Biology Reference
In-Depth Information
a population will reach equilibrium, and that equilibrium state will be
the single state most beneficial to the species. Obviously, this does
not happen. Change is a fact of life. So what is wrong with our
model? A good place to start is by reexamining the model's
assumptions.
First of all, the Hardy-Weinberg assumptions and the model we
developed to study the effect of a maladaptive gene are only
approximations that do not fully reflect reality. Mating is usually not
totally random, and one does not often have a population free from both
immigration and emigration. Catastrophic events, such as disease or a
meteor striking the earth, may wipe out entire species. Mutations occur,
and some may propagate, becoming a non-negligible part of a
population's gene pool—especially if the mutations confer some
advantage. Of course, what may be an advantageous mutation for some
members of a species may be a disadvantage for others, as amply
illustrated by the sickle-cell anemia allele.
In a way, we are back to where we were modeling population growth,
with initial attempts yielding limited insights, but failing to fully
describe what actually happens. For better descriptions and more
refined outcomes, our models need to be modified by adding new
variables and/or parameters. Although carrying out such modifications
falls beyond the scope of this text, a detailed discussion may be found in
Falconer (1989). We shall now move forward to another application of
mathematics to genetics.
VI. QUANTITATIVE TRAITS
A. Discontinuous Versus Continuous Traits
In each of our previous examples, genetic variations within the
population were due to different genotypes involving one or two loci.
These examples give rise to discontinuous traits, where just a few
distinct phenotypes are observed: The seed coats of pea plants are either
round or wrinkled; the seedpods are green or yellow; etc. Because of the
relatively small number of phenotypes, they are easily separated from
one another. Studying the phenotypes of the parents and offspring and
examining the phenotypic ratios can readily establish the connection
between the traits and the genes.
Many other traits, known as continuous traits, do not follow this pattern.
These traits, such as human weight and height, exhibit a wide range of
possible phenotypes. The color of human eyes, for example, varies
from the lightest shades of green and blue to deep dark brown to nearly
black. The branch of genetics that examines the inheritance of
continuous traits is called quantitative genetics. It employs a variety of
quantitative methods to study the genetic make-up of continuous traits
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