The Ins and Outs of Pedigree Analysis, Genetic Diversity,
and Genetic Disease Control
Jerold S. Bell, DVM, Tufts University School of Veterinary
Medicine
As breeders, you engage in genetic "experiments" each time
you plan a mating. The type of mating selected should coincide
with your goals. Outbreeding brings together two animals
less related than the average for the breed. This promotes
more heterozygosity, and usually more variation in a litter.
A reason to outbreed would be to bring in new genes or traits
that your breeding stock does not possess. Outbreeding can
also mask the expression of recessive genes, and allow their
propagation in the carrier state.
Linebreeding attempts to concentrate the genes of a specific
ancestor or ancestors through their appearance multiple times
in a pedigree. The ancestor should appear behind more than
one offspring in the sire and dam's pedigree. Otherwise you
are only linebreeding on the single offspring. A linebreeding
may produce an offspring with magnificent qualities. However,
if those qualities are not present in any of the ancestors
that have been linebred on, the individual may have a wonderful
show career, but it may not breed true. Careful selection
of mates is important, but careful selection of offspring
from the resultant litter is also important to fulfill your
genetic goals. Without this, you are reducing your chances
of concentrating the genes of the linebred ancestor.
Inbreeding significantly increases homozygosity, and therefore
uniformity in litters. Inbreeding can cause the expression
of both beneficial and detrimental recessive genes through
pairing up. Inbreeding cannot change, or create undesirable
genes. It only exposes them through homozygosity. Inbreeding
can also exacerbate a tendency toward disorders controlled
by multiple genes, such as hip dysplasia and congenital heart
anomalies. Unless you have prior knowledge of what milder
linebreeding on the common ancestors has produced, inbreeding
may expose the offspring (and buyers) to extraordinary
risk of genetic defects. Research has shown that inbreeding
depression, or diminished health and viability through inbreeding
is directly related to the amount of detrimental recessive
genes present. Some lines can thrive with inbreeding, and
some cannot.
The inbreeding coefficient is an estimate of the percentage
of all the variable gene pairs that are homozygous due to
inheritance from common ancestors. It is also the average
chance that any single gene pair is homozygous due to inheritance
from a common ancestor. In order to determine whether a particular
mating is an outbreeding or inbreeding relative to your breed,
you must determine the breed's average inbreeding coefficient.
The average inbreeding coefficient of a breed will vary depending
on the breed's popularity or the age of its breeding population.
For the calculated inbreeding coefficient of a pedigree
to be accurate, it must be based on several generations.
Inbreeding in the fifth and later generations (background
inbreeding) often has a profound effect on the genetic
makeup of the offspring represented by the pedigree. In pedigree
studies, the difference in inbreeding coefficients based
on four versus eight-generation pedigrees varied immensely.
A four-generation pedigree containing 28 unique ancestors
for 30 positions in the pedigree could generate a low inbreeding
coefficient, while eight generations of the same pedigree,
which contained 212 unique ancestors out of 510 possible
positions, had a considerably higher inbreeding coefficient.
What seemed like an outbred mix of genes in a couple of generations
appeared as a linebred concentration of genes from influential
ancestors in extended generations.
Many breeders plan matings solely on the appearance of an
animal and not on its pedigree or the relatedness of the
prospective parents. This is called assortative mating. Breeders
use positive assortative matings (like-to-like) to
solidify traits, and negative assortative matings (like-to-unlike) when
they wish to correct traits. Some individuals may share desirable
characteristics, but they inherit them differently. This
is especially true of polygenic traits, such as ear set,
bite or length of forearm. Breeding two phenotypically similar
but genotypically unrelated individuals together would not
necessarily reproduce these traits. Conversely, each individual
with the same pedigree will not necessarily look or breed
alike. Therefore, matings should be based on a combination
of appearance and ancestry.
Rare breeds with small gene pools have concerns about genetic
diversity. Some breed clubs advocate codes of ethics that
discourage linebreeding or inbreeding, as an attempt to increase
breed diversity. The types of matings utilized do not cause
the loss of genes from a breed gene pool. It occurs through
selection; the use and non-use of offspring. Regardless of
the popularity of the breed, if everyone is breeding to a
single stud, (the popular sire syndrome) the gene
pool will drift in that individual's direction and there
will be a loss of genetic diversity. The frequency of his
genes will increase, possibly fixing breed related genetic
disease through the founder's effect. If some breeders linebreed
to certain individuals that they favor, and others linebreed
to other individuals that they favor, then breed-wide genetic
diversity is maintained. Animals who are poor examples of
the breed should not be bred simply to maintain diversity.
Related individuals with desirable qualities will maintain
diversity, and improve the breed.
If you linebreed and are not happy with what you have produced,
breeding to a less related line immediately creates an outbred
line and brings in new traits. Repeated outbreeding to attempt
to dilute detrimental recessive genes is not a desirable
method of control. Recessive genes cannot be diluted; they
are either present or not. If an individual is a known carrier
or a high carrier risk through pedigree analysis, it can
be retired from breeding, and replaced with one or two quality
offspring. Those offspring can be bred, and replaced with
quality offspring of their own, with the hope of losing the
defective gene.
Trying to develop your breeding program scientifically can
be an arduous, but rewarding, endeavor. By taking the time
to understand the types of breeding schemes available, you
can concentrate on your goals towards producing a healthy
and worthy representative of your breed. |
