Biology Reference
In-Depth Information
quantitative, this does not necessarily imply that it is controlled by
multiple loci. Instead, other factors, such as the environment, proper
nutrition, or socioeconomic behaviors, may have a smoothing effect on
the trait, introducing phenotypes that are not entirely determined by
genetic inheritance. These observations raise some interesting and
important questions.
D. GENES, ENVIRONMENTS, AND VARIATION
IN A POPULATION
In the first part of this chapter, we examined the genetics of
discontinuous traits. These are the kinds of traits described by Gregor
Mendel as either dominant or recessive: the alternate forms of tall or
short pea plants with green or yellow seeds which may be smooth or
wrinkled. However, not all inheritance patterns are so straightforward.
As discussed above, it has been experimentally determined that many
traits, such as height, assume their values from a continuum of
possibilities. Additionally, some of the variation in human height is not
caused by genetic factors alone. Environmental factors, such as the
availability and nutritive value of the food consumed by individuals
during childhood, will certainly play a part in determining the height
distribution of a population. Because of the continuous nature of
quantitative traits such as height, it is much more difficult to determine
the relative contributions of heredity and environment to the resulting
variation observed in the population.
It is important to be able to assess the relative contributions of heredity
and environment, because environmental factors have considerable
impact on the development of many phenotypes. For example, genetic
predisposition is known to increase the possibility for developing
type II diabetes, but obesity is another critical factor. Thus, the
development of the disease phenotype depends upon the interaction of
genetic and environmental (including behavioral) factors. Knowing how
much of this predisposition is genetic and how much is caused by
environmental factors would allow individuals to make informed
decisions about diet and exercise, and help public health officials to
make appropriate recommendations to minimize chances of developing
the disease.
Consider all of the women whose mothers had breast cancer. How can
we quantify their risk of developing breast cancer? How would that
quantification change if we were talking about women who pursue low-
fat diets and healthy lifestyles? How would it change if the women in
question were alcoholic chain-smokers? How much of their risk is
caused by genetics, and how much is caused by environmental
(specifically behavioral) factors? In the next chapter, we shall examine
techniques that will help us quantify these kinds of questions.
