediately indicates that the
dog is heterozygous for color (Bb) and it is expected that on average, 50% of the
pups will be red. One note of caution - while we can determine the probability or chance
of an event happening, that does not mean that it will happen with that frequency. It is
possible (although unlikely) that a heterozygote would never produce a red pup in test
matings.
While every mating is potentially a testcross, in actuality matings are generally not
made with the intent of discovering the presence of recessive alleles (except in a
situation designed to detect the carrier of a recessive genetic disease). Nevertheless,
surprises happen and the occurrence of one or more red puppies in a litter does happen
even when there is no indication from the pedigree that any of the immediate ancestors
were red or red-factored. In large, randomly breeding populations, the largest number of
recessive alleles often occur in heterozygous individuals. And while purebred dogs are not
random-breeding, a sizeable number of the recessive red alleles (b) are found in
individuals of the dominant phenotype. Because of this, forms of traits controlled by
recessive alleles follow a typical pattern:
- Expression of the recessive phenotype may skip generations;
- All of the offspring of parents of the recessive phenotype (bb)are also of the recessive phenotype (bb);
- In litters where both parents are of the dominant phenotype but heterozygous (Bb), approximately 1/4 of the offspring will be of the recessive phenotype (bb); and,
- There will be approximately equal numbers of males and females of each color. The gene for color is not associated with the chromosomes that determine sex.
The opposite situation exists for the forms of traits controlled by dominant alleles:
- The dominant phenotype is seen in each generation;
- Every offspring with the dominant phenotype has at least one parent with the dominant phenotype;
- In litters where both parents are of the dominant phenotype (BB or Bb),
all or 3/4 of the offspring will be of the dominant phenotype, respectively; and,
- As with recessive alleles, there will be approximately equal numbers of males and females of
each color.
Another example of a trait in Aussies that is controlled by one gene with two alleles,
one dominant and one recessive, is ticking (the T locus). Ticks appear as copper or
black spots in the white areas of the dog; this is especially noticeable on the feet and
muzzle. The presence of ticks is determined by the dominant allele (T) while the
absence of ticks is due to the recessive allele (t). Two dogs without ticking (tt)
cannot produce pups with ticks because the dogs are each homozygous recessive.
Inheritance of Pattern In the Australian Shepherd
There are two patterns possible in Aussies, merle and solid. This trait is controlled
by the gene at the M locus which is an entirely different gene than the one for
base color. The gene controlling pattern is even on a different chromosome than the gene
for color so that any and all combinations of color and pattern are possible: blue merle,
black, red merle, and red.
Pattern is a little more complicated than color in Aussies, because while there are two
alleles for pattern (M for merle and m for solid), the heterozygous merle (Mm)
is distinguishable from the homozygous merle (MM) and the solid (mm).
Homozygous merle Aussies are almost always excessively white, and to complicate the
situation even more, may suffer from eye, ear and other problems and should be euthanized.
Many breeders avoid this situation by only breeding a merle to a solid thereby eliminating
any possibility of a homozygous merle. For genes where the heterozygote is intermediate
between and distinguishable from both homozygotes the interaction between the alleles is
known as incomplete dominance or lack of dominance. Two of the most common examples of
this type of interaction are Roan coloration in cattle and pink coloration in Four-o'
Clock flowers where the heterozygotes (roan and pink, respectively) are intermediate
between the red and white homozygotes. Heterozygous merles are considered to be normal and
not to have the problems associated with homozygous merles.
Although the interaction among the alleles for pattern is more complicated than those
for color, mating a heterozygous merle (Mm) to a solid (mm) is essentially a
testcross. Since each pup has a 50/50 chance of inheriting the M or the m
allele, one half of the litter is expected to be merle (Mm) and the other half is
expected to be solid (mm). If only one parent is a merle the complication of the
homozygous merle is avoided; there can be none.
If both color and pattern are considered together, because two genes are being
considered, the situation is known as a dihybrid. The Punnett Square would now be composed
of sixteen cells, and the probabilities associated with the various genotypes would be
measured in sixteenths. Each parent would pass on an allele for color as well as an allele
for pattern. The greatest amount of variation possible in the litter would be achieved by
mating a red-factored black dog (Bb) to a red dog (bb), one of the dogs
being merle (Mm) and the other being solid (mm). It would make no difference
which color was solid and which was merle; all color and pattern combinations are possible
regardless of parental color/pattern because the genes are not linked on the same
chromosome. There would be a 25% chance (4 of 16 cells of the Punnett Square) of producing
either a blue merle, red merle, black solid or red solid dog.
Extensions of Mendelian Genetics
The principles of Mendelian or transmission genetics can be applied to more complex
situations such as the genes in Aussies responsible for the presence and distribution of
white and copper trim. These genes have multiple alleles and present a broader range of
outcomes than the two or three associated with color and merling, respectively, presented
here. Nevertheless, they can be analyzed in the same way.
The recent decision by ASCA's Board of Directors to phase-in the DNA fingerprinting of
all breeding Aussies is one of the most exciting applications of the principles of
transmission genetics. Although the DNA segments known as markers used to fingerprint dogs
do not represent Mendelian genes, the variants of the markers act like alleles and are
transmitted in Mendelian fashion and can be traced through the pedigree. Therefore, the
sire and dam of a pup transmit one of the two alleles that they have at each marker
(locus) to each pup, just as they did for the alleles of color and pattern. The alleles
present in each pup can be attributed to the sire and dam thereby confirming that they are
the parents of record. DNA fingerprinting and multiple alleles will be the focus of a
future article in this series.
Questions from the Membership
One of the purposes of this series of articles is to answer questions from ASCA's
membership. If you have a question about something in one of these articles or anything
related to the genetics of the Australian Shepherd, contact me at the address below, by
phone, or by email. If I don't know the answer I will search until I do. Your question
will be answered privately or in an upcoming issue of the Aussie
Times.
George P. Johnson
Department of Biological Sciences
Arkansas Tech University
Russellville, AR 72801
Phone: 501-968-0312
FAX: 501-964-0837
Email: george.johnson@mail.atu.edu
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