Biology Reference
In-Depth Information
population. We examine this question in more detail in the laboratory
manual project Selection in Genetics: The Effect of a Maladaptive or
Lethal Gene.
Another question is why certain genetic diseases that confer a severe
genetic disadvantage are still prevalent in the population, given that our
results show that they should be decreasing and gradually disappear.
For instance, CF is extremely common among the reproductively isolated
population of the Old Order Amish, with frequencies as high as 1 in 500
live births. Why should an allele with such devastating effects be present
at such a high frequency? The answer may lie in a phenomenon called
heterozygote advantage. Some research suggests that individuals who
have one copy of the CFTR allele are partially resistant to the devastating
effects of cholera, typhoid fever, or other gastrointestinal infections.
So individuals with two copies of the normal allele might be at a
disadvantage whenever contracting bacterial diarrhea was a possibility.
An analogous example is sickle-cell anemia among people of African
heritage. Sickle-cell anemia, a life-threatening disease, occurs at high
frequencies in individuals whose ancestors come from those areas of the
world where malaria is endemic. Sickle-cell anemia is caused by a single
mutation in one of the genes for the oxygen-carrying red blood cell
protein hemoglobin. Individuals inheriting two copies of the mutant
allele will have sickle-cell disease, which causes red blood cells to change
shape and clog capillaries, starving the tissues served by those capillaries
of food and oxygen. On the other hand, individuals having two copies of
the normal allele will have normal, circular, biconcave red blood cells,
which carry lots of oxygen and provide a perfect environment for the
malarial protozoan parasite. Malaria, like sickle-cell anemia, is life
threatening. In areas where malarial infection is likely, heterozygotes
with one normal and one mutant allele have a selective advantage in that
the slightly reduced oxygen-carrying capacity of their red blood cells
provides an inhospitable environment for the malarial parasite and thus
gives them resistance against the disease. Among African Americans,
however, the heterozygous state confers no advantage, because malaria
is not common in the United States. For further details on the balance
between some genetic and infectious human diseases, see Dean,
Carrington, and O'Brien (2002).
The last two examples show once again that decisions regarding the use
of a mathematical model should always be made with care. The above
examples show there are situations when the use of the models
developed in this section would not be prudent, as they may fail to
capture the more complex genetic dynamics in a population.
V. MORE COMPLEX HEREDITARY PATTERNS
So far, we have been dealing with only the simplest Mendelian genetic
systems, in which there are only two alleles at a particular locus, one of
