The basis for order in life lies in a very large molecule called deoxyribonucleic
acid, mercifully abbreviated to DNA. A related molecule, ribonucleic acid
(RNA) provides the genetic material for some microbes, and also helps read
the DNA to make proteins.
Read?
Yes, read.
DNA has a shape rather like a corkscrewed ladder. The "rungs" of the
ladder are of four different types. The information in DNA comes in how
those types are ordered along the molecule, just as the information in
Morse code comes in how the dashes and dots are ordered. The information
in three adjacent rungs is "read" by a kind of RNA that hooks onto a particular
triad of rungs at one end and grabs a particular amino acid at the other.
Special triads say "start here" and "end here" and mark off regions of
the DNA molecule we call discrete genes. The eventual result is a chain
of amino acids that makes up a protein, with each amino acid corresponding
to a set of three rungs along the DNA molecule. There are also genes that
tell the cell when to turn on or turn off another gene. The proteins produced
may be structural or they may be enzymes that facilitate chemical reactions
in the body.
We now know that chromosomes are essentially DNA molecules. In an advanced
(eukaryotic) cell, these chromosomes appear as threadlike structures packaged
into a more or less central part of the cell, bound by a membrane and called
the nucleus. What is more important is that the chromosomes in a body cell
are arranged in pairs, one from the father and one from the mother. Further,
the code for a particular protein is always on the same place on the same
chromosome. This place, or location, is called a locus (plural loci.)
There are generally a number of slightly different genes that code
for forms of the same protein, and fit into the same locus. Each of these
genes is called an allele. Each locus, then, will have one allele from
the mother and one from the father. How?
When an animal makes an egg or a sperm cell (gametes, collectively)
the cells go through a special kind of division process, resulting in a
gamete with only one copy of each chromosome. Unless two genes are very
close together on the same chromosome, the selection of which allele winds
up in a gamete is strictly random. Thus a dog who has one gene for black
pigment and one for brown pigment may produce a gamete which has a gene
for black pigment OR for brown pigment. If he's a male, 50% of the sperm
cells he produces will be B (black) and 50% will be brown (b).
When the sperm cell and an egg cell get together, a new cell is created
which once again has two of each chromosome in the nucleus. This implies
two alleles at each locus (or, in less technical terms, two copies of each
gene, one derived from the mother and one from the father,) in the offspring.
The new cell will divide repeatedly and eventually create an animal ready
for birth, the offspring of the two parents. How does this combination
of alleles affect the offspring?
There are several ways alleles can interact. In the example above,
we had two alleles, B for black and b for brown. If the animal has two
copies of B, it will be black. If it has one copy of B and one of b, it
will be just as black. Finally, if it has two copies of b, it will be brown,
like a chocolate Labrador. In this case we refer to B as dominant to b
and b as recessive to B. True dominance implies that the dog with one B
and one b cannot be distinguished from the dog with two B alleles. Now,
what happens when two black dogs are bred together?
We will use a diagram called a Punnett square. For our first few examples,
we will stick with the B locus, in which case there are two possibilites
for sperm (which we write across the top) and two for eggs (which we write
along the left side. Each cell then gets the sum of the alleles in the
egg and the sperm. To start out with a very simple case, assume both parents
are black not carrying brown, that is, they each have two genes for black.
We then have:
B B
B BB (black) BB (black)
B BB (black) BB (black)
All of the puppies are black if both parents are BB (pure for black.
Now suppose the sire is pure for black but the dam carries a recessive
gene for brown. In this case she can produce either black or brown gametes,
so
B B
B BB (pure for black) BB (pure for black)
b Bb (black carrying brown) Bb (black carrying brown)
This gives appoximately a 50% probability that any given puppy is pure
for black, and a 50% probability that it is black carrying brown. All puppies
appear black. We can get essentially the same diagram if the sire is black
carrying brown and the dam is pure for black. Now suppose both parents
are blacks carrying brown:
B b
B BB (pure for black Bb (black carrying brown)
b Bb (black carrying brown) bb (brown)
This time we get 25% probabilty of pure for black, 50% probability
of black carrying brown, and - a possible surprise if you don't realize
the brown gene is present in both parents - a 25% probability that a pup
will be brown. Note that only way to distinguish the pure for blacks from
the blacks carrying brown is test breeding or possibly DNA testing - they
all look black.
Another possible mating would be pure for black with brown:
B B
b Bb (black carrying brown) Bb (black carrying brown)
b Bb (black carrying brown) Bb (black carrying brown)
In this case, all the puppies will be black carrying brown.
Suppose one parent is black carrying brown and the other is brown:
B b
b Bb (black carrying brown) bb (brown)
b Bb (black carrying brown) bb (brown)
In this case, there is a 50% probability that a puppy will be black
carrying brown and a 50% probability that it will be brown.
Finally, look at what happens when brown is bred to brown:
b b
b bb (brown) bb (brown)
b bb (brown) bb (brown)
Recessive to recessive breeds true - all of the pups will be brown.
Note that a pure for black can come out of a mating with both parents
carrying brown, and that such a pure for black is just as pure for black
as one from ten generations of all black parentage. THERE IS NO MIXING
OF GENES. They remain intact through their various combinations, and B,
for instance, will be the same B no matter how often it has been paired
with brown. This, not the dominant-recessive relationship, is the real
heart of Mendelian genetics.
This type of dominant-recessive inheritance is common (and at times
frustrating if you are trying to breed out a recessive trait, as you can't
tell by looking which pups are pure for the dominant and which have one
dominant and one recessive gene.) Note that dominant to dominant can produce
recessive, but recessive to recessive can only produce recessive. The results
of a dominant to recessive breeding depends on whether the dog that looks
to be the dominant carries the recessive. A dog that has one parent expressing
the recessive gene, or that produces a puppy that shows the recessive gene,
has to be a carrier of the recessive gene. Otherwise, you really don't
know whether or not you are dealing with a carrier, bar genetic testing
or test breeding.
One more bit of terminology before we move on - an animal that has
matching alleles (BB or bb) is called homozygous. An animal that has two
different alleles at a locus (Bb) is called heterozygous.
A pure dominant-recessive relationship between alleles implies that
the heterozygous state cannot be distinguished from the homozygous dominant
state. This is by no means the only possibility, and in fact as DNA analysis
advances, it may become rare. Even without such analysis, however, there
are many loci where three phenotypes (appearances) come from two alleles.
An example is merle in the dog. This is often treated as a dominant, but
in fact it is a type of inheritance in which there is no clear dominant
- recessive relationship. It is sometimes called overdominance, if the
heterozyote is the desired state. I prefer incomplete dominance, recognising
that in fact neither of the alleles is truly dominant or recessive relative
to the other.
As an example, we will consider merle. Merle is a diluting gene, not
really a color gene as such. If the major pigment is eumelanin, a dog with
two non-merle genes (mm) is the expected color - black, liver, blue, tan-point,
sable, recessive red. If the dog is Mm, it has a mosaic appearance, with
random patches of the expected eumelanin pigment in full intensity against
a background of diluted eumelanin. Phaeomelanin (tan) shows little visual
effect, though there is a possibility that microscopic examination of the
tan hair would show some effect of M. Thus a black or black tan-point dog
is a blue merle, a brown or brown tan-point dog is red merle, and a sable
dog is sable merle, though the last color, with phaeomelanin dominating,
may be indistinguishable from sable in an adult. (The effect of merle on
recessive red is unknown, and I can't think of a breed that has both genes.)
What makes this different from the black-brown situation is that an MM
dog is far more diluted than is an Mm dog. In those breeds with white markings
in the full-color state the MM dog is often almost completely white with
a few diluted patches, and has a considerable probablity of being deaf,
blind, and/or sterile. Even in the daschund, which generally lacks white
markings, the so-called double dapple (MM) has extensive white markings
and may have reduced eye size. Photographs of Shelties with a number of
combinations of merle with other genes are available on this site, but
the gene also occurs in Australian Shepherds, Collies, Border Collies,
Cardiganshire Welsh Corgis, Beaucerons (French herding breed), harlequin
Great Danes, Catahoula leopard dogs, and Daschunds, at the least.
Note that both of the extremes - normal color and double merle white
- breed true when mated to another of the same color, very much like the
Punnett squares above for the mating of two browns or two pure for blacks.
I will skip those two and go to the more interesting matings involving
merles.
First, consider a merle to merle mating. Remember both parents are
Mm, so we get:
M m
M MM (sublethal double merle) Mm (merle)
m Mm (merle) mm (non-merle)
Assuming that merle is the desired color, this predicts that each pup
has a 25% probability of inheriting the sublethal (and in most cases undesirable
by the breed standards) MM combination, only 50% will be the desired merle
color, and 25% will be acceptable full-color individuals. (In fact there
is some anecdotal evidence that MM puppies make up somewhat less than 25%
of the offspring of merle to merle breedings, but we'll discuss that separately.)
Merle, being a heterozygous color, cannot breed true.
Merle to double merle would produce 50% double merle and is almost
never done intentionally. The Punnet square for this mating is:
M M
M MM (sublethal double merle) MM (sublethal double merle)
m Mm (merle) Mm (merle)
Merle to non-merle is the "safe" breeding, as it produces no MM individuals:
m m
M Mm (merle) Mm (merle)
m mm (non-merle) mm (non-merle)
We get exactly the same probability of merle as in the merle to merle
breeding (50%) but all of the remaining pups are acceptable full-colored
individuals.
There is one other way to breed merles, which is in fact the only way
to get an all-merle litter. This is to breed a double merle (MM) to a non-merle
(mm). This breeding does not a use a merle as either parent, but it produces
all merle puppies. (The occasional exception will be discussed elsewhere.)
In this case,
M M
m Mm (merle) Mm (merle)
m Mm (merle) Mm (merle
The problem with this breeding is that it requires the breeder to maintain
a dog for breeding which in most cases cannot be shown and which may be
deaf or blind. Further, in order to get that one MM dog who is fertile
and of outstanding quality, a number of other MM pups will probably have
been destroyed, as an MM dog, without testing for vision and hearing, is
a poor prospect for a pet. In Shelties, the fact remains that several double
merles have made a definite contribution to the breed. This does not change
the fact that the safe breeding for a merle is to a nonmerle.
Thus far, we have concentrated on single locus genes, with two alleles
to a locus. Even something as simple as coat color, however, normally involves
more than one locus, and it is quite possible to have more than two alleles
at a locus. What happens when two or more loci are involved in one coat
color?