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Polygenic Inheritance
Prelab Reading
Thoroughbred Winners Through Quantitative Genetics
For more than 300 years, thoroughbred horses have
been raised for a single purpose — to win at the
racetrack. The origin of these horses can be traced to a
small group that was imported to England from North
Africa and the Middle East in the 1600s. The
population of racing horses remained small until the
1800s, when horse racing became increasing popular;
today there are approximately half a million
thoroughbred horses worldwide.
Breeding and racing thoroughbred horses is a
multibillion-dollar industry that relies on the idea that
a horse’s speed is inherited. Speed is not, however, a
simple genetic characteristic such as seed shape in peas.
Numerous genes and nongenetic factors such as diet,
training, and the jockey who rides the horse all
contribute to a horse’s success in a race. The inheritance
of racing speed in thoroughbreds is more complex than
that of any of the characteristics that we have studied
up to this point. Can the inheritance of a complex
characteristic such as racing speed be studied? Is it
possible to predict the speed of a horse on the basis of
its pedigree? The answers are yes—at least in part—but
these questions cannot be addressed with the methods
that we’ve been using for simple genetic characteristics.
Instead, we must use statistical procedures that have
been developed for analyzing complex characteristics.
The genetic analysis of complex characteristics such as
racing speed of thoroughbreds is known as quantitative
genetics.
Although the mathematical methods for analyzing
complex characteristics may seem to be imposing at
first, most people can intuitively grasp the underlying
logic of quantitative genetics. We all recognize family
resemblance: we talk about inheriting our father’s
height or our mother’s intelligence. Family resemblance
lies at the heart of the statistical methods used in
Adapted from Genetics: A Conceptual Approach (B. A. Pierce)
quantitative genetics. When genes influence a
characteristic, related individuals resemble one another
more than unrelated individuals. Closely related
individuals (such as siblings) should resemble one
another more than distantly related individuals (such as
cousins). Comparing individuals with different degrees
of relatedness, then, provides information about the
extent to which genes influence a characteristic.
This type of analysis has been applied to the
inheritance of racing speed in thoroughbreds. In 1988,
Patrick Cunningham and his colleagues examined
records of more than 30,000 three-year-old horses that
raced between 1961 and 1985. They reasoned that, if
genes influence racing success, a horse’s racing success
should be more similar to that of its parents than to
that of unrelated horses. Similarly, the racing speeds of
half-brothers and half-sisters should be more similar
than the speeds of unrelated horses are. When
Cunningham and his colleagues statistically analyzed
the racing records for thoroughbreds, they found that a
considerable amount of variation in track performance
was due to genetic differences — racing speed is
heritable. With the use of statistics, it is possible to
estimate, with some degree of accuracy, the track
performance of a horse from the performance of its
relatives.
Quantitative Characteristics
Qualitative, or discontinuous, characteristics possess
only a few distinct phenotypes (Fig. 1a); these
characteristics are the types studied by Mendel and
have been the focus of our attention thus far. However,
many characteristics vary continuously along a
spectrum, with many overlapping phenotypes (Fig. 1b).
Imagine a type of plant in which its height is
determined by the amount of plant hormone its cells
produce. There are three genes that are involved in
hormone production. At each genetic location, a
dominant allele produces enough hormone to add 1 cm
of height onto a 10-cm baseline. A recessive allele does
not produce any hormone. If you were asked how high
a plant with the genotype AABbCc would be, here is
how you would figure it out:
Fig. 1. Discontinuous and continuous characteristics differ
in the number of phenotypes exhibited.
They are referred to as continuous characteristics; they
are also called quantitative characteristics because any
individual’s phenotype must be described with a
quantitative measurement (a number). Quantitative
characteristics might include height, weight, and blood
pressure in humans, growth rate in mice, seed weight in
plants, and milk production in cattle. Each of these
would be measured with a number rather than a words
like purple flowers or wrinkled seeds.
Quantitative characteristics arise from two
phenomena. First, many are polygenic—they are
influenced by genes at many loci*. If many loci take
part, many genotypes are possible, each producing a
slightly different phenotype. Second, quantitative
characteristics often arise when environmental factors
affect the phenotype, because environmental
differences result in a single genotype producing a
range of phenotypes. Most continuously varying
characteristics are both polygenic and influenced by
environmental factors, and these characteristics are said
to be multifactorial.
The influence of an organism’s environment also can
complicate the relation between genotype and
phenotype. Because of environmental effects, the same
genotype may produce a range of potential phenotypes.
The phenotypic ranges of different genotypes may
overlap, making it difficult to know whether individuals
differ in phenotype because of genetic or
environmental differences.
In summary, the simple relation between genotype and
phenotype that exists for many qualitative
(discontinuous) characteristics is absent in quantitative
characteristics, and it is impossible to assign a genotype
to an individual on the basis of its phenotype alone.
The methods used for analyzing qualitative
characteristics (examining the phenotypic ratios of
offspring from a genetic cross) will not work with
quantitative characteristics. Our goal remains the same,
though: we wish to make predictions about the
phenotypes of offspring produced in a genetic cross.
We may also want to know how much of the variation
in a characteristic results from genetic differences and
how much results from environmental differences. To
answer these questions, we have to turn to more
advanced statistical methods that allow us to make
predictions about the inheritance of phenotypes in the
absence of information about the underlying
genotypes.
*A locus (loci, plural) is the specific location of a gene on a chromosome. It is derived from the Latin term for place.