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Multiple Gene Traits
Simple traits such as coat color and the merle pattern are due
to the actions of single genes. Individual traits can also be
affected by multiple genes. In the preceding article, two forms
of genetic heterogeneity were discussed where multiple genes
were responsible for slightly different but discrete forms of
the same trait or for the same form of a trait.
There
are two other forms of multiple gene based inheritance of a
single trait. For both of these forms, the genetic effect is
additive. For one form the phenotypic effect of the genes is
continuously additive while for the other form, the additive
effect upon phenotype is only manifested after the genetic component
accumulates to a threshold level. For purpose of discussion,
I will refer to these types of multigenic traits as quantitative
traits and threshold traits.
Quantitative
Traits
Quantitative
traits are usually measurable. In people, height, weight, and
skin color are examples of quantitative traits. Height can be
measured as the number of feet and inches, weight can be measured
as the number of pounds, and skin color can be measured as the
degree of dark pigmentation. Note that these traits can not
be grouped into discrete categories but rather these traits
represent a continuous gradient of variation within the population.
While we may arbitrarily classify people as short, medium or
tall, in reality, there are no discrete height measurements
into which all people fall. If heights of all individuals were
measured and the data plotted, the plot would look like the
one in the diagram below. Note the continuous variation in height
and the arbitrary categorization.
Quantitative traits are determined by the additive action of
many genes. To oversimplify a bit for the sake of explanation,
let us assume that height is determined by 5 different genes
called A, B, C, D, and E. Each gene has 2 alleles. The "recessive"
alleles a, b, c, d, and e make no contribution to height and
the "dominant" alleles A, B, C, D, and E make equal
contribution to height. An individual who has the genotype aabbccddee
would be of the shortest possible height and an individual who
has the genotype AABBCCDDEE would be of the tallest possible
height. Individuals who have the genotype AaBbCcDdEe would be
of average height. Note, that individuals who are AabbCCddEe
and individuals who are aaBBccDDEe would also be of average
height. Why, because the effect of the "dominant"
alleles is additive and all three genotypes that give rise to
average height individuals contain exactly 5 "dominant"
out of the possible 10 "dominant" alleles. Which of
the dominant alleles are present is not important. They all
contribute equally to the final phenotype. As another example,
note that individuals with Aabbccddee, aabbCcddee, and aabbccddEe
genotypes would be all the same height and just a bit taller
than aabbccddee individuals.
How
are quantitative traits inherited? For example, let's say that
dad is of the tallest possible height (AABBCCDDEE) and mom is
of the shortest possible height (aabbccddee). Dad's gamete will
contain all "dominant" alleles (ABCDE) and mom's gametes
will contain all "recessive" alleles. All offspring
that result from the fusion of these gametes will have the genotype
AaBbCcDdEe and thus will be of average height.
Now,
if these average height offspring mate with each other (brother-sister
matings [this is illegal in real life, but this is only an example]),
the gametes they produce can be of all possible types ranging
from all "dominant" (ABCDE) to all "recessive"
(abcde) and all combinations in between (AbcDE, aBCDe, abcdE,
etc.). Therefore, the offspring of these brother-sister matings
will be of all possible sizes, and if a large enough number
of offspring were to be conceived, the distribution would look
very much like the bell shape curve shown in the diagram earlier
in this article.
Ever
wonder why mating two perfectly proportioned dogs does not result
in all perfectly proportioned offspring? That's because most
desirable conformation characteristics are governed by multiple
genes with additive effects. The most desirable dogs are those
which are the most average for many traits. And as the height
example illustrates, the average genotype produces the most
variety of gametes with regard to gene combinations and hence
the greatest variety of phenotypes in the offspring produced
by those gametes.
Threshold
Traits
In
dogs, an accepted example of a threshold trait is hip dysplasia.
Please note that the actual genetics of canine hip dysplasia
have not been worked out and the following is a hypothetical
example made up to explain the principle of inheritance of this
threshold trait.
Let
us assume that 5 additive genes are involved in determining
this trait. Let's call these genes G, H, I, J, and K. Only the
dominant alleles contribute to hip dyspalsia. Individuals who
are gghhiijjkk are not dysplastic and have no chance of being
dysplastic. Individuals who are GGHHIIJJKK are dysplastic. If
dogs of these two genotypes were mated, all offspring would
be GgHhIiJjKk and would not be dysplastic. Why would they not
be dysplastic? Because they only have 5 dominant alleles and
the threshold of expression is 7 dominant alleles. That is,
the trait is phenotypically expressed only if 7 or more of the
dominant alleles are present in the genotype of an individual.
Which dominant alleles are present is not important. For example,
both ggHHIIJJKk and GGHHIIJjkk genotypes would produce hip dyspalsia
because both genotypes contain 7 dominant alleles.
When
trying to minimize diseases such as hip dysplasia that are inherited
as threshold traits, the common practice is to make sure that
the parents are not affected. However, this strategy does very
little to eliminate the condition from the breed. Non-dysplastic
dogs can have a very high propensity to produce dysplastic offspring
if they have a fairly high number of the dominant alleles. For
example, two dogs that each have 6 dominant alleles can produce
a sizable proportion of offspring with 7 or more dominant alleles.
So,
how do you pick breeding stock that minimizes production of
dysplastic offspring? Certainly, you want to check that the
sire and dam are non-dysplastic, but you also want to make sure
that none of their parents had dysplastic siblings and their
own siblings were not dysplastic. The two pedigrees below illustrate
the concept.
The pedigree on the left shows no history of hip dysplasia and,
therefore, it can be concluded that individuals in this pedigree
have a very small number of the dyspalsia producing dominant
alleles. The odds are that offspring produced by individuals
from this pedigree with individuals from similar pedigrees would
have a very low probability of being dyspalstic.
The
pedigree on the right shows a fairly high incidence of hip dysplasia.
The affected individuals have 7 or more of the dysplasia producing
dominant alleles. Any of the phenotypically normal individuals
in this pedigree, while not dysplastic themselves, probably
have a fairly large number of dysplasia producing dominant alleles.
These normal individuals who have dyspalstic siblings, such
as the male marked by "x" should probably not be bred.
Of course if the male marked "x" is an exceptional
specimen that the breeder feels must be bred, then care should
taken to find a female from a hip dysplasia free pedigree (such
as the one on the left) to minimize the probability of production
of dysplastic offspring.
Pedigree
analysis is not in itself sufficient to determine if a trait
is inherited as a threshold trait. However, a comprehensive
database of Australian Shepherd pedigree information should
be of tremendous value to better understand the inheritance
of those traits shown to be inherited as threshold traits.
Disclaimer
Please
note that the above examples are hypothetical. They were made
up to explain the basic principles of inheritance of additive
traits and threshold traits. Also note that the notion of all
dominant alleles having an equal effect toward the phenotype
is an oversimplification. In actuality, alleles of different
genes can have major and minor additive effects. Current research
being carried out to identify genes responsible for canine hip
dyspalsia is geared toward identifying those genes that have
the greatest effect.
Permission is granted for publication of this article in regional
Australian Shepherd Club newsletters provided that the article
is copied in its entirety and the source of the article is acknowledged
as http://aussie-health.westga.edu.
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