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Dr. Sponenberg is Professor of Pathology and Genetics at
the Virginia-Maryland Regional College of Veterinary Medicine.
Genetics contributions includes publications in peer-reviewed
journals and the book Equine Color Genetics. He is the convener
of the color group of the International Committee on Genetic
Nomenclature of Sheep and Goats. He is active in rare breed
conservation, and serves as the technical coordinator for
the American Livestock Breeds Conservancy. dpsponen@vt.edu;
(540) 231-4805.
ANIMAL BREEDING
Concepts and Basics with Suri Llamas in Mind
Genetics is the science of how traits pass down generation
to generation. Many people think that genetics is a predictive
science-accurately predicting the next baby to hit the ground.
Alas, genetics does not work that way. Instead, genetics can
be viewed as a science of possibilities. Genetics can help
with predicting the overall range of expected types within
the offspring of certain pairs. Genetics is pretty good at
predicting what will happen over the next 100 babies, but
not very good (in most instances) at predicting the details
of the next one. Knowledge of genetics is an extremely powerful tool for animal breeders, although its strengths
and weaknesses both need to be appreciated for it to yield
the greatest benefits.
HOW GENES WORK
Genes, with few exceptions, work in pairs. This is an essential
concept. Each individual gets one member of the pair from
its sire, one from its dam. Each individual, in its own turn,
donates one of each pair to its offspring. Genetics “works”
on the basis of these pairs, and the interactions of the members
of each pair, as well as the interactions of the different
pairs with each other.
Each individual is the result of genes and environment, and
the genetic component is the sum total of all those pairs.
If the genes are considered to be the units of interest, then
the population can be imagined as a “jiggling” of
the genes down through the generations as they are mixed up
and recombined into new combinations at each generational
step. The animal breeder's task is to use the favorable combinations
more heavily than the unfavorable ones, so that the “jiggling”
goes in a positive direction. Differential reproduction (some
animals more than others) is the essence of selective breeding.
The pairs of genes can interact in different ways. Members
of the pair can either be identical or different. If identical,
then obviously that is the character (phenotype) that is expressed.
If different, then a few different things can happen. One
is that only one (a specific one) of the pair is expressed,
and the other is hidden. In this case the one that is expressed
is called “dominant” and the one not expressed is
called “recessive”. This is a key issue - dominant
genes essentially cover up recessive genes. This means that
recessive genes can trail along for many generations without
being expressed, until they are paired up with another identical
recessive gene and are therefore able to be expressed. As
a result, recessive phenotypes (what is expressed) tend to
show up as surprises, and tend to not reproduce themselves
very well unless mated to the same recessive phenotype, or
a dominant phenotype that carries the recessive.
In some situations where members of the gene pair are different,
each member shows up in the phenotype. These situations are
called incompletely dominant, or codominant. Blood types are
a great example of a codominant system - everything is expressed,
nothing is hidden. Good examples of incomplete dominance are
not documented in llamas, but are common in other species.
Palomino horses are a good example - if both genes are “normal”
or dark, the horse is chestnut (reddish). If one “dark”
and one “light” gene are present, the horse is palomino
(yellowish). If two doses of the “light” gene are
present, then the horse is cream with blue eyes.
The critical concept for the way genes interact is that various
mechanisms exist for hiding portions of the genome. The hidden
parts may be good or may be bad, and breeding strategies can
use them to advantage if carefully constructed.
A very common misconception is that common phenotypes are
dominant, and uncommon ones are recessive. The relative frequency
of a trait is simply a matter of gene frequency, or how many
copies of a specific gene are represented in a population.
The relative frequency has absolutely nothing to do with the
dominance or recessiveness of a system. A good example is
white horses. White in horses is dominant, and yet this color
is very, very rare due to the gene having a very low frequency
among horse populations. Chestnut in horses is recessive,
and yet some entire breeds (such as the Suffok) are chestnut
because selector has fixed the gene frequency at 100%
BREEDING PHILOSOPHY
A crucial first step for breeding programs is to decide upon
a philosophy. Philosophies include conservation, improvement,
and a host of others. A conservation philosophy is going to
dictate different goals and actions than a strict animal improvement
philosophy, which is also going to be different than a companion
animal philosophy. No single philosophy is wrong, they are
just each different. Many discussions, and even some heated
arguments, can stem from different breeders having different
philosophies. The variety of philosophies is probably good
for the overall health of the genetic resource, since each
breeder is doing something slightly different and this helps
the population have desired levels of genetic diversity.
Philosophy drives goals. Why are animals being bred, and
what is the mental picture of the ideal animal? Is the goal
show wins? Conformation? Certain fiber characteristics? Certain
colors? Without answering these questions (honestly) very
little progress is possible in a breeding program. Progress
is difficult enough as it is - and is definitely enhanced
by acknowledging a philosophy and the goals that go along
with it.
SELECTION
Selection simply means that some animals get to reproduce
more than do other animals. Selection differential indicates
the relative proportion of animals that do reproduce. In alpacas
the selection differential for females is pretty high, since
close to one hundred percent. That is, nearly every female
is used for reproduction and therefore gets a chance to pass
along her genes for good or ill. For llamas this is much lower.
For males the selection differential is smaller, but how
much smaller varies with individual breeders. The selection
differential for dairy bulls is probably the smallest of the
common domesticated species, since by artificial insemination
only one bull in thousands is used. For llamas the selection
differential for males is smaller than for females, but still
only moderate when compared to many other species.
The point of selection is that it is what dictates the form
of the succeeding generations. Selection determines which
traits get passed along and which do not. The results of selection
are easy to demonstrate for Peruvian versus North American
alpacas. While certainly individual breeder’s goals differ
in both locations, the general trend is that Peruvians favor
white, and favor huacaya fiber. In North America the opposite
is generally true (with exceptions, of course). The result
of the selection exerted in the two areas is that the gene
frequencies, and phenotypic frequencies, of the two alpaca
populations are going to differ because the selection pressures
are different.Selection changes gene frequencies, and that
limits the component genes in the population that can jiggle
down to the next generations. The desirability of this is
hardly debatable for disease traits (get rid of the genes,
get rid of the disease), but is more subjective for other
traits such as color and fleece variants.
BREEDING STRATEGIES
Breeding strategies include inbreeding and outbreeding. There
are varying levels of these, and each has an appropriate place
in a healthy population structure. Each does something different,
and they are value neutral - being good or bad in different
situations and for different goals.
Inbreeding includes any mating in which the mated animals
have ancestors in common. That is, the mating “doubles
up” on certain ancestors. This can happen to varying
degrees. When first-degree relatives (parent to offspring,
sibling to sibling) are mated, the result is generally regarded
as inbreeding. When more distant matings are accomplished
(grandparent to grand offspring, aunt to nephew) the matings
are more likely to be considered linebreeding. There is no
magic point at which the boundary between inbreeding and linebreeding
is drawn.
Inbreeding tends to make animals more genetically uniform.
That is, the pairs of genes are more likely to be similar
than they are likely to be different. This has a variety of
consequences, which can be good or bad depending on what goes
into the mix. That is, good things become consistent, or bad
things become consistent. Therefore, inbreeding must be accompanied
by selection. Very, very good and consistent populations of
animals in a variety of species have been accomplished by
inbreeding to varying degrees. The key strength of an inbred
or linebred animal is that since the gene pairs are generally
alike, the animal produces very uniform offspring. This is
one of the main strengths of a linebred animal - predictability.
A very important aspect of inbreeding is that as it proceeds
and the gene pool gets narrower and narrower, traits of general
fitness tend to suffer in a population. These include reproductive
traits, milk production, growth rates, and size traits. Also
disease resistance traits may well suffer, although this is
going to vary. The point here is that inbreeding, especially
if not associated with selection, has consequences that may
not be all that good.
Outbreeding tends to do the opposite of inbreeding. It tends
to make populations more variable by matching up unlike members
in the gene pairs. Outbred animals, since they have unlike
gene pairs, tend to produce variable offspring.
Outbred matings are those that do not have ancestors in common.
Outbreeding or outcrossing can vary in extent, just like inbreeding.
The widest outbreeding is to mate a llama to an alpaca, guanaco,
or vicuña. The trick to outbreeding is that the products
of the initial cross are very likely to be very uniform. If
100 babies were produced, they may actually end up looking
like near copies of one another (to the extent possible in
any animal related endeavor). So where is the variability?
It is locked up in the fact that for each of these outbred
animals the gene pairs are unlike, and so when these uniform
animals are used for reproduction they in turn produce extreme
variability.
Outbred animals can therefore be very, very productive animals.
The initial outbred product can be uniform, and they also
have excellence for those very traits that suffer under inbreeding:
vitality, reproduction, and growth. The peculiar qualities
of inbreeding and outbreeding are used to great advantage
in some animal industries. Egg laying chickens, for example,
are the result of crossing inbred parental or grandparental
lines. The resulting hens are uniform as a consequence of
the linebreeding behind the parents, which constrains each
gene pair to be one each of specific genes. They are also
vigorous since the gene pairs are unlike. And - they are useless
for anyone else to breed from, since they will produce uneven
offspring. This tactic protects the investment of the breeder
companies; since it does not matter into whose hands the actual
laying hens fall.
So which is best - inbreeding or outbreeding? Depends entirely
on the breeder’s goals. Inbreeding tends to bring recessive
genes to the light of day by forcing them into pairing with
one another. That can be good or bad, depending on the trait
and the selection imposed on it. Alternatively, outbreeding
tends to hide recessive genes. Note well, though, that these
genes are still in the population, and in a form against which
selection cannot occur since they are not expressed. Some
deleterious genes could therefore become very widespread in
a population before even discovered. A good example is the
combined immunodeficiency of Arabian foals. About 20% of Arabian
horses carry this gene, resulting in about 4% affected foals
being born. The gene was allowed to get to this high frequency
by lack of selection on the part of breeders.
FIBER CHARACTERISTICS
Fiber characteristics are very important to alpacas, especially
if they are to enter mainstream production agriculture. The
alpaca fiber is unique, and its uniqueness is important to
foster and enhance. Llamas can have fiber the equal of alpacas,
though when taken as a whole, llamas in general have poorer
quality fiber. That means that llama breeders interested in
high-quality fiber production have their work cut out for
them.
Fleece quality varies in a host of ways, many of them strongly
influenced by genes. The main list of traits that are largely
genetic includes growth rate, density, fineness, uniformity,
handle (texture, feel), and color. Color is the easiest, but
the most important traits are probably growth rate, density,
uniformity, and fineness. All of these are affected by environment
as well as by genes, but fortunately the genetic component
is relatively large and so selection can be based on individual
performance. That is, looking at the animal itself is accurate
enough, and progeny testing does not add much.
SURI
The suri fleece variant is a most interesting variant, since
the resultant fleece is unlike any other mammalian fiber.
As a spinner I find it something like silk, and very different
from other mammalian fibers. Suri inheritance is complicated.
The suri variant is reported to be inherited as a dominant
trait by some Australian researchers. This means that huacaya
to huacaya should never (and one is reluctant to use that
word) produce suris, while suri to suri could well produce
a proportion of huacaya offspring.
Unfortunately this simple pattern does not tell the whole
truth, since (if rarely) huacaya to huacaya matings produce
suri offspring. In addition, the results of mating suri to
huacaya consistently produce more huacayas than expected.
These phenomena point to a genetic mechanism for inhibiting
suri expression in at least some huacayas.
Given the present state of knowledge on Suri genetics, the
best recommendation at this time is that most matings involving
Suris should be Suri to Suri. This avoids complicating the
gene pool by producing ‘hidden’ Suris within the
Huacaya gene pool. It is important to realize that in some
instances it makes perfect sense to use Suri x Huacaya matings.
This is especially for introduction of certain fiber or color
characters into the Suri, or for a handful of other helpful
and appropriate reasons. Matings between the two types, though,
should generally be for reasons other than simply increasing
Suri numbers, since that strategy will eventually complicate
things more than it will help them.
Suri llamas, in contrast, bring a host of more complicated
issues with them - largely because of their rarity. For suri
llamas it is unreasonable to eliminate suri x nonsuri (one
hesitates to indicate “huacaya” in this sense) matings.
However, the nonsuri mates should be carefully chosen in order
to maximize the production of high quality suri offspring.
A few strategies that can do this are to assure minimal guard
hair is present. One very good strategy is to preferentially
use nonsuris that have been produced by suri parents. These
animals, by virtue of their genetic background, are likely
to have a number of characteristics that lead to good suri
phenotype when paired up with the major suri-producing genes.
DEFECTS
A variety of physical defects occur in llamas, and are important
to breeders of llamas since the production of defective babies
has two negative aspects. One negative is the loss or suffering
of the baby. The second loss is the tarnished image of the
parents producing the defective baby. Few (if any) defects
have yet to be proven genetic in origin, but certainly some
are very good candidates: choanal atresia, angular limb deformities.
In the event that some defects are shown to be due to simple
single genes, then selection becomes pretty easy. The affected
animals can be culled, and with modern genetic techniques
it is reasonable to expect there to be blood or DNA tests
developed to spot carriers. Carriers can then be used wisely
in reproduction. If a carrier is only average, the best idea
is to cull. If a carrier has some other excellent traits,
then the carrier could be used on a limited basis, hoping
to replace the carrier with a noncarrier offspring that is
excellent. The key to the single gene traits is that on average
half of the offspring will be carriers, but the other half
will not. A single gene can therefore be tracked, and eliminated
with careful breeding practices.
Other defects are due to polygenes. These traits include
some, such as cardiac defects, where animals have no defect
until the number of genes passes some threshold. Above the
threshold the defect is expressed, and with increasing numbers
of genes the severity of the defect is increased. The trick
with these, though, is that since the defect is associated
with many genes it is impossible to use breeding practice
to eliminate these. Any animal with the defect, and any animal
that is a first degree relative (parent, sibling) is more
likely to have lots of these genes than is a random member
of the population. That means that with few exceptions selection
should be sure and firm against bearers of such defects as
well as their first degree relatives. Again, philosophy will
come into play here.
It is critically important to react to defects appropriately
- worry about them when it is worthwhile, and ignore them
if they are very rare. The incidence rate of defects is therefore
important, and usually unknown. If a defect occurs in only
one of 200 births or fewer, it is probably not worth worrying
about. If 1 in 100 or 1 in 50, then it is worth worrying about.
Some sort of anonymous, accurate tracking system is needed
simply to track the incidence of these, so that an increase
can be met with appropriate action, while rare ones can largely
be ignored.
MATING OF EXTREMES (assortative mating)
The issue of defects brings up the subject of “assortative
mating”. This simply means the deliberate mating of animals
that are similar (positive assortative mating) or very different
(negative assortative mating).
Positive assortative mating, when accomplished with conformational
traits, translates into the mating of similarly extreme animals.
This, in many species, can include the mating of the very
large to the very large, very small to very small, or any
other peculiarities of conformation (cute, short heads, on
and on). In many species, dogs being the best example, this
ends up giving us extreme breeds such as the dachshund, Boston
terriers, Irish Wolfhounds, Saint Bernards, and a host of
others that fall outside the norm for the original species.
This can be bad or good, but frequently brings along associated
defects. Some of this depends on exactly what goes into the
original mix.
In cats, for instance, short-headedness is desired in the
Persian, and these animals have very little problem (unless
the extreme end has some trouble breathing). Selection for
a similar head shape in the Burmese has included a single
gene that results in brain abnormalities in some of the kittens.
The lesson here is that extremes can cause animals to trespass
over the limit of soundness. This is especially so for conformational
extremes, and usually not for extremes of fiber quality.
The take-home lesson is that overall soundness and conformational
quality needs to be the bottom line minimum when selecting
breeding stock, and that slow progress with soundness is better
than fast, extreme progress that might well leave overall
soundness behind.
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