OF PEAS AND PUPS
PART III
Dear Dr. McCue: …This
was a well done paper. Several of the ideas you developed are difficult to get
across to the layman, but you did a good job.
The "Mother
Line" idea is difficult to defend from a pure genetical
point, but I feel it is worthwhile, if for no other reason than to focus
attention on the importance of "good bitches", which has been the
weakness in many breeding programs. The idea of "breeding any bitch"
to a "big name stud" won't get the job done.
Sincerely, Dr. R.C. Busteed
ED NOTE: Dr. Busteed received First Honorable
Award at the annual awards dinner of the Dog Writers Association of America
(DWAA). One of the three judges was internationally known Dr. Clarence C.
Little, scientist, author and researcher.
INTRODUCTION
WITHOUT SOME KNOWLEDGE
of the basis of heredity an understanding of life itself is impossible and some
of the joys of a full life cannot be realized.
The Laws of Heredity are
as much a part of Nature as the rising of the sun. We cannot fail to use them
even if we try. They made us what we are and out children what they are. These
latter reasons alone should awaken our interest...Every living cell, plant and
animal, since time began, is governed by these laws, and here, as elsewhere,
"ignorance is no excuse for the law". Let us together make an effort
to understand the few rules which have been uncovered for us and to employ them
wherever possible to the advantage of our short-tailed GSP companions.
TRIHYBRIDS
In discussing hybrids
in Part II, we crossed a GWP and a GSP, suggesting that they were the same
breed genetically, except for the texture of their coats. At this point let us
consider Trihybrids (heterozygoteous
for three pairs of alleles) and make similar mating....Coat colors are used as examples not because they hold the
greatest interest and not because they offer most for breed improvement...but
simply because the color and texture genes are among
the very few genes explored, discussed and understood in the dog....Since they
show themselves more readily, they are more easily explained. Knowledge of the behavior of these genes have some practical value but their
greatest contribution lies in helping us more fully understand the action of
all genes. It is thought that the genes controlling these qualitative factors (color, texture, et al) operate in much the same way as
those controlling the more important (to us) quantative
factors (nose, run, weight, et al) which we will discuss later. In this latter
case, the explanation is not easy, since the phenotype is so difficult to
measure and the genotype is so much more complicated...Liver is liver, white is
white, rough is not smooth but...how do we describe accurately the degree of
love of water, nose, run, stamina, "intelligence", trainability and
the like??
To discuss Trihybrids,
we must first "make" some. This can be accomplished by mating a
WWTTRR and a wwttrr, a homozygous dominant and a homozygous recessive. A Wirehaired
(WW) bitch, sold (TT) liver with a small ticked (RR) area on the chest and a
Shorthair male with smooth coat (ww), spotted (tt) but without ticking (rr). Now all the pups are going to be WwRrTt. What are they going to look like? The dominants
indicated by capital letters are going to show us. They will look just like
their dam...Ww Wirehaired, Tt
solid liver, while whatever tiny patch of white does show, will be
ticked....Ordinarily, we would mate these siblings or sibs (brothers &
sisters), however, since this might pain the moral sensibilities of some of our
more timid readers, we will make two of the above matings
with unrelated parents and then there will be plenty of puppies to breed from
each litter without resorting to inbreeding...The male puppies will form eight
genetic types of sperms (as far as these genes are concerned) and the little
bitches will form eight types of eggs in their independent assortment (assuming
that each gene lies on a different chromosome) as shown here...Fig 18
A checkerboard
involving these gametes give us a very cumbersome 64 squared (8x8) diagram with
a phenotypic ratio of 27:9:9:9:3:3:3:1. This is not so
complicated as it at first appears. It can all be boiled and broken down to Mendels original 3:1 ratio. To
simplify the details and emphasize the 3:1 ratio again this diagram has been
adapted from the German pedigree forms. The GWP are all shown above the
horizontal midline, where the sires lineage usually falls, and the GSP are all
shown in the area where generally rests the ancestors of the dam of the litter
in question....Fig 19.
Note there are three
times as many Wirehairs (48) as Shorthairs (16). Of these Wirehairs there are
three times as many solid livers (36) as spotteds
(12). Of the solids there are three times as many ticked (24) where there is a
spot of white, as there are without that ticking (12) in those small areas.
Follow the GSP's and note again the 3:1 ratio all the
way through. The Phenotype shows...The Genotype shows sometimes but we must
breed again to learn where in the case of the dominants. If the Phenotype gives
us the recessive, we know it is homozygous. We learned all of this from Mendel
and his round, wrinkled seed experiment and it makes no difference which
characteristics are under discussion, these
same rules apply....The overall picture becomes more complicated numerically
but the 3:1 phenotypic ratio is still present - two hybrids will yield
3:1....Two phenotypes in a ratio of 3 to 1 and three genotypes in a ratio of
1:2:1, when both are heterozygous. What if one is heterozygous Ww and one homozygous ww, instead
of both factors being heterozygous as in the hybrids?...Since there are only
two possible outcomes, the same as the original, regardless of how we try to
rearrange them we get a 1:1 ratio. In this case we have but two genotypes and
two phenotypes. A breeding which gives us such results could very well be used
in test matings, and so it is, but before going into
it, lets return to Fig. #19 and this time, with the colors the same, make the Shorthair smoothhaired
instead of wirehaired. To do this we must change one gene...We mated WwTtRr to WwTtRr, they both
looked like Wirehairs but when we change one gene, W to w, we then mate WwTtRr and wwTtRr and the whole
picture changes...Ww to ww
yields smooth hair to wirehair 1:1 and we come up with 32 Shorthairs and 32
Wirehairs....RrTt and RrTt
continue to give the 3:1 phenotype of course. Make either one
of the remaining hybrid characteristics homozygous, wither the ticking or the
solidness and they will go to 1:1, rather that the 3:1 they now exhibit.
TEST MATING
WHEN WE JOIN HYBRIDS
whether they be peas or pups, we get 1 homozygous dominant, 2 heterozygous
dominants, 1 homozygous recessive, these are the genotypes.....The phenotype is
3:1 but what is the genotype of the 3, how can we tell, they all look alike? With
the 1:1 key the lock is not too difficult to open....For a little variety lets
change the genes we've been playing with and use long hair kk,
which is recessive to shorthair KK. F1 from such breeding give all long hair
hybrids, Kk. F2 will give 1 KK˝ 2 Kk:
1kk....The long hair recessives we can see but we note there are 3 pups with
shorthair. Which of these pups is pure and which are hybrid? If we breed our
recessive kk, to one of its litter mates and get all
shorthaired pups, we know in all probability that we mated our kk to KK and got Kk. If, on the
other foot, we get some shorthairs and some longhairs (ideally it should be
1:1) we have mated our kk to a Kk.
kk to KK can give us NO longhaired puppies, kk to Kk SHOULD give us some kk's longhaired puppies.
The monohybrids
gave us two types of sex cells, the dihybrids four
kinds, the trihybrids eight....This can become pretty
complex fast. A dog and bitch heterozygous for only ten pairs will each produce
1024 genetically different gametes. A checkerboard for that (1024 x 1024) would
contain over a MILLION squares. Since we know that the dog possesses many
thousands of genes in his 78 chromosomes, the more of them that are homozygous
the happier we should be....As was said in Part 1, "whatever minute contribution we can make
toward the goal of genetic purity will be to the everlasting advantage of the
breed in the generations to come."...Some might ask if
homozygosis is always desirable. "Always" covers a lot of territory,
but generally I would say "yes". Recalling our beads for genes
discussion earlier, it is apparent that some qualifications are
indicated....Our long range intention should be the 'improvement of the breed'.
We all agree in this point, the controversy lies in how to achieve the end....
How can the breed be
improved by producing pups which are homozygous recessive for some defective
genes??........What possible contribution can a
Pottering, Scentless, Scrawny, Water Fearing, Gun Shy creature make to better
our breed? How can this "thing" lend aid in our quest for the ideal
GSP? The unthinking reply would be a better dog if he were heterozygous....and
he would be, a hell of a lot better....at least he'd look a lot better in the
field and that's what we're after...But that's a shortsighted
opinion, remember our real goal is breed improvement and for that purpose
heterozygote is limited.
This homozygous
recessive, wretched, little pup contributes in a very real sense, not of course
as a stud but by acting as a sieve or strainer and capturing these defects...By
spotlighting these defective genes which we would be unable to see if they were
not HOMOZYGOUS....The apparently worthless little pup is a martyr to breed
betterment, for surely he should be put away, and with him goes these
previously hidden defectives which have risen to plague us. These harmful
recessives have ruined the dog but they have aided the breeder and benefitted,
the breed. We now know that each parent carries the defect and we can make our
future breeding plans in light of this knowledge. Our further plans can be
based on fact rather than trial and error as was the mating which produced our
little "sieve".
When 10 dissimilar
pairs of alleles are mated and result in over a million genotypes, we become
more aware than ever of the complications of hybridization. Occasionally the
short term gains of such a breeding program may be phenomenal...but, the long
range effects are disappointing. it is safe to say
that if 'genetic purity' is our goal, as I believe it is, hybridization is not the
path to that door.
Lest we despair at this
complexity, let us remember the GSP is already genetically pure for thousands
and thousands of allemorphs or alleles (pairs of
determiners)...that we no longer need concern ourselves with these well-aligned
genes; this is due, in no small part,
to the efforts of the earliest breeders...These Shorthair pioneers have already
aligned for us, many of the most important genes, and even before them, Nature
herself, through natural selection, had already aligned many....That the early
Germans succeeded at all, without even knowing the gene, is further proof of
the importance of ART in breeding...To repeat, no breeder, regardless of his genetic knowledge, can succeed
without an "EYE FOR A DOG"....Yet with that knowledge,
his task is made easier...it does not assure success but it contributes to
success. Knowledge of the action of every gene is not important to our
discussion nor is it necessary to the success of a genetically sound breeding
program. The important consideration, is an understanding of the general
operation of the genetic system...Upon such understanding can be based the
general rules of wise breeding, which we are trying to formulate from this
discussion. We are not trying to be specific. This new science, has not
particularly where the dog is concerned, reached specifics and we must be
satisfied with the assistance it can lend us in broad general terms...Genetics
need not apologize for that which it cannot tell us but can take pride in the
knowledge it has provided. It is we who should realize, that we are not making
use of a fraction of the knowledge provided...Let us not become so engrossed
with the genes themselves, that we lose sight of the geneti
system itself...Preaching the word that knowledge of every gene is necessary
before we can employ the genetic system is sheer nonsense! It may lead many
down the old trail and error (mostly the latter) trail to the disadvantage of
the Shorthair....
The recently considered
mating of the solid liver Wirehair bitch and the liver and spotted Shorthair
stud was not discussed to provide the probably results for an individual who
may be contemplating such a mating but rather to demonstrate the overall
workings of the genetic system by showing the action of three pair of genes,
located on three different chromosomes.
SEX
DETERMINATION
WHENEVER THE SUBJECT
BECOMES DULL, we can usually brighten it a bit with the addition of SEX. Almost
any article or book written today which hope to achieve great interest or
success must bow at the feet of the 'sex goddess'. Oft times the sincerity of
that devotion determines its success to a far greater degree than it s literary
value or the importance of its message. Now I have nothing against sex, in
fact, I am a staunch admirer of the institution but, like other great American
past times, it has its proper time and place...If books had genes, the
homozygous dominant, SS (I'm sure its dominant), could
be found high on the best seller list. Since genetics cannot be discussed
without sex (ss, sexless) we must admit that this is
a hybrid series, Ss, which in this case and a few others, it better than either
SS or ss.
Since sex occurs in
approximately equal numbers (1:1) we must suspect that one of the partners is
heterozygous (like the heterozygous Wirehair, Ww
& the pure Shorthair, ww, recently mated). If
they were homozygous the offspring would all be the same sex, if both were
heterozygous, we would have Mendels is old 3:1 ratio
back...three males to each female or visa versa. So it must be that one is
homozygous and one heterozygous. And so it is, but we are talking as if sex was
decided by a couple of pair of genes. This is not the case, however, since in
all of our discussions to date, we have considered the genes as being associated
with different chromosomes the mechanism is the same. Actually, complete
chromosomes determine sex, rather than individual genes.
The dog has 78
chromosomes...39 pair. That is, the bitch has 39 pair. The dog has 38˝ pair.
More clearly, the female dog has 38 pair plus an W and
a Y. These chromosomes separate in meiosis and are joined again at
fertilization just as the genes we have already discussed. After meiosis, each
egg contains 38 Autosomal Chromosomes (all the chromosomes except
the sex chromosomes) plus an X chromosome. One half the sperms
carry the autosomes plus an X and the other half of
the sperms the 38 plus a Y. When an W egg (they are
all X eggs) is fertilized by an X sperm a female results. When an X egg is
fertilized by a Y sperm a male is produced. Thus the female is homozygous XX,
and the male is heterozygous XY. It can be seen from this that the male sperm
determines (by chance) the sex of the puppies. It is, of course, the female, by
the number of eggs she produces to be fertilized by those sperm,
that determines the size of the litter.
Since there are but two
possible outcomes, as in the flipping of our coin, each litter should
theoretically produce an equal number of male and female puppies. This is the
probability, the law of averages. The fact that this does not materialize in
each litter, yet closely approaches that average over the long haul, gives us a
better understanding of the statistical side of heredity. Failure to produce an
equal number of males and females in a given litter in no way invalidates the
rule...
Imagine, if you can,
some five or six million spermatozoa, like tiny tadpoles, battling their way
from the sea up the mighty Columbia, into the great and twisting Snake, on up
through the gorges of the Salmon in search of 8 or 10 microscopic
eggs...hiding, resting peacefully in little trickle or marsh in the vast
expanse of the high country of central Idaho. It is surprising that they make
it at all. It is a matter of pure chance whether an X sperm or a Y sperm
reaches a given egg first. When the sperm enters the egg an immediate chemical
reaction takes place which bar the entrance of any further sperms. At the
moment of entrance the die is cast. Although it cannot be told microscopically,
the sex of the pup has been decided, indeed the entire
blueprint has been drawn. In the earliest stages the development of the two
sexes parallel each other before any differentiation is observable.
Amateurs as well as
professional scientists, have been working for a long
time attempting to push sex one way or another. Little progress has been made.
Some success has been claimed for the alteration of the pH of the vagina
immediately prior to copulation (acidity favors the
males, alkalinity the females) but until some firm data is established, we had
best leave it in the hands of the gods.
It would appear that
the Y chromosome (sometimes called the HETEROSOME, different chromosome) is of
but secondary importance. In some organisms maleness is decided by the single X
femaleness by the pair of X's. Thus, it may be that the sex of the dog is
determined more by the absence of the X than the presence of the Y.
It is thought that
fewer genes are carried in the sex chromosomes than in the autosomes.
This is unfortunate for breeding, since the heredity in these SHOULD be easier
to follow. The genes in the male X are going to shuttle back and forth between
the sexes through the generations. First in the grandfather, then in the
mother, from the mother to her sone and from the sons
to their daughters and so on....This crisscross inheritance is the basis of
"Mother-line" breeding, for the sire is unimportant in alternate
generations with regard to this chromosome and its genes....The Y chromosome
passes in a direct line father to son, no females ever receiving a Y. Thus with
regard to this chromosome, we can rightly say "like father, like son"
but since the Y is apparently of so little consequence relatively and when we
consider the four sex chromosomes together. We would be more correct to say
that daughters take after their fathers since they receive his most important
sex chromosome (the chromosome which "showed" most) and sons take
after their mother because the only X they have came from her. This should also
be true because that X which came from the dam has no other chromosomes to mask
its effects whether the genes it contains are dominant or recessive. It must be
remembered however, that each chromosome only represents 1/78th of the total
organisms.
LINKAGE
The characteristics
which are controlled by the genes in these sex chromosomes and follow this
pattern are called "sex-linked". Actually the genes in every
chromosome are LINKED in that they remain together in a specific chromosome and
are passed from parent to offspring in that chromosome...They do not exhibit
the independent assortment to the same degree that we have discussed regarding
genes on different chromosomes. They "ignore" to a large extent
Mendel's Second Law.
Let us pursue the
action of a couple of sex linked genes in the dog. Unfortunately, we will have
to borrow from man (which follows the same mechanism) because the only sex
linked characteristic known in the dog, is the hemophelic
gene (as in man) and so will take red-green color
blindness from man (sex linked) and make our dog hemophelic
and color blind. Please don't ask who examined the
eyes of the dog....These two factors are classic genetic examples of sex
linkage in man. Hemophilia, also observed in dogs, is
a condition in which the blood clots very slowly or not at all, and is
considered to be controlled by a recessive mutant gene, as is colorblindness, which has not been observed in the dog. For
the sake of our explanation, we are placing both these recessive genes on the
same X chromosome of our dog. It is possible for each gene to be on a different
X and they would still exhibit sex linkage...but both faults would then not
show up in any male at the same time (because he has only one X) but a female
could have both defects, it would be a rare case indeed since the recessives
would both have to be paired to show....
Actually, hemophilia has been reported in bitches but only rarely has
a case been reported in a human female. These recessives (all recessives) will
show up only, if there is no dominant allemorph to
hide them...In the following figure the black round bead is the normal gene for
sight, the squared black bead represents the normal gene for blood clotting.
The white beads indicate the defective, recessive, mutants....Fig.20.
In (20A) the dam did not
exhibit the defects, although they were present. The father was normal.
One-half the sons were hemophelic and color blind and half the daughters were carriers of both
conditions.
|
|
In
Figure (20B) the father was the defective one....we can use the defective son
from 20A...he showed it because he had no masking dominant. The mother was
normal. All the sons are normal because the only X they got came from their
normal dam....both little bitches were carriers, since the only X they got
came from their defective sire...It is possible to place each defect on a
different X in the above diagram and follow it through, noting that in such a
case, no male would have both defects very few females would show either
defect. The most famous human hemophelic carrier
was Queen |
|
|
|
Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14 | Corrections to Part 12 & 13
Copyright 2001. Dr. James G. McCue, Jr. All rights
reserved. Postscript: And his legacy lives on in the German Shorthaired
Pointers of today. May they always be
healthy and bred with forethought and planning.
Return
Home